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