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/*
Copyright (C) 2000, Entity Cyber, Inc.
Authors: Gary Byers (gb@thinguin.org)
Marty Connor (mdc@thinguin.org)
This software may be used and distributed according to the terms
of the GNU Public License (GPL), incorporated herein by reference.
Description:
This is just a little bit of code and data that can get prepended
to an Etherboot ROM image in order to allow LILO to load the
result as if it were a Linux kernel image.
A real Linux kernel image consists of a one-sector boot loader
(to load the image from a floppy disk), followed a few sectors
of setup code, followed by the kernel code itself. There's
a table in the first sector (starting at offset 497) that indicates
how many sectors of setup code follow the first sector and which
contains some other parameters that aren't interesting in this
case.
When LILO loads the sectors that comprise a kernel image, it doesn't
execute the code in the first sector (since that code would try to
load the image from a floppy disk.) The code in the first sector
below doesn't expect to get executed (and prints an error message
if it ever -is- executed.) LILO's only interested in knowing the
number of setup sectors advertised in the table (at offset 497 in
the first sector.)
Etherboot doesn't require much in the way of setup code.
Historically, the Linux kernel required at least 4 sectors of
setup code. Current versions of LILO look at the byte at
offset 497 in the first sector to indicate how many sectors
of setup code are contained in the image.
*/
#define SETUPSECS 4 /* Minimal nr of setup-sectors */
#define PREFIXSIZE ((SETUPSECS+1)*512)
#define PREFIXPGH (PREFIXSIZE / 16 )
#define BOOTSEG 0x07C0 /* original address of boot-sector */
#define INITSEG 0x9000 /* we move boot here - out of the way */
#define SETUPSEG 0x9020 /* setup starts here */
#define SYSSEG 0x1000 /* system loaded at 0x10000 (65536). */
.text
.code16
.arch i386
.org 0
.section ".prefix", "ax", @progbits
.globl _prefix
_prefix:
/*
This is a minimal boot sector. If anyone tries to execute it (e.g., if
a .lilo file is dd'ed to a floppy), print an error message.
*/
bootsector:
jmp $BOOTSEG, $go - _prefix /* reload cs:ip to match relocation addr */
go:
movw $0x2000, %di /* 0x2000 is arbitrary value >= length
of bootsect + room for stack */
movw $BOOTSEG, %ax
movw %ax,%ds
movw %ax,%es
cli
movw %ax, %ss /* put stack at BOOTSEG:0x2000. */
movw %di,%sp
sti
movw $why_end-why, %cx
movw $why - _prefix, %si
movw $0x0007, %bx /* page 0, attribute 7 (normal) */
movb $0x0e, %ah /* write char, tty mode */
prloop:
lodsb
int $0x10
loop prloop
freeze: jmp freeze
why: .ascii "This image cannot be loaded from a floppy disk.\r\n"
why_end:
.org 497
setup_sects:
.byte SETUPSECS
root_flags:
.word 0
syssize:
.word _verbatim_size_pgh - PREFIXPGH
swap_dev:
.word 0
ram_size:
.word 0
vid_mode:
.word 0
root_dev:
.word 0
boot_flag:
.word 0xAA55
/*
We're now at the beginning of the second sector of the image -
where the setup code goes.
We don't need to do too much setup for Etherboot.
This code gets loaded at SETUPSEG:0. It wants to start
executing the Etherboot image that's loaded at SYSSEG:0 and
whose entry point is SYSSEG:0.
*/
setup_code:
pushl $0 /* No parameters to preserve for exit path */
pushw $0 /* Use prefix exit path mechanism */
/* Etherboot expects to be contiguous in memory once loaded.
* LILO doesn't do this, but since we don't need any
* information that's left in the prefix, it doesn't matter:
* we just have to ensure that %cs:0000 is where the start of
* the Etherboot image *would* be.
*/
ljmp $(SYSSEG-(PREFIXSIZE/16)), $_start
.section ".text16", "ax", @progbits
.globl prefix_exit
prefix_exit:
int $0x19 /* should try to boot machine */
.globl prefix_exit_end
prefix_exit_end:
.previous
.org (PREFIXSIZE-1)
.byte 0
prefix_end:
/*
That's about it.
*/
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