~ubuntu-branches/ubuntu/wily/gargoyle-free/wily-proposed

#ifdef RCSID
static char RCSid[] =
"$Header$";
#endif

/* 
 *   Copyright (c) 1999, 2002 Michael J. Roberts.  All Rights Reserved.
 *   
 *   Please see the accompanying license file, LICENSE.TXT, for information
 *   on using and copying this software.  
 */
/*
Name
  vmdbg.cpp - T3 VM debugger API
Function
  
Notes
  
Modified
  11/23/99 MJRoberts  - Creation
*/

#include <stdio.h>
#include <string.h>

#include "t3std.h"
#include "os.h"
#include "charmap.h"
#include "vmglob.h"
#include "vmtype.h"
#include "vmdbg.h"
#include "vmrun.h"
#include "vmsrcf.h"
#include "vmhash.h"
#include "vmpool.h"
#include "vmop.h"
#include "vmlst.h"
#include "tctarg.h"
#include "tcglob.h"
#include "tctok.h"
#include "tcprs.h"
#include "tcmain.h"
#include "resload.h"
#include "tchostsi.h"
#include "tchost.h"
#include "tcgen.h"
#include "vmimage.h"
#include "vmhost.h"
#include "vmvec.h"
#include "vmbignum.h"
#include "vmanonfn.h"


/* ------------------------------------------------------------------------ */
/*
 *   Special override host interface for compiling expressions in the
 *   debugger.  This host interface allows us to capture error messages
 *   generated during compilation for display in the debugger user
 *   interface. 
 */
class CTcHostIfcDebug: public CTcHostIfc
{
public:
    CTcHostIfcDebug()
    {
        /* initially clear the message buffer */
        reset_messages();
    }

    /* clear the message buffer */
    void reset_messages()
    {
        errmsg_[0] = '\0';
        errmsg_free_ = errmsg_;
        have_errmsg_ = FALSE;
    }

    /* get the text of the first error message since we cleared the buffer */
    const char *get_error_msg() const { return errmsg_; }
    
    /*
     *   CTcHostIfc interface 
     */
    
    /* display an informational message */
    virtual void v_print_msg(const char *, va_list)
    {
        /* ignore informational messages */
    }

    /* display a process step message */
    virtual void v_print_step(const char *, va_list)
    {
        /* ignore process step messages */
    }

    /* display an error message */
    virtual void v_print_err(const char *msg, va_list args)
    {
        /* if we haven't already captured an error message, capture it */
        if (!have_errmsg_)
        {
            char buf[1024];

            /* format the message */
            t3vsprintf(buf, sizeof(buf), msg, args);

            /* append as much of it as possible to our buffer */
            strncpy(errmsg_free_, buf,
                    sizeof(errmsg_) - (errmsg_free_ - errmsg_) - 1);

            /* ensure the message is null-terminated */
            errmsg_[sizeof(errmsg_) - 1] = '\0';

            /* advance the free pointer past what we just added */
            errmsg_free_ += strlen(errmsg_free_);

            /* 
             *   if there's a newline in the appended message, we're done
             *   gathering this message line 
             */
            if (strchr(buf, '\n') != 0)
            {
                char *p;
                
                /* note that we have the complete first error line now */
                have_errmsg_ = TRUE;

                /* terminate the message at the newline */
                p = strchr(errmsg_, '\n');
                if (p != 0)
                    *p = '\0';
            }
        }
    }

private:
    /* our error message buffer */
    char errmsg_[256];

    /* pointer to next available byte of message buffer */
    char *errmsg_free_;

    /* flag: we've captured an error message */
    uint have_errmsg_ : 1;
};


/* ------------------------------------------------------------------------ */
/*
 *   Compiler parser symbol table interface.  This provides an
 *   implementation of the compiler parser's debugger symbol table
 *   interface, which allows the compiler to look up symbols in the image
 *   file's debug records.  
 */
class CVmDbgSymtab: public CTcPrsDbgSymtab
{
public:
    CVmDbgSymtab()
    {
        globals_ = 0;
        frame_id_ = 0;
    }
    
    /* initialize in a given scope */
    CVmDbgSymtab(VMG_ const CVmDbgTablePtr *debug_table, uint frame_id,
                 uint stack_level)
    {
        /* initialize with the given parameters */
        init(vmg_ debug_table, frame_id, stack_level);
    }

    /* find a symbol - implementation of CTcPrsDbgSymtab interface */
    virtual int find_symbol(const textchar_t *sym, size_t len,
                            tcprsdbg_sym_info *info);

    void init(VMG_ const CVmDbgTablePtr *debug_table, uint frame_id,
              uint stack_level)
    {
        /* save the globals if necessary */
        globals_ = VMGLOB_ADDR;

        /* save my debug table pointer */
        debug_ptr_.copy_from(debug_table);

        /* save my frame ID */
        frame_id_ = frame_id;

        /* save my stack level */
        stack_level_ = stack_level;
    }

private:
    /* VM globals */
    vm_globals *globals_;
    
    /* my debug records table */
    CVmDbgTablePtr debug_ptr_;

    /* active frame ID for this scope */
    uint frame_id_;

    /* stack frame level */
    uint stack_level_;
};

/*
 *   Find a symbol 
 */
int CVmDbgSymtab::find_symbol(const textchar_t *sym, size_t len,
                              tcprsdbg_sym_info *info)
{
    CVmDbgFramePtr frame;
    VMGLOB_PTR(globals_);

    /* if I have no local frame, there's nothing to do */
    if (frame_id_ == 0)
        return FALSE;
    
    /* 
     *   search the enclosing frames, starting with my innermost frame and
     *   working outwards 
     */
    debug_ptr_.set_frame_ptr(vmg_ &frame, frame_id_);
    for (;;)
    {
        CVmDbgFrameSymPtr sym_ptr;
        uint i;
        
        /* iterate over the symbols in this frame */
        for (i = frame.get_sym_count(), frame.set_first_sym_ptr(&sym_ptr) ;
             i != 0 ;
             --i, sym_ptr.inc(vmg0_))
        {
            /* if this one matches, we found it */
            if (sym_ptr.get_sym_len(vmg0_) == len
                && memcmp(sym_ptr.get_sym(vmg0_), sym, len) == 0)
            {
                /* fill in the type information for the caller */
                if (sym_ptr.is_ctx_local())
                {
                    /* it's a context local */
                    info->sym_type = TC_SYM_LOCAL;
                    info->ctx_arr_idx = sym_ptr.get_ctx_arr_idx();

                    /* fill in the local variable ID and stack level */
                    info->var_id = sym_ptr.get_var_num();
                    info->frame_idx = stack_level_;
                }
                else if (sym_ptr.is_local())
                {
                    /* it's a local */
                    info->sym_type = TC_SYM_LOCAL;
                    info->ctx_arr_idx = 0;

                    /* fill in the local variable ID and stack level */
                    info->var_id = sym_ptr.get_var_num();
                    info->frame_idx = stack_level_;
                }
                else if (sym_ptr.is_param())
                {
                    /* it's a parameter variable */
                    info->sym_type = TC_SYM_PARAM;
                    info->ctx_arr_idx = 0;

                    /* fill in the parameter ID and stack level */
                    info->var_id = sym_ptr.get_var_num();
                    info->frame_idx = stack_level_;
                }
                else
                {
                    /* unknown symbol type */
                    info->sym_type = TC_SYM_UNKNOWN;
                }

                /* tell the caller we found it */
                return TRUE;
            }
        }
        
        /* if there's no enclosing frame, we're done */
        if (frame.get_enclosing_frame() == 0)
            break;

        /* move on to the enclosing frame */
        debug_ptr_.set_frame_ptr(vmg_ &frame, frame.get_enclosing_frame());
    }

    /* failure */
    return FALSE;
}


/* ------------------------------------------------------------------------ */
/*
 *   Creation
 */
CVmDebug::CVmDebug(VMG0_)
{
    /* we have not yet been initialized during program load */
    program_inited_ = FALSE;

    /* we have no UI context yet */
    ui_ctx_ = 0;

    /* not currently in the debugger */
    in_debugger_ = FALSE;

    /* there's no valid debug pointer yet */
    dbg_ptr_valid_ = FALSE;

    /* no method headers loaded yet */
    method_hdr_ = 0;
    method_hdr_cnt_ = 0;

    /* 
     *   start in step-in mode, so that we break as soon as we start
     *   executing the program
     */
    set_step_in();

    /* note in step-over-breakpoint mode yet */
    step_over_bp_ = FALSE;
    step_frame_depth_ = 0;

    /* no global breakpoints yet */
    global_bp_cnt_ = 0;

    /* 
     *   we have no valid source line bounds yet, so set both ends to zero
     *   - zero is never a valid PC address (even if a function is at
     *   address zero, its method header will precede any executable code
     *   in the function, hence the program counter could never be at
     *   zero) 
     */
    cur_stm_start_ = cur_stm_end_ = 0;

    /* create our reverse-lookup hash tables */
    obj_rev_table_ = new CVmHashTable(256, new CVmHashFuncDbgRev, TRUE);
    prop_rev_table_ = new CVmHashTable(256, new CVmHashFuncDbgRev, TRUE);
    func_rev_table_ = new CVmHashTable(256, new CVmHashFuncDbgRev, TRUE);
    enum_rev_table_ = new CVmHashTable(256, new CVmHashFuncDbgRev, TRUE);

    /* create a host interface for the compiler */
    hostifc_ = new CTcHostIfcDebug();

    /* no halt requested yet */
    G_interpreter->set_halt_vm(FALSE);
}

/*
 *   Deletion
 */
CVmDebug::~CVmDebug()
{
    /* delete the host interface object */
    delete hostifc_;

    /* delete our reverse-lookup hash tables */
    delete obj_rev_table_;
    delete prop_rev_table_;
    delete func_rev_table_;
    delete enum_rev_table_;

    /* if we allocated a method header array, delete it */
    if (method_hdr_ != 0)
        t3free(method_hdr_);
}

/* ------------------------------------------------------------------------ */
/*
 *   VM initialization 
 */
void CVmDebug::init(VMG_ const char *image_fname)
{
    /* note that the program has been initialized */
    program_inited_ = TRUE;

    /* tell the UI to initialize */
    CVmDebugUI::init(vmg_ image_fname);

    /* initialize the compiler */
    CTcMain::init(hostifc_, G_host_ifc->get_cmap_res_loader(), 0);
}

/*
 *   Initialization phase 2 - after loading the image file 
 */
void CVmDebug::init_after_load(VMG0_)
{
    /* tell the UI to initialize */
    CVmDebugUI::init_after_load(vmg0_);
}

/*
 *   VM termination 
 */
void CVmDebug::terminate(VMG0_)
{
    CVmDebugBp *bp;
    size_t i;

    /* if we were never initialized, there's nothing to terminate */
    if (!program_inited_)
        return;

    /* tell the UI to terminate */
    CVmDebugUI::terminate(vmg0_);

    /* tell the breakpoint objects we're about to terminate */
    for (i = 0, bp = bp_ ; i < VMDBG_BP_MAX ; ++i, ++bp)
        bp->do_terminate(vmg0_);

    /* terminate the compiler */
    CTcMain::terminate();
}


/* ------------------------------------------------------------------------ */
/*
 *   Determine if the image file has debugger information 
 */
int CVmDebug::image_has_debug_info(VMG0_) const
{
    /* ask the image loader if there's a GSYM block */
    return G_image_loader->has_gsym();
}


/* ------------------------------------------------------------------------ */
/*
 *   Add an entry to the reverse-lookup hash tables 
 */
void CVmDebug::add_rev_sym(const char *sym, size_t sym_len,
                           tc_symtype_t sym_type, ulong sym_val)
{
    CVmHashTable *table;
    CVmHashEntryDbgRev *entry;
    
    /* figure out which table to use, based on the symbol type */
    switch(sym_type)
    {
    case TC_SYM_FUNC:
        /* function */
        table = func_rev_table_;
        break;
        
    case TC_SYM_OBJ:
        /* object */
        table = obj_rev_table_;
        break;

    case TC_SYM_PROP:
        /* property */
        table = prop_rev_table_;
        break;

    case TC_SYM_ENUM:
        /* enumerator */
        table = enum_rev_table_;
        break;

    default:
        /* 
         *   ignore other symbol types - we don't maintain reverse-lookup
         *   tables for anything else 
         */
        return;
    }

    /* create the symbol */
    entry = new CVmHashEntryDbgRev(sym_val, sym, sym_len);

    /* add it to the table */
    table->add(entry);
}

/*
 *   Look up a symbol in one of our reverse-mapping tables 
 */
const char *CVmDebug::find_rev_sym(const CVmHashTable *tab,
                                   ulong val) const
{
    CVmHashEntryDbgRev *entry;
    
    /* look up the symbol */
    entry = (CVmHashEntryDbgRev *)tab->find((char *)&val, sizeof(val));

    /* if we found it, return the name pointer, otherwise null */
    return (entry != 0 ? entry->get_sym() : 0);
}

/*
 *   Find the actual symbol for a 'modify' chain. 
 */
const char *CVmDebug::get_modifying_sym(const char *sym_name) const
{
    /* if there's no original symbol, there's obviously no modifier */
    if (sym_name == 0)
        return 0;

    /* keep going until we find the base object */
    for (;;)
    {
        CTcSymObj *sym;
        
        /* look up the symbol in the global symbol table */
        sym = (CTcSymObj *)G_prs->get_global_symtab()
              ->find(sym_name, strlen(sym_name));

        /* 
         *   if we found it, and it has a modifier object, proceed up to the
         *   modifier object 
         */
        if (sym != 0
            && sym->get_type() == TC_SYM_OBJ
            && sym->get_modifying_obj_id() != VM_INVALID_OBJ)
        {
            const char *new_sym_name;
            
            /* 
             *   Forget the current object, and use the modification object
             *   instead.  Look up the symbol name for the modification
             *   object, and replace the previous symbol - which was just a
             *   fake symbol that the compiler synthesized anyway - with the
             *   modifying object.  At the top of the chain of modifying
             *   objects is the actual symbol name used in the source code,
             *   which is what we're after.  
             */
            new_sym_name = objid_to_sym(sym->get_modifying_obj_id());

            /* 
             *   if we can't find the modifying object name, stop at the
             *   previous symbol 
             */
            if (new_sym_name == 0)
                return sym_name;

            /* we got a symbol - use it */
            sym_name = new_sym_name;
        }
        else
        {
            /* 
             *   There's either no symbol or no modifier, so we can stop
             *   looking.  Simply return what we have.  
             */
            return sym_name;
        }
    }
}


/* ------------------------------------------------------------------------ */
/*
 *   Get information on the source location at a given stack level 
 */
int CVmDebug::get_source_info(VMG_ const char **fname,
                              unsigned long *linenum, int level) const
{
    CVmFuncPtr func_ptr;
    CVmDbgLinePtr line_ptr;
    CVmSrcfEntry *srcf_entry;
    ulong stm_start;
    ulong stm_end;

    /* if there's no source file table, we can't get any information */
    if (G_srcf_table == 0)
        return 1;

    /* get information on the execution location at the requested level */
    if (get_stack_level_info(vmg_ level, &func_ptr, &line_ptr,
                             &stm_start, &stm_end))
        return 1;

    /* get the source file entry for this line's source file index */
    srcf_entry = G_srcf_table->get_entry(line_ptr.get_source_id());

    /* if we didn't find an entry, we can't return any information */
    if (srcf_entry == 0)
        return 1;

    /* fill in the caller's filename pointer */
    *fname = srcf_entry->get_name();

    /* fill in the caller's line number record */
    *linenum = line_ptr.get_source_line();

    /* success */
    return 0;
}

/* ------------------------------------------------------------------------ */
/*
 *   Enumerate local variables at the given stack level 
 */
void CVmDebug::enum_locals(VMG_ void (*cbfunc)(void *, const char *, size_t),
                           void *cbctx, int level)
{
    CVmFuncPtr func_ptr;
    CVmDbgLinePtr line_ptr;
    CVmDbgTablePtr dbg_ptr;
    CVmDbgFramePtr frame_ptr;
    CVmDbgFrameSymPtr sym;
    ulong stm_start;
    ulong stm_end;
    uint i;
    uint frame_id;

    /* get information on the execution location at the requested level */
    if (get_stack_level_info(vmg_ level, &func_ptr, &line_ptr,
                             &stm_start, &stm_end))
        return;

    /* set up a debug table pointer for the function */
    if (!func_ptr.set_dbg_ptr(&dbg_ptr))
        return;

    /* if there's a 'self' in this context, add it to the list */
    if (G_interpreter->get_self_at_level(vmg_ level) != VM_INVALID_OBJ)
        (*cbfunc)(cbctx, "self", 4);

    /* iterate over the enclosing frames, starting with the innermost */
    for (frame_id = line_ptr.get_frame_id() ; frame_id != 0 ;
         frame_id = frame_ptr.get_enclosing_frame())
    {
        /* set up a frame table pointer for the current local frame */
        dbg_ptr.set_frame_ptr(vmg_ &frame_ptr, frame_id);

        /* set up a pointer to the first symbol */
        frame_ptr.set_first_sym_ptr(&sym);

        /* iterate through the frame and call the callback for each symbol */
        for (i = frame_ptr.get_sym_count() ; i != 0 ; --i, sym.inc(vmg0_))
            (*cbfunc)(cbctx, sym.get_sym(vmg0_), sym.get_sym_len(vmg0_));
    }
}

/* ------------------------------------------------------------------------ */
/*
 *   Build a stack traceback listing 
 */
void CVmDebug::build_stack_listing(VMG_
                                   void (*cbfunc)(void *, const char *, int),
                                   void *cbctx, int source_info)
{
    ulong method_ofs;
    vm_val_t *fp;
    vm_obj_id_t self_obj;
    pool_ofs_t entry_addr;
    int level;
    
    /* start at the current function */
    fp = G_interpreter->get_frame_ptr();
    self_obj = G_interpreter->get_self(vmg0_);
    entry_addr = entry_ofs_;
    method_ofs = pc_ - entry_ofs_;

    /* iterate through the frames */
    for (level = 0 ; fp != 0 ;
         fp = G_interpreter->get_enclosing_frame_ptr(vmg_ fp), ++level)
    {
        vm_obj_id_t def_obj;
        int argc;
        char buf[256];
        int i;
        char *p;
        char *open_paren_ptr = 0;
        char *close_paren_ptr = 0;
        size_t rem;

        /* get the current 'self' object */
        self_obj = G_interpreter->get_self_from_frame(vmg_ fp);

        /* get the current number of arguments */
        argc = G_interpreter->get_argc_from_frame(vmg_ fp);
        
        /* get the current function's defining object */
        def_obj = G_interpreter->get_defining_obj_from_frame(vmg_ fp);

        /* determine whether we have an object.method or a function call */
        if (method_ofs == 0)
        {
            /* 
             *   a zero method offset indicates a native caller making a
             *   recursive entry into the byte-code interpreter 
             */
            sprintf(buf, "<System>");

            /* 
             *   we don't know anything about the arguments to a native
             *   routine 
             */
            argc = 0;
        }
        else if (def_obj == VM_INVALID_OBJ)
        {
            const char *func_sym;
            
            /* it's a function call */
            if ((func_sym = funcaddr_to_sym(entry_addr)) != 0)
                sprintf(buf, "%.255s", func_sym);
            else
                sprintf(buf, "%08lx", entry_addr);
        }
        else
        {
            const char *def_obj_sym;
            const char *self_obj_sym;
            const char *prop_sym;
            vm_prop_id_t prop_id;
            int is_anon_fn = FALSE;
            
            /* it's an object.method call */
            def_obj_sym = objid_to_sym(def_obj);
            self_obj_sym = objid_to_sym(self_obj);

            /* translate the object to the original 'modify' object name */
            def_obj_sym = get_modifying_sym(def_obj_sym);

            /* 
             *   Add the object name.  If we have a name for 'self', use that
             *   name; otherwise, if we have a name for the defining object,
             *   use that, and show the self object number after it;
             *   otherwise, if this is an anonymous function object, note
             *   that and show it specially; otherwise just show the object
             *   number.  
             */
            if (def_obj != VM_INVALID_OBJ
                && CVmObjAnonFn::is_anonfn_obj(vmg_ def_obj))
            {
                /* 
                 *   it's an anonymous function - note this, and use a
                 *   special format for the display 
                 */
                is_anon_fn = TRUE;
                sprintf(buf, "{anonfn:%lx}", def_obj);
            }
            else if (self_obj_sym != 0)
            {
                /* 'self' has a name, so show that */
                strcpy(buf, self_obj_sym);
            }
            else if (def_obj_sym != 0)
            {
                /* 
                 *   'self' has no name, but the defining object does, so
                 *   show the defining object name and add the 'self' object
                 *   number 
                 */
                sprintf(buf, "%.240s [%lx]", def_obj_sym, self_obj);
            }
            else
            {
                /* there's no name to be had, so use the object number */
                sprintf(buf, "[%lx]", self_obj);
            }

            /* prepare to add to the buffer */
            p = buf + strlen(buf);

            /* get the property ID */
            prop_id = G_interpreter->get_target_prop_from_frame(vmg_ fp);
            if (is_anon_fn)
            {
                /* it's an anonymous function - there's no property part */
                *p = '\0';
            }
            else if (prop_id != VM_INVALID_PROP
                     && (prop_sym = propid_to_sym(prop_id)) != 0)
            {
                /* we got the property symbol - add it */
                sprintf(p, ".%.255s", prop_sym);
            }
            else
            {
                /* unknown property name */
                sprintf(p, ".prop#%x", (uint)prop_id);
            }
        }

        /* get the remainder of the buffer */
        p = buf + strlen(buf);
        rem = sizeof(buf) - (p - buf);

        /* add the open paren */
        if (entry_addr != 0)
        {
            open_paren_ptr = p;
            *p++ = '(';
            --rem;
        }

        /* add the arguments */
        for (i = 0 ; i < argc ; ++i)
        {
            size_t cur_len;

            /* 
             *   if we're running out of space, just add an indication
             *   that more arguments follow, and stop here 
             */
            if (rem <= 10)
            {
                strcpy(p, " ...");
                p += 4;
                rem -= 4;
                break;
            }

            /* 
             *   add this argument, making sure we leave room over for the
             *   closing paren, newline, null terminator, and if necessary
             *   an ellipsis indicating we ran out of room 
             */
            format_val(vmg_ p, rem - 10,
                       G_interpreter->get_param_from_frame(vmg_ fp, i));

            /* move past the latest addition */
            cur_len = strlen(p);
            rem -= cur_len;
            p += cur_len;

            /* add a comma if this isn't the last argument */
            if (i + 1 < argc)
            {
                *p++ = ',';
                *p++ = ' ';
                rem -= 2;
            }
        }

        /* add a close paren */
        if (entry_addr != 0)
        {
            close_paren_ptr = p;
            *p++ = ')';
        }

        /* if we have room, add the method offset */
        if (entry_addr != 0 && rem > 12)
        {
            sprintf(p, "+ %lx", method_ofs);
            p += strlen(p);
        }

        /* add a newline and a null terminator */
        *p++ = '\n';
        *p = '\0';

        /* 
         *   if the method offset is non-zero, and they want source line
         *   information, add it 
         */
        if (method_ofs != 0 && source_info)
        {
            const char *fname;
            unsigned long linenum;
            
            /* get the source information */
            if (!get_source_info(vmg_ &fname, &linenum, level))
            {
                char numbuf[32];
                size_t fname_len;
                size_t num_len;
                
                /* convert the line number to a string */
                sprintf(numbuf, " line %ld]\n", linenum);

                /* get the lengths of what we're adding */
                fname_len = strlen(fname);
                num_len = strlen(numbuf);

                /* 
                 *   if we don't have room, try getting the root of the
                 *   filename, in case it has a long path prefix 
                 */
                if (fname_len + num_len + 2 >= rem)
                {
                    /* skip to the root of the filename */
                    fname = os_get_root_name((char *)fname);

                    /* get the new length */
                    fname_len = strlen(fname);
                }

                /* 
                 *   If it's still too long, we must have a really long
                 *   argument list - try removing some of the argument list,
                 *   adding "..." in place of the removed bits, to make room
                 *   for the line number.  Don't bother if we can't free up
                 *   enough space even doing this.  
                 */
                if (fname_len + num_len + 2 >= rem
                    && close_paren_ptr != 0
                    && (fname_len + num_len + 2
                        < rem + (close_paren_ptr - open_paren_ptr - 4)))
                {
                    char *dst;
                    size_t adjust;

                    /* 
                     *   get the adjustment size - back up by as much as we
                     *   need to make the filename/number fit, plus space
                     *   for the terminating null and the ellipsis 
                     */
                    adjust = (fname_len + num_len + 2 - rem + 1 + 3);
                    
                    /* back up by the adjustment */
                    dst = close_paren_ptr - adjust;

                    /* insert the ellipsis */
                    memcpy(dst, "...", 3);

                    /* move the part after the close paren */
                    memmove(dst + 3, close_paren_ptr,
                            strlen(close_paren_ptr));

                    /* 
                     *   adjust by the amount we moved, not counting the
                     *   ellipsis we added 
                     */
                    rem += adjust - 3;
                    p -= adjust - 3;
                }
                
                /* add it if there's room */
                if (fname_len + num_len + 2 < rem)
                {
                    /* change the newline to a space */
                    *(p-1) = ' ';

                    /* add the filename and line number in brackets */
                    *p = '[';
                    memcpy(p + 1, fname, fname_len);
                    memcpy(p + 1 + fname_len, numbuf, num_len + 1);
                }
            }
        }

        /* invoke the callback for this level */
        (*cbfunc)(cbctx, buf, strlen(buf));

        /* move on to the enclosing frame */
        entry_addr = G_interpreter
                     ->get_enclosing_entry_ptr_from_frame(vmg_ fp);
        method_ofs = G_interpreter->get_return_ofs_from_frame(vmg_ fp);
    }
}

/* ------------------------------------------------------------------------ */
/*
 *   Toggle a breakpoint 
 */
int CVmDebug::toggle_breakpoint(VMG_ ulong code_addr,
                                const char *cond, int change,
                                int *bpnum, int *did_set,
                                char *errbuf, size_t errbuflen)
{
    char *code_ptr;
    CVmDebugBp *bp;

    /* if there's a code address, get a pointer to it */
    if (code_addr != 0)
    {
        /* get a writable pointer to the code location */
        code_ptr = G_code_pool->get_writable_ptr(code_addr);

        /* if that failed, we can't set a breakpoint */
        if (code_ptr == 0)
        {
            /* set a message if appropriate */
            if (errbuf != 0 && errbuflen != 0)
            {
                strncpy(errbuf, "unable to write code address", errbuflen - 1);
                errbuf[errbuflen - 1] = '\0';
            }
            
            /* return failure */
            return 1;
        }
    }
    else
    {
        /* global breakpoint - no code address is required */
        code_ptr = 0;
    }

    /* search for a breakpoint at this same address */
    bp = find_bp(code_addr);

    /* 
     *   if we found it, we're deleting the old one; otherwise, we're
     *   creating a new one 
     */
    if (bp == 0)
    {
        int err;
        
        /* setting a new breakpoint - allocate a new record */
        bp = alloc_bp();
        if (bp == 0)
        {
            /* generate a message */
            if (errbuf != 0 && errbuflen != 0)
            {
                strncpy(errbuf, "out of internal breakpoint records",
                        errbuflen - 1);
                errbuf[errbuflen - 1] = '\0';
            }

            /* return failure */
            return 2;
        }

        /* initialize the breakpoint */
        if ((err = bp->set_info(vmg_ code_addr, cond, change, FALSE,
                                errbuf, errbuflen)) != 0)
        {
            /* un-allocate the breakpoint */
            bp->set_in_use(FALSE);
            
            /* return the error */
            return err;
        }

        /* indicate to the caller that we're creating a breakpoint */
        *did_set = TRUE;

        /* 
         *   fill in the breakpoint ID for the caller - this is just the
         *   1-based index into our breakpoint array of the breakpoint
         *   record 
         */
        *bpnum = (int)(bp - bp_) + 1;

        /* if it's global, count it */
        if (bp->is_global())
            ++global_bp_cnt_;
    }
    else
    {
        /* if it's global, count the deletion */
        if (bp->is_global() && !bp->is_disabled())
            --global_bp_cnt_;
        
        /* free the breakpoint record */
        bp->do_delete(vmg0_);

        /* indicate to the caller that we're deleting the breakpoint */
        *did_set = FALSE;

        /* let the caller know which breakpoint we deleted */
        *bpnum = (int)(bp - bp_) + 1;
    }

    /* success */
    return 0;
}

/*
 *   Toggle a breakpoint's enabled/disabled status 
 */
void CVmDebug::toggle_breakpoint_disable(VMG_ int bpnum)
{
    CVmDebugBp *bp;

    /* if the breakpoint ID is invalid, ignore the request */
    if (bpnum < 1 || bpnum > VMDBG_BP_MAX)
        return;

    /* get the breakpoint record (the breakpoint ID is a 1-based index) */
    bp = bp_ + (bpnum - 1);

    /* toggle the disabled flag */
    set_breakpoint_disable(vmg_ bpnum, !bp->is_disabled());
}

/*
 *   Set a breakpoint's disabled status 
 */
void CVmDebug::set_breakpoint_disable(VMG_ int bpnum, int disable)
{
    CVmDebugBp *bp;

    /* if the breakpoint ID is invalid, ignore the request */
    if (bpnum < 1 || bpnum > VMDBG_BP_MAX)
        return;

    /* get the breakpoint record (the breakpoint ID is a 1-based index) */
    bp = bp_ + (bpnum - 1);

    /* set the new state */
    bp->set_disabled(disable);

    /* add or remove the breakpoint from the code if necessary */
    bp->set_bp_instr(vmg_ !bp->is_disabled(), FALSE);

    /* if it's global, count the change */
    if (bp->is_global())
    {
        if (bp->is_disabled())
            --global_bp_cnt_;
        else
            ++global_bp_cnt_;
    }
}

/*
 *   Determine if a breakpoint is disabled 
 */
int CVmDebug::is_breakpoint_disabled(VMG_ int bpnum)
{
    CVmDebugBp *bp;

    /* if the breakpoint ID is invalid, ignore the request */
    if (bpnum < 1 || bpnum > VMDBG_BP_MAX)
        return FALSE;

    /* get the breakpoint record (the breakpoint ID is a 1-based index) */
    bp = bp_ + (bpnum - 1);

    /* return the status */
    return (bp->is_disabled() != 0);
}

/*
 *   Set a breakpoint's condition expression 
 */
int CVmDebug::set_breakpoint_condition(VMG_ int bpnum,
                                       const char *cond, int change,
                                       char *errbuf, size_t errbuflen)
{
    CVmDebugBp *bp;
    int err;

    /* if the breakpoint ID is invalid, ignore the request */
    if (bpnum < 1 || bpnum > VMDBG_BP_MAX)
    {
        /* set the error message if needed */
        if (errbuf != 0 && errbuflen != 0)
        {
            strncpy(errbuf, "invalid breakpoint", errbuflen - 1);
            errbuf[errbuflen - 1] = '\0';
        }

        /* return failure */
        return 1;
    }

    /* get the breakpoint record (the breakpoint ID is a 1-based index) */
    bp = bp_ + (bpnum - 1);

    /* set the condition */
    if ((err = bp->set_condition(vmg_ cond, change, errbuf, errbuflen)) != 0)
        return err;

    /* success */
    return 0;
}

/*
 *   Delete a breakpoint 
 */
void CVmDebug::delete_breakpoint(VMG_ int bpnum)
{
    CVmDebugBp *bp;
    
    /* if the breakpoint ID is invalid, ignore the request */
    if (bpnum < 1 || bpnum > VMDBG_BP_MAX)
        return;

    /* get the breakpoint record (the breakpoint ID is a 1-based index) */
    bp = bp_ + (bpnum - 1);

    /* 
     *   If the breakpoint is global, update the global bp count.  (Don't
     *   bother if the breakpoint is disabled, as we don't include disabled
     *   breakpoints in the count in the first place.) 
     */
    if (bp->is_global() && !bp->is_disabled())
        --global_bp_cnt_;

    /* clear the breakpoint */
    bp->do_delete(vmg0_);
}

/*
 *   Allocate a new breakpoint record 
 */
CVmDebugBp *CVmDebug::alloc_bp()
{
    size_t i;
    CVmDebugBp *bp;

    /* scan our list for an entry not currently in use */
    for (i = 0, bp = bp_ ; i < VMDBG_BP_MAX ; ++i, ++bp)
    {
        /* if this one isn't in use, allocate it */
        if (!bp->is_in_use())
        {
            /* mark it in use */
            bp->set_in_use(TRUE);

            /* return the new record */
            return bp;
        }
    }

    /* there are no free records - return failure */
    return 0;
}

/*
 *   Find a breakpoint at a given address 
 */
CVmDebugBp *CVmDebug::find_bp(ulong code_addr)
{
    size_t i;
    CVmDebugBp *bp;
    
    /* 
     *   if the code address is zero, we know there can't be a breakpoint
     *   there 
     */
    if (code_addr == 0)
        return 0;

    /* scan our list */
    for (i = 0, bp = bp_ ; i < VMDBG_BP_MAX ; ++i, ++bp)
    {
        /* if this one is in use, and it matches the address, it's the one */
        if (bp->is_in_use() && bp->get_code_addr() == code_addr)
            return bp;
    }

    /* we didn't find it */
    return 0;
}


/* ------------------------------------------------------------------------ */
/*
 *   Format a value into the given buffer 
 */
void CVmDebug::format_val(VMG_ char *dst, size_t dstlen, const vm_val_t *val)
{
    const char *p;
    char buf[128];

    /* if there's no buffer, we can't do anything */
    if (dstlen == 0)
        return;

    /* format the value based on the type */
    switch(val->typ)
    {
    case VM_NIL:
        /* constant 'nil' value */
        p = "nil";
        break;
        
    case VM_TRUE:
        /* constant 'true' value */
        p = "true";
        break;

    case VM_OBJ:
        /* object reference - get the object's name */
        p = objid_to_sym(val->val.obj);

        /* if there's no symbol name, try alternative display formats */
        if (p == 0)
        {
            /* try it as a string */
            if ((p = vm_objp(vmg_ val->val.obj)->get_as_string(vmg0_)) != 0)
                goto format_string;

            /* try it as a list */
            if ((p = vm_objp(vmg_ val->val.obj)->get_as_list()) != 0)
                goto format_list;

            /* try it as a BigNumber */
            if (CVmObjBigNum::is_bignum_obj(vmg_ val->val.obj))
            {
                CVmObjBigNum *bn;
                char *nump;

                /* cast it to a BigNumber object */
                bn = (CVmObjBigNum *)vm_objp(vmg_ val->val.obj);

                /* set up a prefix to flag it as a BigNumber value */
                sprintf(buf, "obj#%lx (BigNumber ", (ulong)val->val.obj);

                /* 
                 *   write at the end of our prefix, minus two bytes for
                 *   the length prefix that the string converter will
                 *   write into the buffer (we'll fix up the ovewritten
                 *   bytes later) 
                 */
                nump = buf + strlen(buf) - 2;

                /* format it into our buffer */
                if (bn->cvt_to_string_buf(vmg_ nump,
                                          sizeof(buf) - (nump - buf) - 4,
                                          64, -1, -1, -1, 0) != 0)
                {
                    size_t len;
                    
                    /* get the length prefix */
                    len = vmb_get_len(nump);

                    /* put the close of the string */
                    strcpy(nump + len + 2, ")");

                    /* fix up the bytes we overwrote with the length prefix */
                    *nump = 'r';
                    *(nump + 1) = ' ';
                    
                    /* use the buffer */
                    p = buf;
                    
                    /* handled */
                    break;
                }
            }

            /* 
             *   It's a dynamically-created object with no name.  Show it
             *   by object it. 
             */
            sprintf(buf, "obj#%lx", (ulong)val->val.obj);

            /* if possible, add the name of the of the superclass */
            if (vm_objp(vmg_ val->val.obj)
                ->get_superclass_count(vmg_ val->val.obj) > 0)
            {
                vm_obj_id_t sc_obj;

                /* get the superclass */
                sc_obj = vm_objp(vmg_ val->val.obj)
                         ->get_superclass(vmg_ val->val.obj, 0);

                /* if it's valid, look up the superclass name */
                if (sc_obj != 0 && (p = objid_to_sym(sc_obj)) != 0)
                {
                    /* add "(superclass name)" after the numeric ID */
                    sprintf(buf + strlen(buf), " (%.100s)", p);
                }
            }

            /* use the buffer we formatted */
            p = buf;
        }
        break;

    case VM_PROP:
        /* property ID - get the property name */
        p = propid_to_sym(val->val.prop);
        if (p != 0)
        {
            /* we have a symbol - use it with an ampersand operator */
            sprintf(buf, "&%.127s", p);
        }
        else
        {
            /* there's no symbol, so use the numeric property ID */
            sprintf(buf, "prop#%x", (uint)val->val.prop);
        }

        /* use the text we built in the buffer */
        p = buf;
        break;

    case VM_ENUM:
        /* enum ID - get the enum name */
        p = enum_to_sym(val->val.enumval);

        /* if there's no s ymbol, use the numeric enum ID */
        if (p == 0)
        {
            sprintf(buf, "enum#%x", (uint)val->val.enumval);
            p = buf;
        }
        break;

    case VM_INT:
        /* integer */
        sprintf(buf, "%ld", val->val.intval);
        p = buf;
        break;

    case VM_SSTRING:
        /* length-prefixed string - get a pointer to the data */
        p = G_const_pool->get_ptr(val->val.ofs);

    format_string:
        /* 
         *   first, add an open quote, if we minimally have room for the
         *   quote plus a null terminator 
         */
        if (dstlen > 1)
        {
            *dst++ = '\'';
            --dstlen;
        }
        
        /* copy as much as we can */
        {
            size_t rem;
            utf8_ptr strp;

            /* read and skip the length prefix */
            rem = osrp2(p);
            p += 2;

            /* set up a utf-8 pointer to the source string */
            strp.set((char *)p);

            /* 
             *   keep going until we run out of source text or space in
             *   our output buffer - leave ourselves 5 extra bytes (close
             *   quote, "..." if we run out of space, and null terminator) 
             */
            for ( ; rem != 0 && dstlen > 5 ; strp.inc(&rem))
            {
                wchar_t uni;
                int mappable;

                /* get the current unicode character code */
                uni = strp.getch();

                /* 
                 *   determine if the character is mappable to the local
                 *   character set 
                 */
                mappable = G_cmap_to_ui->is_mappable(uni);

                /* 
                 *   If there's no mapping, insert a '\u' sequence for the
                 *   character.  If the character is in the non-printable
                 *   control character range, use a backslash
                 *   representation as well.  If the character is a
                 *   backslash or single quote, also represent it as a
                 *   backslash sequence, since it would need to be quoted
                 *   in a string submitted to the compiler.  
                 */
                if (uni < 32 || uni == '\\' || uni == '\'')
                {
                    char esc;
                    unsigned int dig;

                    switch(uni)
                    {
                    case 9:
                        /* tab - '\t' */
                        esc = 't';

                    do_escape:
                        /* we need two bytes, plus our 5 reserve */
                        if (dstlen < 2 + 5)
                            goto out_of_str_space;

                        /* add the escape sequence */
                        *dst++ = '\\';
                        *dst++ = esc;

                        /* consume the two bytes */
                        dstlen -= 2;
                        break;
                        
                    case 10:
                        /* newline - '\n' */
                        esc = 'n';
                        goto do_escape;
                        
                    case 0x000F:
                        /* caps - '\^' */
                        esc = '^';
                        goto do_escape;

                    case 0x000E:
                        /* uncaps - '\v' */
                        esc = 'v';
                        goto do_escape;

                    case 0x000B:
                        /* blank - '\b' */
                        esc = 'b';
                        goto do_escape;

                    case 0x0015:
                        /* quoted space - '\ ' */
                        esc = ' ';
                        goto do_escape;

                    case '\\':
                    case '\'':
                        esc = (char)uni;
                        goto do_escape;

                    default:
                        /* 
                         *   represent anything else as a hex sequence -
                         *   we need four bytes, plus the 5 reserve 
                         */
                        if (dstlen < 4 + 5)
                            goto out_of_str_space;

                        /* add the backslash and 'x' */
                        *dst++ = '\\';
                        *dst++ = 'x';

                        /* add the the first hex digit */
                        dig = (uni >> 4) & 0xF;
                        *dst++ = dig + (dig < 10 ? '0' : 'A' - 10);

                        /* add the second hex digit */
                        dig = (uni & 0xF);
                        *dst++ = dig + (dig < 10 ? '0' : 'A' - 10);

                        /* consume the space */
                        dstlen -= 4;
                        break;
                    }
                }
                else if (!mappable)
                {
                    int i;
                    unsigned int c;
                    
                    /* 
                     *   the '\u' sequence requires six bytes (backslash,
                     *   'u', and the four hex digits of the character
                     *   code), plus the 5 reserve we always leave, so if
                     *   we don't have room stop now 
                     */
                    if (dstlen < 5 + 6)
                        break;

                    /* consume the space */
                    dstlen -= 6;

                    /* add the '\u' */
                    *dst++ = '\\';
                    *dst++ = 'u';

                    /* add the four hex digits */
                    for (c = (unsigned int)uni, i = 0 ; i < 4 ; ++i)
                    {
                        unsigned int dig;
                        
                        /* get the current most significant digit's value */
                        dig = (c >> 12) & 0xf;
                        
                        /* generate the next hex digit */
                        *dst++ = dig + (dig < 10 ? '0' : 'A' - 10);

                        /* shift up so the next digit is in current */
                        c <<= 4;
                    }
                }
                else
                {
                    char buf[20];
                    size_t xlat_len;
                    
                    /* get the translation of this character */
                    xlat_len = G_cmap_to_ui->map_char(uni, buf, sizeof(buf));

                    /* if there's no room, stop now */
                    if (dstlen < xlat_len + 5)
                        break;

                    /* copy the translation */
                    memcpy(dst, buf, xlat_len);

                    /* advance past the space we've used */
                    dst += xlat_len;
                    dstlen -= xlat_len;
                }
            }

        out_of_str_space:
            /* add an ellipsis at the end if we ran out of space */
            if (rem != 0 && dstlen >= 5)
            {
                *dst++ = '.';
                *dst++ = '.';
                *dst++ = '.';
                dstlen -= 3;
            }

            /* add the close quote */
            if (dstlen > 1)
                *dst++ = '\'';

            /* add the null terminator */
            if (dstlen > 0)
                *dst++ = '\0';
        }

        /* we're done - don't bother with the normal copying */
        return;

    case VM_LIST:
        /* constant list - get the data pointer */
        p = G_const_pool->get_ptr(val->val.ofs);

    format_list:
        {
            size_t i;
            size_t cnt;
            
            /* get the element count */
            cnt = vmb_get_len(p);

            /* add the open bracket */
            if (dstlen >= 2)
            {
                *dst++ = '[';
                --dstlen;
            }

            /* scan through the elements (using a 1-based counter) */
            for (i = 1 ; i <= cnt && dstlen > 7 ; ++i)
            {
                vm_val_t val;
                size_t cur_len;

                /* get this element */
                CVmObjList::index_list(vmg_ &val, p, i);

                /* add this element, leaving room for closing data */
                format_val(vmg_ dst, dstlen - 7, &val);

                /* skip past this value */
                cur_len = strlen(dst);
                dst += cur_len;
                dstlen -= cur_len;

                /* add a comma between elements if possible */
                if (dstlen > 6 && i != cnt)
                {
                    *dst++ = ',';
                    --dstlen;
                }
            }

            /* 
             *   if we didn't exhaust the list, and we have room, indicate
             *   that more follows 
             */
            if (i <= cnt && dstlen >= 5)
            {
                *dst++ = '.';
                *dst++ = '.';
                *dst++ = '.';
                dstlen -= 3;
            }

            /* add the closing bracket if possible */
            if (dstlen >= 2)
                *dst++ = ']';

            /* add the null terminator */
            *dst = '\0';
        }

        /* we're done - don't bother with normal copying */
        return;

    case VM_FUNCPTR:
        /* function pointer - get the function name */
        p = funcaddr_to_sym(val->val.ofs);

        /* if there's no symbol, use the numeric function address */
        if (p == 0)
        {
            sprintf(buf, "function#%08lx", (ulong)val->val.ofs);
            p = buf;
        }
        break;

    default:
        p = "?";
        break;
    }

    /* copy as much of the value as possible into the buffer */
    strncpy(dst, p, dstlen);

    /* make sure the buffer is null-terminated */
    dst[dstlen - 1] = '\0';
}

/* ------------------------------------------------------------------------ */
/*
 *   Callback context structure for enum_props_eval_cb 
 */
struct enum_props_eval_cb_ctx
{
    /* the UI callback function to invoke and its own context */
    void (*ui_cb)(void *, const char *, int, const char *);
    void *ui_cb_ctx;

    /* the original object whose properties we're enumerating */
    vm_obj_id_t obj;

    /* debugger object */
    CVmDebug *dbg;
};

/*
 *   Object property enumeration callback for evaluating an object-valued
 *   expression.  
 */
static void enum_props_eval_cb(VMG_ void *ctx0,
                               vm_obj_id_t self, vm_prop_id_t prop,
                               const vm_val_t *val)
{
    enum_props_eval_cb_ctx *ctx = (enum_props_eval_cb_ctx *)ctx0;
    const char *prop_name;
    vm_val_t ov_val;
    vm_obj_id_t src_obj;

    /* 
     *   The property enumerator tells us about all of the properties
     *   throughout the entire inheritance tree.  Make sure this one isn't
     *   overridden - if it is, we'll already have shown (or suppressed
     *   showing) the overriding copy of the property.  Since overridden
     *   properties can't be seen in the actual object, we don't want to show
     *   them here.  
     */
    if (vm_objp(vmg_ ctx->obj)->get_prop(vmg_ prop, &ov_val,
                                         ctx->obj, &src_obj, 0)
        && src_obj != self)
    {
        /* 
         *   we found the property, but it was defined in a different object
         *   than the object whose properties we're enumerating - this must
         *   be an inherited version of the property which the object
         *   overrides, so we don't want to show it among the original
         *   object's properties 
         */
        return;
    }

    /* get the name of this property */
    prop_name = ctx->dbg->propid_to_sym(prop);

    /* 
     *   if we couldn't get a name for the property, don't bother invoking
     *   the UI callback, since it won't be able to use the property
     *   anyway 
     */
    if (prop_name == 0)
        return;

    /* ignore methods and properties containing self-printing strings */
    switch(val->typ)
    {
    case VM_NIL:
    case VM_TRUE:
    case VM_OBJ:
    case VM_PROP:
    case VM_INT:
    case VM_ENUM:
    case VM_SSTRING:
    case VM_LIST:
    case VM_FUNCPTR:
        /* 
         *   these types are all valid for enumeration - invoke the UI
         *   callback
         */
        (*ctx->ui_cb)(ctx->ui_cb_ctx, prop_name, strlen(prop_name), ".");
        break;

    default:
        /* do not enumerate any other types */
        break;
    }
}

/* ------------------------------------------------------------------------ */
/*
 *   Evaluate an expression 
 */
int CVmDebug::eval_expr(VMG_ char *buf, size_t buflen, const char *expr,
                        int level, int *is_lval, int *is_openable,
                        void (*aggcb)(void *, const char *,
                                      int, const char *),
                        void *aggctx, int speculative)
{
    int err;
    CVmPoolDynObj *code_obj;
    vm_val_t expr_val;
    CVmDbgSymtab local_symtab;
    vmdbg_step_save_t old_step;
    vm_obj_id_t self_obj;
    vm_obj_id_t orig_target_obj;
    vm_obj_id_t defining_obj;
    vm_prop_id_t target_prop;
    vmrun_save_ctx run_ctx;
    
    /* presume it won't be an lvalue or openable */
    if (is_lval != 0)
        *is_lval = FALSE;
    if (is_openable != 0)
        *is_openable = FALSE;

    /* clear the buffer in case we can't evaluate the expression */
    buf[0] = '\0';
    
    /* get information on the indicated stack level */
    {
        CVmFuncPtr func_ptr;
        CVmDbgLinePtr line_ptr;
        CVmDbgTablePtr dbg_ptr;
        ulong stm_start;
        ulong stm_end;

        if (get_stack_level_info(vmg_ level, &func_ptr, &line_ptr,
                                 &stm_start, &stm_end)
            || !func_ptr.set_dbg_ptr(&dbg_ptr))
            return 1;
    
        /* set up our local symbol table interface for this context */
        local_symtab.init(vmg_ &dbg_ptr, line_ptr.get_frame_id(), level);
    }

    /* get the 'self' object for the selected level */
    self_obj = G_interpreter->get_self_at_level(vmg_ level);

    /* get the target property for the level */
    target_prop = G_interpreter->get_target_prop_at_level(vmg_ level);

    /* get the original target object and defining object at the level */
    orig_target_obj = G_interpreter->get_orig_target_obj_at_level(vmg_ level);
    defining_obj = G_interpreter->get_defining_obj_at_level(vmg_ level);

    /* compile the expression */
    if ((err = compile_expr(vmg_ expr, level, &local_symtab,
                            self_obj != VM_INVALID_OBJ,
                            speculative, is_lval,
                            &code_obj, buf, buflen)) != 0)
        return err;

    /* note the pre-call stack depth */
    G_interpreter->save_context(vmg_ &run_ctx);

    /* set up for the recursive execution */
    prepare_for_eval(&old_step);

    /* execute the code in a protected block */
    err_try
    {
        /* execute the code in a recursive call to the VM */
        G_interpreter->do_call(vmg_ 0, code_obj->get_ofs(), 0,
                               self_obj, target_prop,
                               orig_target_obj, defining_obj, "dbg eval");
    }
    err_catch(exc)
    {
        const char *msg;
        size_t msg_len;
        static const char prefix[] = "error: ";

        /* copy the prefix into the buffer if there's room */
        if (buflen > sizeof(prefix) + 20)
        {
            memcpy(buf, prefix, sizeof(prefix));
            buflen -= sizeof(prefix) - 1;
            buf += sizeof(prefix) - 1;
        }
        
        /* note the error */
        err = exc->get_error_code();

        /* 
         *   if it's "unhandled exception," get the message from the
         *   exception object itself; otherwise, format the message for
         *   the error 
         */
        if (err == VMERR_UNHANDLED_EXC)
        {
            /* get the message from the exception */
            msg = CVmRun::get_exc_message(vmg_ exc, &msg_len);
            if (msg == 0)
                msg = "Unhandled program exception";

            /* limit the size to the caller's buffer size */
            if (msg_len > buflen - 1)
                msg_len = buflen - 1;

            /* copy it into the caller's buffer and null-terminate it */
            memcpy(buf, msg, msg_len);
            buf[msg_len] = '\0';
        }
        else
        {
            /* format the VM error message into the caller's result buffer */
            msg = err_get_msg(vm_messages, vm_message_count, err, FALSE);
            err_format_msg(buf, buflen, msg, exc);
        }
    }
    err_end;

    /* restore the original execution mode */
    restore_from_eval(&old_step);

    /* restore the interpreter context */
    G_interpreter->restore_context(vmg_ &run_ctx);

    /* delete the byte code object */
    G_code_pool->get_dynamic_ifc()->dynpool_delete(code_obj);

    /* get the return value, if any */
    if (err == 0)
    {
        /* get the value from R0 */
        expr_val = *G_interpreter->get_r0();

        /* leave the value on the stack for gc protection */
        G_stk->push(&expr_val);

        /* format the value into the buffer */
        format_val(vmg_ buf, buflen, &expr_val);
    }
    else
    {
        /* return failure */
        return 1;
    }

    /* if an aggregation callback is provided, use it */
    if (aggcb != 0)
    {
        const char *p;
        size_t cnt;
        size_t i;
        
        /* determine if the object has contents to be iterated */
        switch(expr_val.typ)
        {
        case VM_LIST:
            /* get the pointer to the constant list data */
            p = G_const_pool->get_ptr(expr_val.val.ofs);

        agg_list:
            /* get the element count */
            cnt = vmb_get_len(p);

            /* iterate through the elements */
            for (i = 1 ; i <= cnt ; ++i)
            {
                char idxbuf[30];
                
                /* format an index operator for this index */
                sprintf(idxbuf, "[%u]", i);

                /* invoke the callback with this subitem */
                (*aggcb)(aggctx, idxbuf, strlen(idxbuf), "");
            }
            break;

        case VM_OBJ:
            /* if it's a list object, use the list contents */
            if ((p = vm_objp(vmg_ expr_val.val.obj)->get_as_list()) != 0)
                goto agg_list;

            /* if it's an array, handle it specially */
            if (CVmObjVector::is_vector_obj(vmg_ expr_val.val.obj))
            {
                CVmObjVector *vec;
                size_t cnt;

                /* cast it to an array object */
                vec = (CVmObjVector *)vm_objp(vmg_ expr_val.val.obj);

                /* iterate through the elements */
                cnt = vec->get_element_count();
                for (i = 1 ; i <= cnt ; ++i)
                {
                    char idxbuf[30];
                    
                    /* foramt an index operator for this index */
                    sprintf(idxbuf, "[%u]", i);

                    /* invoke the callback with this subitem */
                    (*aggcb)(aggctx, idxbuf, strlen(idxbuf), "");
                }

                /* we've handled the item */
                break;
            }

            /* determine if the object provides a property list */
            if (vm_objp(vmg_ expr_val.val.obj)->provides_props(vmg0_))
            {
                enum_props_eval_cb_ctx cb_ctx;

                /* set up our callback context */
                cb_ctx.ui_cb = aggcb;
                cb_ctx.ui_cb_ctx = aggctx;
                cb_ctx.dbg = this;
                cb_ctx.obj = expr_val.val.obj;

                /* enumerate the properties */
                vm_objp(vmg_ expr_val.val.obj)
                    ->enum_props(vmg_ expr_val.val.obj, &enum_props_eval_cb,
                                 &cb_ctx);
            }
            break;

        default:
            /* other types don't have sub-parts to iterate */
            break;
        }
    }

    /* check the openable status if the caller wants to know */
    if (is_openable != 0)
    {
        /* 
         *   check to see if it's openable - it is if it's a TADS object
         *   or a list 
         */
        switch(expr_val.typ)
        {
        case VM_LIST:
            /* lists are always openable */
            *is_openable = TRUE;
            break;
            
        case VM_OBJ:
            /* 
             *   object - if it's a list or TADS object, it's openable;
             *   otherwise it's not 
             */
            if (vm_objp(vmg_ expr_val.val.obj)->get_as_list() != 0
                || CVmObjVector::is_vector_obj(vmg_ expr_val.val.obj)
                || vm_objp(vmg_ expr_val.val.obj)->provides_props(vmg0_))
                *is_openable = TRUE;
            else
                *is_openable = FALSE;
            break;
            
        default:
            /* other types aren't openable */
            *is_openable = FALSE;
            break;
        }
    }

    /* discard the gc protection */
    G_stk->discard();

    /* success */
    return 0;
}


/*
 *   Compile an expression 
 */
int CVmDebug::compile_expr(VMG_ const char *expr,
                           int level, CVmDbgSymtab *local_symtab,
                           int self_valid, int speculative,
                           int *is_lval, CVmPoolDynObj **code_obj,
                           char *dstbuf, size_t dstbuflen)
{
    int err;
    CTcPrsNode *expr_node;
    CTPNStmReturn *ret_stm;
    CTPNCodeBody *code_body;
    tcprsmem_state_t prsmem_state;
    CTcPrsDbgSymtab *old_symtab;
    CVmPoolDynamic *dyn_code;
    size_t copy_len;
    ulong copy_ofs;
    char *dst;
    tcpn_debug_info adjust_info;
    int need_err_msg;

    /* presume we won't need an error message formatted */
    need_err_msg = FALSE;

    /* 
     *   get the dynamic code pool manager - if it's not available, we
     *   have no place to put the compiled code, so we can't continue 
     */
    if ((dyn_code = G_code_pool->get_dynamic_ifc()) == 0)
        return 1;

    /* set up the parser with our local symbol table */
    old_symtab = G_prs->set_debug_symtab(local_symtab);

    /* 
     *   save the parser memory pool state, so we can reset it when we're
     *   done (this allows us to discard any parser memory we allocate
     *   while we're working - we only need it while compiling, and can
     *   discard it when we're done) 
     */
    G_prsmem->save_state(&prsmem_state);

    /* presume no error will occur */
    err = 0;

    /* presume we won't generate an expression node */
    expr_node = 0;

    /* catch any errors that occur during compilation or code generation */
    err_try
    {
        /* set up the tokenizer with the source buffer */
        G_tok->set_source_buf(expr);

        /* read the first token */
        G_tok->next();

        /* reset the compiler error counters for the new expression */
        G_tcmain->reset_error_counts();

        /* 
         *   clear the message buffer in the host interface, so we will
         *   capture the text of the first error message generated after
         *   this point 
         */
        hostifc_->reset_messages();

        /* compile the expression */
        expr_node = G_prs->parse_expr();

        /* don't proceed if compilation failed for any reason */
        if (expr_node == 0 || G_tcmain->get_error_count() != 0)
            goto compilation_done;

        /* fold constants in the expression */
        expr_node = expr_node->fold_constants(G_prs->get_global_symtab());
        if (expr_node == 0)
            goto compilation_done;

        /* adjust the expression for debugger execution */
        adjust_info.speculative = speculative;
        adjust_info.stack_level = level;
        expr_node = expr_node->adjust_for_debug(&adjust_info);
        if (expr_node == 0)
            goto compilation_done;

        /* check to see if the expression is an lvalue */
        if (is_lval != 0)
        {
            /* check the expression node to see if it's an lvalue */
            *is_lval = expr_node
                       ->check_lvalue_resolved(G_prs->get_global_symtab());
        }

        /* 
         *   Put the expression within a 'return' statement, and put the
         *   'return' statement within a code body.  If 'self' is
         *   available, provide an object statement object, so that the
         *   code generator knows that 'self' is valid within the
         *   generated code.  
         */
        ret_stm = new CTPNStmReturn(expr_node);
        code_body = new CTPNCodeBody(G_prs->get_global_symtab(),
                                     0, ret_stm, 0, FALSE, FALSE, 0, 0,
                                     self_valid, 0);

        /* set the appropriate debug modes in the code generator */
        G_cg->set_debug_eval(speculative, level);

        /* make the global symbol table active for code generation */
        G_cs->set_symtab(G_prs->get_global_symtab());

        /* generate code */
        code_body->gen_code(FALSE, FALSE);

    compilation_done:
        ;
    }
    err_catch(exc)
    {
        /* note the error code */
        err = exc->get_error_code();

        /* 
         *   if the caller provided a result buffer, format the error
         *   message into the result buffer 
         */
        if (G_tcmain->get_error_count() != 0)
        {
            /* 
             *   a compilation error was logged - use the error message we
             *   captured in the host interface 
             */
            need_err_msg = TRUE;
        }
        else if (dstbuf != 0)
        {
            const char *msg;

            /* get the message for the compiler error */
            msg = tcerr_get_msg(err, FALSE);
            err_format_msg(dstbuf, dstbuflen, msg, exc);
        }
    }
    err_end;

    /* if a compilation error occurred, return failure */
    if (err != 0)
    {
        /* parsing failed - no need to go on */
        goto done;
    }
    else if (G_tcmain->get_error_count() != 0 || expr_node == 0)
    {
        /* parse errors were reported - no need to go on */
        err = G_tcmain->get_first_error();
        need_err_msg = TRUE;
        goto done;
    }

    /* finish the code - if there's a value, return it */
    if (G_cg->get_sp_depth() != 0)
    {
        /* there's a value on the stack - return it */
        G_cg->write_op(OPC_RETVAL);
    }
    else
    {
        /* no value - just return nil */
        G_cg->write_op(OPC_RETNIL);
    }

    /* if any errors occurred generating code, fail */
    if (G_tcmain->get_error_count() != 0)
    {
        err = 1;
        need_err_msg = TRUE;
        goto done;
    }

    /* 
     *   if the code stream is too large, we can't save the code in the
     *   code pool 
     */
    if (G_cs->get_ofs() > G_code_pool->get_page_size())
    {
        err = 1;
        need_err_msg = TRUE;
        goto done;
    }

    /* get the amount of data to copy */
    copy_len = (size_t)G_cs->get_ofs();

    /* start reading at offset zero in the code stream */
    copy_ofs = 0;

    /* allocate code pool space for the code */
    *code_obj = dyn_code->dynpool_alloc(copy_len);
    if (*code_obj == 0)
    {
        err = 1;
        need_err_msg = TRUE;
        goto done;
    }

    /* get a writable pointer to the allocated code pool space */
    dst = G_code_pool->get_writable_ptr((*code_obj)->get_ofs());
    if (dst == 0)
    {
        /* release the code pool object */
        dyn_code->dynpool_delete(*code_obj);

        /* give up */
        err = 1;
        need_err_msg = TRUE;
        goto done;
    }

    /* move the generated code into the code pool */
    while (copy_len != 0)
    {
        const char *src;
        ulong avail_len;
        
        /* get the next block */
        src = G_cs->get_block_ptr(copy_ofs, copy_len, &avail_len);

        /* copy the data */
        memcpy(dst, src, (size_t)avail_len);

        /* advance past the copied data */
        dst += (size_t)avail_len;
        copy_len -= (size_t)avail_len;
        copy_ofs += avail_len;
    }

done:
    /* restore the original symbol table in the parser */
    G_prs->set_debug_symtab(old_symtab);

    /* reset the tokenizer */
    G_tok->reset();

    /* reset the code generator streams */
    G_ds->reset();
    G_cs->reset();
    G_os->reset();

    /* 
     *   reset the parser's memory pool, since we don't need any of the
     *   intermediate compilation data any longer 
     */
    G_prsmem->reset(&prsmem_state);

    /* generate an error message if necessary */
    if (need_err_msg && dstbuf != 0)
    {
        /* 
         *   if we logged a compiler error, report the first one we found;
         *   otherwise, use an empty message 
         */
        if (G_tcmain->get_error_count())
        {
            /* get the first error message captured in our host interface */
            strncpy(dstbuf, hostifc_->get_error_msg(), dstbuflen - 1);
            dstbuf[dstbuflen - 1] = '\0';
        }
        else
        {
            /* we don't have an error code, so there's no message */
            dstbuf[0] = '\0';
        }
    }

    /* return the result */
    return err;
}

/* ------------------------------------------------------------------------ */
/*
 *   Set step-over mode 
 */
void CVmDebug::set_step_over(VMG0_)
{
    /* set the mode flags */
    single_step_ = TRUE;
    step_in_ = FALSE;
    step_out_ = FALSE;

    /* 
     *   note the current stack frame depth - we won't stop until the
     *   stack frame is back at this same depth, or at an enclosing level 
     */
    step_frame_depth_ = G_interpreter->get_frame_depth(vmg0_);
}

/*
 *   Set step-out mode 
 */
void CVmDebug::set_step_out(VMG0_)
{
    /* set the mode flags for stepping over source lines */
    single_step_ = TRUE;
    step_in_ = FALSE;
    step_out_ = TRUE;

    /*
     *   Keep stepping until we're at an enclosing stack frame level - to
     *   do this, note the current frame depth, but decrement it, since we
     *   don't want to stop again until the frame depth is lower than it
     *   is now.
     */
    step_frame_depth_ = G_interpreter->get_frame_depth(vmg0_) - 1;
}


/* ------------------------------------------------------------------------ */
/*
 *   Synchronize our internal memory of the last execution point 
 */
void CVmDebug::sync_exec_pos(VMG_ const uchar *pc_ptr,
                             pool_ofs_t method_start_ofs)
{
    CVmFuncPtr func_ptr;
    CVmDbgLinePtr line_ptr;
    ulong stm_start, stm_end;
    unsigned int exec_ofs;

    /* 
     *   if we're already in the debugger, don't change anything - this must
     *   be a recursive invocation due to expression evaluation 
     */
    if (in_debugger_)
        return;

    /* set up a pointer to the current function's header */
    G_interpreter->set_current_func_ptr(vmg_ &func_ptr);

    /* get the byte-code offset of this instruction */
    pc_ = (pc_ptr == 0 ? 0 : G_interpreter->pc_to_code_ofs(vmg_ pc_ptr));

    /* for the UI, compute the offset from the start of the method */
    exec_ofs = (unsigned int)(pc_ - method_start_ofs);
    
    /* get the boundaries of the current source code statement */
    if (CVmRun::get_stm_bounds(vmg_ &func_ptr, exec_ofs, &line_ptr,
                               &stm_start, &stm_end))
    {
        /* remember the current function pointer */
        func_ptr_.copy_from(&func_ptr);

        /* remember the new line pointer */
        cur_stm_line_.copy_from(&line_ptr);

        /* remember the method start offset */
        entry_ofs_ = method_start_ofs;

        /* remember the statement bounds */
        cur_stm_start_ = stm_start;
        cur_stm_end_ = stm_end;

        /* ask the function pointer to set up the debug table pointer */
        dbg_ptr_valid_ = func_ptr_.set_dbg_ptr(&dbg_ptr_);
    }
    else
    {
        /* the statement has no valid bounds; clear the debug info */
        entry_ofs_ = 0;
        cur_stm_start_ = 0;
        cur_stm_end_ = 0;
    }
}

/* ------------------------------------------------------------------------ */
/*
 *   Single-step interrupt 
 */
void CVmDebug::step(VMG_ const uchar **pc_ptr, pool_ofs_t method_start_ofs,
                    int hit_bp, int error_code)
{
    unsigned int exec_ofs;
    int line_ptr_valid;
    CVmFuncPtr func_ptr;
    CVmDbgLinePtr line_ptr;
    int bpnum;
    int hit_global_bp;
    int stop_for_step;
    ulong stm_start, stm_end;
    int trace_over_bp;
    vm_val_t orig_r0;
    int released_stack_reserve = FALSE;

    /*
     *   If we're stepping over a breakpoint, put the breakpoint back into
     *   effect and restore the actual execution mode 
     */
    if (step_over_bp_)
    {
        /* no longer stepping over a breakpoint */
        step_over_bp_ = FALSE;

        /* restore the original execution mode */
        single_step_ = orig_single_step_;
        step_in_ = orig_step_in_;
        step_out_ = orig_step_out_;

        /* restore the breakpoint */
        step_over_bp_bp_->set_bp_instr(vmg_ TRUE, TRUE);
    }

    /* if we're already in the debugger, don't re-enter */
    if (in_debugger_)
        return;

    /* note that we're in the debugger so that we don't recurse into here */
    in_debugger_ = TRUE;

    /* 
     *   if this is a stack overflow error, release the debugger reserve, so
     *   that we can look at expressions and otherwise go about our business
     *   without encountering additional stack overflows 
     */
    if (error_code == VMERR_STACK_OVERFLOW)
        released_stack_reserve = G_stk->release_reserve();

    /* 
     *   Remember the value of R0, so we can restore it when we leave; also
     *   push it onto the stack, so that it's protected against any garbage
     *   collection that occurs while the debugger has control.  
     */
    orig_r0 = *G_interpreter->get_r0();
    G_stk->push(&orig_r0);

    /* presume we will not trace over a breakpoint at exit */
    trace_over_bp = FALSE;

    /* get the byte-code offset of this instruction */
    pc_ = (pc_ptr == 0 ? 0 : G_interpreter->pc_to_code_ofs(vmg_ *pc_ptr));

    /* 
     *   If we're in single-step mode, assume we'll stop for stepping.  If
     *   we're stopping for an error, also set single-step mode for now. 
     */
    stop_for_step = single_step_ || (error_code != 0);

    /* if we hit a breakpoint, find it */
    if (hit_bp)
    {
        CVmDebugBp *bp;

        /* find the breakpoint */
        bp = find_bp(pc_);

        /* get the breakpoint number */
        bpnum = (bp == 0 ? 0 : (int)(bp - bp_) + 1);

        /* 
         *   If the breakpoint has a condition attached to it, stop only
         *   if the condition is true.  
         */
        if (bp != 0 && bp->has_condition() && !bp->eval_cond(vmg0_))
        {
            /* 
             *   The condition is false - this means we must ignore the
             *   breakpoint; forget that we hit the breakpoint.  (We can't
             *   just decide to keep running now because there are
             *   numerous other reasons we might want to stop now, and we
             *   must consider the other possibilities before we jump back
             *   into the program.)  
             */
            hit_bp = FALSE;
            
            /* we don't have a valid breakpoint number after all */
            bpnum = 0;

            /*
             *   Since we're skipping this breakpoint, we need to trace
             *   over it to continue running. 
             */
            trace_over_bp = TRUE;
        }
    }
    else
    {
        /* we don't have a valid breakpoint */
        bpnum = 0;
    }

    /* presume we won't hit a global breakpoint */
    hit_global_bp = FALSE;

    /*
     *   If there are any global breakpoints, and we didn't hit a code
     *   breakpoint, check for a hit.  Don't bother with this if we've hit
     *   a real breakpoint, since we'll certainly stop in that case.
     *   Also, don't bother checking global breakpoints if we're in native
     *   code (i.e., pc_ptr == 0), since we can't stop right now if we
     *   are.  
     */
    if (!hit_bp && pc_ptr != 0 && global_bp_cnt_ != 0)
    {
        CVmDebugBp *bp;
        size_t i;
        
        /* scan all of the breakpoints and look for a global breakpoint */
        for (i = 0, bp = bp_ ; i < VMDBG_BP_MAX ; ++i, ++bp)
        {
            /* 
             *   if this breakpoint is enabled and global, evaluate its
             *   condition 
             */
            if (bp->is_in_use() && !bp->is_disabled() && bp->is_global())
            {
                /* evaluate the condition */
                if (bp->eval_cond(vmg0_))
                {
                    /* 
                     *   we've hit a global breakpoint - note it, and
                     *   compute the breakpoint number (it's just the
                     *   breakpoint index adjusted to a 1-based index) 
                     */
                    hit_global_bp = TRUE;
                    bpnum = (int)(i + 1);

                    /* 
                     *   If this is a stop-when-true condition, automatically
                     *   disable the breakpoint.  Once a global
                     *   stop-when-true breakpoint hits, its condition will
                     *   probably remain true for some time, and it would be
                     *   pointless to have it keep hitting over and over
                     *   again now.  Note that this isn't necessary for
                     *   stop-on-change breakpoints, as the fact that the
                     *   value just changed doesn't tell us anything about
                     *   the likelihood of future changes.  
                     */
                    if (!bp->stop_on_change())
                        bp->set_disabled(TRUE);

                    /* 
                     *   there's no need to look further - we only need to
                     *   have one global breakpoint hit in order to stop 
                     */
                    break;
                }
            }
        }
    }

    /* 
     *   Check to see if we're within the same source code statement that
     *   we were in the last time we were in this routine.  If so, we
     *   already have most of the information we need about the execution
     *   location, so we can avoid a little set-up work.  If we're not in
     *   the same statement any more, we have to figure out where we are.
     *   Note that we will also break if we're at a different stack level
     *   than we were last time, because in this case we're stepping out
     *   of a recursive invocation.  
     */
    if (stop_for_step
        && pc_ >= cur_stm_start_ && pc_ < cur_stm_end_
        && G_interpreter->get_frame_depth(vmg0_) == step_frame_depth_)
    {
        /* 
         *   No matter what source-stepping mode we're in, we never stop
         *   twice consecutively at the same source statement.  Make a
         *   note that we don't need to stop here.  However, don't just
         *   return now, since we might need to stop for a breakpoint or a
         *   global breakpoint.  
         */
        stop_for_step = FALSE;
    }

    /*
     *   If we're in step-over mode, there's no need to stop if we're
     *   executing at any stack level nested within the original stack frame.
     *   If this is the case, we can simply return now.  Of course, if we're
     *   stopping because we hit an error, don't worry about our step mode.  
     */
    if (stop_for_step
        && error_code == 0
        && single_step_ && !step_in_
        && G_interpreter->get_frame_depth(vmg0_) > step_frame_depth_)
    {
        /* we don't need to stop for stepping */
        stop_for_step = FALSE;
    }

    /* figure out where we're tracing, if we're in byte code */
    if (pc_ptr != 0)
    {
        /* set up a pointer to the current function's header */
        G_interpreter->set_current_func_ptr(vmg_ &func_ptr);
        
        /* for the UI, compute the offset from the start of the method */
        exec_ofs = (unsigned int)(pc_ - method_start_ofs);
        
        /* get the boundaries of the current source code statement */
        line_ptr_valid =
            CVmRun::get_stm_bounds(vmg_ &func_ptr, exec_ofs, &line_ptr,
                                   &stm_start, &stm_end);
        
        /* adjust the statement boundaries to absolute addresses */
        stm_start += method_start_ofs;
        stm_end += method_start_ofs;
    }
    else
    {
        /* we're in native code, so there is no valid byte code location */
        exec_ofs = 0;
        line_ptr_valid = FALSE;
        stm_start = stm_end = 0;
    }

    /* 
     *   If we're within the same source line and at the same stack depth
     *   that we were at last time - even if the apparent byte-code location
     *   has us in a different line - don't stop for a single-step
     *   operation.  The same source line can generate byte code at
     *   different places in certain types of complex statements, and we
     *   don't want to confuse the user by stopping twice in a row at the
     *   same line (thus showing no apparent change in the execution
     *   location) in such cases.
     *   
     *   However, if we're stopping due to a run-time error, do stop even if
     *   we're in the same location - it just means we got another error at
     *   the same place.  
     */
    if (stop_for_step
        && error_code == 0
        && G_interpreter->get_frame_depth(vmg0_) == step_frame_depth_
        && line_ptr_valid
        && cur_stm_start_ != 0
        && line_ptr.get_source_id() == cur_stm_line_.get_source_id()
        && line_ptr.get_source_line() == cur_stm_line_.get_source_line())
    {
        /* note that we don't want to stop for single-step mode */
        stop_for_step = FALSE;
    }

    /*
     *   If we're in native code and we decided that we should stop,
     *   remember the current code location as the last stop location.
     *   Even though we can't actually stop here, we want to act as though
     *   we did, so that next time we step through actual byte code we'll
     *   notice that we are at a different location than we were for the
     *   last stop. 
     */
    if (pc_ptr == 0 && stop_for_step)
    {
        /* 
         *   set the current statement bounds to invalid, to ensure that
         *   we will notice we are in a different location for the next
         *   valid byte code location 
         */
        cur_stm_start_ = cur_stm_end_ = 0;
    }

    /*
     *   If we're stopping for any reason (single-stepping, breakpoint, or
     *   global breakpoint), enter the interactive debugger user
     *   interface.  In any case, only stop if we have a valid line
     *   pointer, since we can't do anything at a location without a valid
     *   source line.  
     *   
     *   Note that we can never take control when we're stepping through
     *   native code (i.e., pc_ptr == 0).  We're called for native code
     *   execution only to advise us of the change in stack levels so that
     *   we can properly track step in/over/out modes properly through
     *   native code traversal.  
     */
    if (line_ptr_valid && (stop_for_step || hit_bp || hit_global_bp))
    {
        /* remember the current function pointer */
        func_ptr_.copy_from(&func_ptr);

        /* remember the current frame depth */
        step_frame_depth_ = G_interpreter->get_frame_depth(vmg0_);

        /* remember the new line pointer */
        cur_stm_line_.copy_from(&line_ptr);

        /* remember the method start offset */
        entry_ofs_ = method_start_ofs;

        /* remember the statement bounds */
        cur_stm_start_ = stm_start;
        cur_stm_end_ = stm_end;

        /* ask the function pointer to set up the debug table pointer */
        dbg_ptr_valid_ = func_ptr_.set_dbg_ptr(&dbg_ptr_);

        /* 
         *   remove all breakpoints from the code while we have control,
         *   so that the code is all the original instructions while the
         *   user is looking at it 
         */
        suspend_all_bps(vmg0_);

        /* 
         *   call the UI interactive command loop - this won't return
         *   until the user tells us to resume execution 
         */
        CVmDebugUI::cmd_loop(vmg_ bpnum, error_code, &exec_ofs);

        /* restore breakpoints now that we're resuming execution */
        restore_all_bps(vmg0_);

        /* 
         *   convert the UI's method offset back into an absolute code
         *   offset (we can never move the execution point outside of the
         *   current method, so we merely have to add the current method
         *   start entry address to the method offset) 
         */
        pc_ = entry_ofs_ + exec_ofs;

        /* 
         *   before returning, re-translate the code pool offset of the
         *   current execution point back into a memory address, in case
         *   we moved the execution point 
         */
        *pc_ptr = (const uchar *)G_code_pool->get_ptr(pc_);

        /* 
         *   get the boundaries of the current statement, in case it
         *   changed - we need this for the next time we enter the
         *   debugger, so we can tell (if we're stepping) whether we've
         *   left the most recent statement or not 
         */
        if (CVmRun::get_stm_bounds(vmg_ &func_ptr_, exec_ofs,
                                   &cur_stm_line_,
                                   &cur_stm_start_, &cur_stm_end_))
        {
            /* adjust the statement boundaries to absolute addresses */
            cur_stm_start_ += entry_ofs_;
            cur_stm_end_ += entry_ofs_;
        }
        else
        {
            /* we have no information for this statement */
            cur_stm_start_ = cur_stm_end_ = 0;
        }

        /* 
         *   note that we must trace over any breakpoint at this new
         *   location, since we want to execute the original instruction
         *   rather than just stopping again 
         */
        trace_over_bp = TRUE;
    }

    /* 
     *   if we released the stack reserve, put it back in reserve now that
     *   we're leaving the debugger - the reserve is for the debugger's use
     *   only, so we don't need it once we resume execution 
     */
    if (released_stack_reserve)
        G_stk->recover_reserve();

    /* note that we're leaving the debugger */
    in_debugger_ = FALSE;

    /*
     *   If appropriate, check the current code location for a breakpoint;
     *   if there's a breakpoint, we need to go through a couple of extra
     *   steps to resume execution so that we don't jump right back into
     *   the debugger.
     *   
     *   The problem is that if we go or step after hitting a breakpoint,
     *   and we were to simply leave the breakpoint in effect, we'd just
     *   hit the same breakpoint again.  What we actually want to do is
     *   execute the original instruction.  To do this, we must first
     *   remove the breakpoint, restoring the original instruction, then
     *   single-step the instruction, then, when the debugger regains
     *   control after the single-step, put the breakpoint back.  Finally,
     *   we can resume with whatever kind of stepping we were going to do
     *   in the first place.  
     */
    if (trace_over_bp && **pc_ptr == (uchar)OPC_BP)
    {
        CVmDebugBp *bp;
        
        /* find the breakpoint */
        bp = find_bp(pc_);
        
        /* if we found the breakpoint, remove it temporarily */
        if (bp != 0)
        {
            /* restore the original instruction */
            bp->set_bp_instr(vmg_ FALSE, TRUE);
            
            /* note that we're in step-over-bp mode */
            step_over_bp_ = TRUE;
            
            /* remember the breakpoint we're stepping over */
            step_over_bp_bp_ = bp;
            
            /* remember the original execution mode */
            orig_single_step_ = single_step_;
            orig_step_in_ = step_in_;
            orig_step_out_ = step_out_;
            
            /* 
             *   temporary set single-step mode, so that we get control
             *   immediately after executing this single instruction 
             */
            single_step_ = TRUE;
            step_in_ = TRUE;
            step_out_ = FALSE;
        }
    }

    /* 
     *   restore the original value of R0, and discard the copy we pushed
     *   onto the stack for protection against garbage collection 
     */
    *G_interpreter->get_r0() = orig_r0;
    G_stk->discard();
}

/* ------------------------------------------------------------------------ */
/*
 *   Suspend all breakpoints - removes all BP instructions from the code
 *   and restores the original instructions 
 */
void CVmDebug::suspend_all_bps(VMG0_)
{
    CVmDebugBp *bp;
    size_t i;

    /* loop over all breakpoints */
    for (i = 0, bp = bp_ ; i < VMDBG_BP_MAX ; ++i, ++bp)
    {
        /* 
         *   if this breakpoint is active, restore its original
         *   instruction (since this routine sets up for switching in and
         *   out of the debugger, force the instruction update even if
         *   we're in debugger mode)
         */
        if (bp->is_in_use() && !bp->is_disabled())
            bp->set_bp_instr(vmg_ FALSE, TRUE);
    }
}

/*
 *   Restore all breakpoints - restores all BP instructions removed by
 *   suspend_all_bps() 
 */
void CVmDebug::restore_all_bps(VMG0_)
{
    CVmDebugBp *bp;
    size_t i;

    /* loop over all breakpoints */
    for (i = 0, bp = bp_ ; i < VMDBG_BP_MAX ; ++i, ++bp)
    {
        /* if this breakpoint is active, put it back */
        if (bp->is_in_use() && !bp->is_disabled())
            bp->set_bp_instr(vmg_ TRUE, TRUE);
    }
}

/* ------------------------------------------------------------------------ */
/*
 *   Determine if a code location is within the current active method 
 */
int CVmDebug::is_in_current_method(VMG_ unsigned long code_addr)
{
    ulong end_addr;
    
    /* 
     *   if the current debug pointer isn't valid, we can't make this
     *   determination, so indicate that we're not in the active method 
     */
    if (!dbg_ptr_valid_)
        return FALSE;

    /* 
     *   Determine the endpoint of the method's code.  If the method has
     *   an exception table, it's the endpoint; otherwise, the debug table
     *   is the endpoint. 
     */
    end_addr = entry_ofs_ + (func_ptr_.get_exc_ofs() != 0
                             ? func_ptr_.get_exc_ofs()
                             : func_ptr_.get_debug_ofs());

    /* 
     *   if the code address is between the entrypoint address for the
     *   active method and the ending address we just calculated, it's in
     *   the active method; otherwise, it's somewhere else 
     */
    return (code_addr >= entry_ofs_ && code_addr < end_addr);
}



/* ------------------------------------------------------------------------ */
/*
 *   Set up a function pointer and line pointer, and get the statement
 *   bounds, for the function at the given stack level.  Level 0 is the
 *   currently executing function, 1 is the first enclosing stack level
 *   (which called the current function), and so on.  Returns true if
 *   successful, false if the information isn't available.  
 */
int CVmDebug::get_stack_level_info(VMG_ int level, CVmFuncPtr *func_ptr,
                                   CVmDbgLinePtr *line_ptr,
                                   ulong *stm_start, ulong *stm_end) const
{
    /* 
     *   if we're at level 0, it's the current method; otherwise, get the
     *   given enclosing frame from the stack 
     */
    if (level == 0)
    {
        /* 
         *   if we don't have a valid debug information pointer, we can't
         *   get any information on the current source location 
         */
        if (!dbg_ptr_valid_)
            return 1;
        
        /* use the current statement */
        func_ptr->copy_from(&func_ptr_);
        line_ptr->copy_from(&cur_stm_line_);
        *stm_start = cur_stm_start_;
        *stm_end = cur_stm_end_;

        /* success */
        return 0;
    }
    else
    {
        vm_val_t *fp;
        ulong ret_ofs;

        /* 
         *   Enclosing level - walk up the stack to the desired level.
         *   Note that we are actually looking for the return address, so
         *   we want to find the frame just within the desired level - for
         *   the first enclosing level, we actually want the current
         *   frame.  
         */
        for (fp = G_interpreter->get_frame_ptr() ; level > 1 && fp != 0 ;
             fp = G_interpreter->get_enclosing_frame_ptr(vmg_ fp), --level) ;

        /* 
         *   if we didn't reach the desired level, or the frame pointer is
         *   null, we didn't find the requested frame - return failure 
         */
        if (level > 1 || fp == 0)
            return 1;

        /* set up a function pointer for the return address from this frame */
        G_interpreter->set_return_funcptr_from_frame(vmg_ func_ptr, fp);

        /* get the return address for the frame */
        ret_ofs = G_interpreter->get_return_ofs_from_frame(vmg_ fp);

        /* 
         *   Find the source line information for the return address in
         *   the frame.  Return failure if there's no debug information
         *   for the method.  
         */
        if (!CVmRun::get_stm_bounds(vmg_ func_ptr, ret_ofs,
                                    line_ptr, stm_start, stm_end))
            return 1;

        /* success */
        return 0;
    }
}

/* ------------------------------------------------------------------------ */
/*
 *   Allocate the method header list 
 */
void CVmDebug::alloc_method_header_list(ulong cnt)
{
    /* 
     *   if we already have a method header list, expand it; otherwise,
     *   allocate a new one 
     */
    if (method_hdr_ != 0)
    {
        /* 
         *   if we're growing the list, expand it; otherwise ignore the
         *   new allocation 
         */
        if (cnt > method_hdr_cnt_)
        {
            /* expand the list */
            method_hdr_ = (ulong *)t3realloc(method_hdr_,
                                             cnt * sizeof(ulong));

            /* note the new size */
            method_hdr_cnt_ = cnt;
        }
    }
    else
    {
        /* allocate the new list */
        method_hdr_ = (ulong *)t3malloc(cnt * sizeof(ulong));

        /* note the size */
        method_hdr_cnt_ = cnt;
    }
}

/*
 *   Given a code pool address, find the method header containing the
 *   address.  This searches the method header list for the nearest method
 *   header whose address is less than the given address.  
 */
pool_ofs_t CVmDebug::find_method_header(pool_ofs_t ofs)
{
    ulong lo;
    ulong hi;

    /* if there are no method headers, there's nothing to find */
    if (method_hdr_cnt_ == 0)
        return 0;
    
    /* perform a binary search of the method header list */
    lo = 0;
    hi = method_hdr_cnt_ - 1;
    while (lo <= hi)
    {
        ulong cur;
        pool_ofs_t addr;
        pool_ofs_t next_addr;

        /* split the difference to get the current entry */
        cur = lo + (hi - lo)/2;

        /* get this entry's address */
        addr = (pool_ofs_t)get_method_header(cur);

        /* 
         *   get the next entry's address - if this is the last entry, the
         *   next entry's address is the highest possible address 
         */
        next_addr = (cur + 1 >= method_hdr_cnt_
                     ? (pool_ofs_t)0xffffffff
                     : (pool_ofs_t)get_method_header(cur + 1));

        /* check how this value compares */
        if (ofs >= next_addr)
        {
            /* we need to go higher */
            lo = (cur == lo ? cur + 1 : cur);
        }
        else if (ofs < addr)
        {
            /* we need to go lower */
            hi = (cur == hi ? hi - 1 : cur);
        }
        else
        {
            /* found it - return this start address */
            return addr;
        }
    }

    /* 
     *   didn't find anything - we don't have any way to indicate this, so
     *   just return the lowest code pool address 
     */
    return 0;
}


/* ------------------------------------------------------------------------ */
/*
 *   Reverse-mapping hash entry 
 */

/*
 *   Construct.  Note that we don't bother to convert the ulong key value
 *   to a portable representation, since this entry is never written to a
 *   file - it's purely an in-memory construct.  Note also that we don't
 *   care whether there are any embedded null bytes in the value, since
 *   the hash entry base class works entirely with counted-length strings.
 *   
 *   Treating a ulong value as a character buffer might seem suspicious
 *   from a portability standpoint, but the C/C++ standards make this
 *   perfectly legitimate as long as we don't make assumptions about the
 *   size or byte ordering of the underlying representation, which we're
 *   not - the byte ordering is irrelevant, since we're simply treating
 *   this value as an identifying string of bytes (i.e., a hash key), and
 *   we're using sizeof() to ensure we use the correct local size for the
 *   type.  What we're doing is exactly the same thing that memcpy() would
 *   do if asked to copy a ulong value, and equally safe.
 */
CVmHashEntryDbgRev::CVmHashEntryDbgRev(ulong sym_val, const char *sym,
                                       size_t len)
    : CVmHashEntry((char *)&sym_val, sizeof(sym_val), TRUE)
{
    /* make a copy of the symbol name */
    sym_ = lib_copy_str(sym, len);
    sym_len_ = len;
}

/*
 *   Destruct 
 */
CVmHashEntryDbgRev::~CVmHashEntryDbgRev()
{
    /* delete our copy of the symbol name */
    lib_free_str(sym_);
}

/*
 *   check for a match 
 */
int CVmHashEntryDbgRev::matches(const char *str, size_t len) const
{
    /*
     *   it's a match if the strings are the same length and all
     *   characters match, treating case as significant 
     */
    return (len == len_
            && memcmp(str, str_, len * sizeof(*str)) == 0);
}

/*
 *   Reverse-lookup hash function 
 */
unsigned int CVmHashFuncDbgRev::compute_hash(const char *s, size_t l) const
{
    uint acc;

    /* add up all the byte values in the string */
    for (acc = 0 ; l != 0 ; ++s, --l)
    {
        uchar c;

        c = (uchar)*s;
        acc += c;
    }

    /* return the accumulated value */
    return acc;
}

/* ------------------------------------------------------------------------ */
/*
 *   Breakpoint object 
 */

/*
 *   instantiate 
 */
CVmDebugBp::CVmDebugBp()
{
    /* not yet in use */
    in_use_ = FALSE;
    
    /* no code address yet */
    code_addr_ = 0;

    /* not disabled */
    disabled_ = FALSE;
    
    /* no original instruction yet */
    orig_instr_ = (char)OPC_NOP;
    
    /* no condition expression or byte code object yet */
    cond_ = 0;
    cond_buf_len_ = 0;
    compiled_cond_ = 0;
    has_cond_ = FALSE;
    stop_on_change_ = FALSE;
    prv_val = 0;
}
   

/*
 *   delete 
 */
CVmDebugBp::~CVmDebugBp()
{
    /* delete any condition text buffer */
    if (cond_ != 0)
        t3free(cond_);
}

/*
 *   terminate - this is called during VM termination so that we can
 *   delete our compiled condition code objects (we can't do this in the
 *   destructor because we need access to the VM globals) 
 */
void CVmDebugBp::do_terminate(VMG0_)
{
    /* release our object table global variable, if we have one */
    if (prv_val != 0)
    {
        G_obj_table->delete_global_var(prv_val);
        prv_val = 0;
    }

#if 0 // $$$
    // It's not necessary to delete my condition object explicitly -
    // when the code pool is unloaded, it'll delete all remaining
    // dynamic objects.  I'm not sure I like this approach - it seems
    // cleaner to have each object's owner explicitly free it, and
    // we're clearly the owner of this object - but there's no
    // practical reason to worry about it.  So, I'll leave it like
    // this for now and maybe revisit it later.
    //
    // (For future reference, what's needed is a pre-termination
    // cleanup pass that vmmain.cpp makes BEFORE calling loader->unload(),
    // because that routine will detach the backing stores and thereby
    // clean up the code pool's dynamic objects.  If we're going to
    // explicitly delete our object here, this needs to be done before
    // the code pool's backing store is detached.  Note that doing things
    // in that order will NOT result in redundant deletions, since deleting
    // our object will remove it from the code pool's dynamic object list
    // and the object thus won't be deleted again by the backing store
    // detach operation.  However, if we detach first and then delete
    // later, we'll have the redundant deletion problem.)
    
    /* if I have a compiled condition object, delete it */
    if (compiled_cond_ != 0)
    {
        /* delete the object */
        G_code_pool->get_dynamic_ifc()->dynpool_delete(compiled_cond_);

        /* forget the object */
        compiled_cond_ = 0;
    }
#endif /* 0 */
}

/*
 *   Set the breakpoint's information 
 */
int CVmDebugBp::set_info(VMG_ ulong code_addr, const char *cond, int change,
                         int disabled, char *errbuf, size_t errbuflen)
{
    int err;
    
    /* remember the code address */
    code_addr_ = code_addr;

    /* remember the disabled status */
    disabled_ = disabled;

    /* remember the expression */
    if ((err = set_condition(vmg_ cond, change, errbuf, errbuflen)) != 0)
        return err;

    /* remember the original instruction at the code address */
    if (code_addr != 0)
    {
        char *code_ptr;

        /* get a writable pointer to the code location */
        code_ptr = G_code_pool->get_writable_ptr(code_addr_);

        /* 
         *   if we got a valid code pointer, remember the original
         *   instruction at the code address, so that we can restore this
         *   instruction when we remove the BP instruction 
         */
        if (code_ptr != 0)
            orig_instr_ = *code_ptr;

        /* set the BP instruction if appropriate */
        set_bp_instr(vmg_ TRUE, FALSE);
    }

    /* success */
    return 0;
}

/*
 *   Set the condition text 
 */
int CVmDebugBp::set_condition(VMG_ const char *new_cond, int change,
                              char *errbuf, size_t errbuflen)
{
    /* if we already have a compiled condition object, delete it */
    if (compiled_cond_ != 0)
    {
        /* delete the object */
        G_code_pool->get_dynamic_ifc()->dynpool_delete(compiled_cond_);

        /* forget the object */
        compiled_cond_ = 0;
    }

    /* 
     *   check for an empty condition string - treat it as no condition if
     *   it's all blanks 
     */
    if (new_cond != 0)
    {
        const char *p;

        /* scan for a non-blank character */
        for (p = new_cond ; is_space(*p) ; ++p) ;

        /* 
         *   if we scanned the entire string without finding anything but
         *   spaces, treat this is no condition 
         */
        if (*p == '\0')
            new_cond = 0;
    }

    /* if there's a new condition, set it up */
    if (new_cond != 0)
    {
        size_t new_cond_len;
        CVmDbgSymtab *symtab;
        CVmDbgSymtab local_symtab;
        int err;

        /* presume we won't find a local symbol table */
        symtab = 0;

        /* 
         *   if there's a code address, set up a local symbol table to
         *   compile the expression in the scope of the code location;
         *   otherwise, compile with no local scope 
         */
        if (code_addr_ != 0)
        {
            pool_ofs_t method_addr;
            CVmFuncPtr func_ptr;
            CVmDbgLinePtr line_ptr;
            CVmDbgTablePtr dbg_ptr;
            ulong stm_start;
            ulong stm_end;

            /* find the entry pointer for the selected code address */
            method_addr = G_debugger->find_method_header(code_addr_);

            /* set up a function pointer at the code location */
            func_ptr.set((const uchar *)G_code_pool->get_ptr(method_addr));

            /* set up the line pointer */
            if (CVmRun::get_stm_bounds(vmg_ &func_ptr,
                                       code_addr_ - method_addr,
                                       &line_ptr, &stm_start, &stm_end)
                && func_ptr.set_dbg_ptr(&dbg_ptr))
            {
                /* 
                 *   We have a valid debug table for the code location -
                 *   set up a local symbol table object for the code.
                 *   Compile the expression for stack level zero, since we
                 *   will always evaluate the expression when this code is
                 *   active.  
                 */
                local_symtab.init(vmg_ &dbg_ptr, line_ptr.get_frame_id(), 0);

                /* we now have a local symbol table for the code - use it */
                symtab = &local_symtab;
            }
        }

        /* 
         *   Compile the expression.  Compile at stack level zero, since
         *   we'll always execute this code immediately upon hitting the
         *   breakpoint, which means that the breakpoint location will be
         *   the active stack level whenever this code is executed.  
         *   
         *   Presume that 'self' will be valid in this context.  If it
         *   isn't, 'self' will be nil at run-time, so no harm will be done.
         */
        err = G_debugger->compile_expr(vmg_ new_cond, 0, symtab, TRUE,
                                       FALSE, 0, &compiled_cond_,
                                       errbuf, errbuflen);

        /* if an error occurred, return the error code */
        if (err != 0)
            return err;

        /* note that we have a condition */
        has_cond_ = TRUE;

        /* note whether or not it's a stop-on-change condition */
        stop_on_change_ = change;

        /* set the size of the new condition, including the trailing null */
        new_cond_len = strlen(new_cond) + 1;

        /* allocate or expand the condition buffer if required */
        if (cond_buf_len_ < new_cond_len)
        {
            /* allocate or reallocate the buffer */
            if (cond_ == 0)
                cond_ = (char *)t3malloc(new_cond_len);
            else
                cond_ = (char *)t3realloc(cond_, new_cond_len);

            /* note the new size of our condition buffer */
            cond_buf_len_ = new_cond_len;
        }

        /* copy the new condition text into our buffer */
        memcpy(cond_, new_cond, new_cond_len);

        /* if it's a stop-on-change condition, initialize the value */
        if (change)
        {
            /* 
             *   Allocate an object table global variable to hold the
             *   previous value, if we haven't already.  We allocate a global
             *   for this so that our value is tracked in the garbage
             *   collector.  
             */
            if (prv_val == 0)
                prv_val = G_obj_table->create_global_var();
            
            /* 
             *   set the value to 'empty' to indicate that we haven't
             *   evaluated our expression for the first time yet 
             */
            prv_val->val.set_empty();

            /* 
             *   evaluate the condition to initialize our memory of the
             *   current value; we'll break the next time we reach this
             *   breakpoint and the new value of the expression differs from
             *   this saved value 
             */
            eval_cond(vmg0_);
        }
    }
    else
    {
        /* note that there's no condition */
        has_cond_ = FALSE;

        /* if we had a global variable for the condition, delete it */
        if (prv_val != 0)
        {
            G_obj_table->delete_global_var(prv_val);
            prv_val = 0;
        }
    }

    /* success */
    return 0;
}

/*
 *   Evaluate my condition 
 */
int CVmDebugBp::eval_cond(VMG0_)
{
    int ret;
    vmrun_save_ctx run_ctx;
    vmdbg_step_save_t old_step;
    
    /* save the interpreter context */
    G_interpreter->save_context(vmg_ &run_ctx);

    /* set the debugger to non-stepping mode */
    G_debugger->prepare_for_eval(&old_step);

    /* 
     *   if there's no condition, just return false, since there's nothing
     *   to evaluate 
     */
    if (compiled_cond_ == 0)
        return FALSE;

    /* execute the code in a protected block */
    err_try
    {
        vm_obj_id_t self_obj;
        vm_obj_id_t orig_target_obj;
        vm_obj_id_t defining_obj;
        vm_prop_id_t target_prop;
        vm_val_t *valp;

        /* get the 'self' object at the current stack level */
        self_obj = G_interpreter->get_self_at_level(vmg_ 0);

        /* get the target property for the level */
        target_prop = G_interpreter->get_target_prop_at_level(vmg_ 0);

        /* get the original target object and defining object at the level */
        orig_target_obj =
            G_interpreter->get_orig_target_obj_at_level(vmg_ 0);
        defining_obj = G_interpreter->get_defining_obj_at_level(vmg_ 0);

        /* execute the code */
        G_interpreter->do_call(vmg_ 0, compiled_cond_->get_ofs(), 0,
                               self_obj, target_prop,
                               orig_target_obj, defining_obj, "dbg cond");

        /* get the return value */
        valp = G_interpreter->get_r0();

        /* check what kind of condition we have */
        if (stop_on_change_)
        {
            /* 
             *   We're a stop-on-change condition - this means that we stop
             *   if the value of the expression differs from the previous
             *   value.  Check to see if the value differs, and return true
             *   (to indicate that the breakpoint condition has been met) if
             *   so.  If the previous value is 'empty', this means we've
             *   never evaluated the value before, so the value can't have
             *   changed; simply remember the new value in this case.  
             */
            ret = (prv_val->val.typ != VM_EMPTY
                   && !prv_val->val.equals(vmg_ valp));

            /* remember the new value for next time */
            prv_val->val = *valp;
        }
        else
        {
            /* see what we have */
            switch(valp->typ)
            {
            case VM_INT:
                /* if the number is zero, it's false, otherwise it's true */
                ret = (valp->val.intval != 0);
                break;
                
            case VM_NIL:
                /* condition is false */
                ret = FALSE;
                break;
                
            default:
                /* anything else is a true condition value */
                ret = TRUE;
                break;
            }
        }
    }
    err_catch(exc)
    {
        /* 
         *   an error occurred - treat the condition as true so that we
         *   stop here 
         */
        ret = TRUE;
    }
    err_end;

    /* restore the original execution mode */
    G_debugger->restore_from_eval(&old_step);

    /* rerstore the interpreter context */
    G_interpreter->restore_context(vmg_ &run_ctx);

    /* return the result */
    return ret;
}


/*
 *   Delete the breakpoint 
 */
void CVmDebugBp::do_delete(VMG0_)
{
    /* remove the BP instruction from the code if necessary */
    set_bp_instr(vmg_ FALSE, FALSE);

    /* mark myself as no longer active */
    in_use_ = FALSE;
}

/*
 *   Set or remove the BP instruction in the code 
 */
void CVmDebugBp::set_bp_instr(VMG_ int set, int always)
{
    /* if I have a code address, set or remove the BP instruction */
    if (code_addr_ != 0)
    {
        char *code_ptr;

        /* get a writable pointer to the code location */
        code_ptr = G_code_pool->get_writable_ptr(code_addr_);

        /* proceed only if we got a valid code pointer */
        if (code_ptr != 0)
        {
            /* check to see if we're setting or removing the BP instruction */
            if (set)
            {
                /* 
                 *   if we're not in the debugger, write the BP
                 *   instruction to the address - don't do this if we're
                 *   in the debugger, since we remove all BP instructions
                 *   from the code while the debugger has control to
                 *   prevent any recursive debugger invocations from
                 *   taking place 
                 */
                if (always || !G_debugger->is_in_debugger())
                    *code_ptr = (char)OPC_BP;
            }
            else
            {
                /* 
                 *   Restore the original instruction to that location.
                 *   This isn't necessary if we're in the debugger, since
                 *   we remove all BP's when the debugger takes control.  
                 */
                if (always || !G_debugger->is_in_debugger())
                    *code_ptr = orig_instr_;
            }
        }
    }
}