source: bootcd/isolinux/syslinux-6.03/gpxe/src/arch/i386/prefix/lkrnprefix.S

/*
        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 a ROM image in order to allow bootloaders 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 a bootloader 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.)

        We don't require much in the way of setup code.  Historically, the
        Linux kernel required at least 4 sectors of setup code.
        Therefore, at least 4 sectors must be present even though we don't
        use them.

*/

FILE_LICENCE ( GPL_ANY )

#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
/* 
        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, $1f   /* reload cs:ip to match relocation addr */
1:
        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, %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: 


/*
        The following header is documented in the Linux source code at
        Documentation/i386/boot.txt
*/
        .org    497
setup_sects: 
        .byte   SETUPSECS
root_flags: 
        .word   0
syssize: 
        .long   -PREFIXPGH

        .section ".zinfo.fixup", "a", @progbits /* Compressor fixups */
        .ascii  "ADDL"
        .long   syssize
        .long   16
        .long   0
        .previous
        
ram_size: 
        .word   0
vid_mode: 
        .word   0
root_dev: 
        .word   0
boot_flag: 
        .word   0xAA55
jump:
        /* Manually specify a two-byte jmp instruction here rather
         * than leaving it up to the assembler. */
        .byte   0xeb
        .byte   setup_code - header
header:
        .byte   'H', 'd', 'r', 'S'
version:
        .word   0x0207 /* 2.07 */
realmode_swtch:
        .long   0
start_sys:
        .word   0
kernel_version:
        .word   0
type_of_loader:
        .byte   0
loadflags:
        .byte   0
setup_move_size:
        .word   0
code32_start:
        .long   0
ramdisk_image:
        .long   0
ramdisk_size:
        .long   0
bootsect_kludge:
        .long   0
heap_end_ptr:
        .word   0
pad1:
        .word   0
cmd_line_ptr:
        .long   0
initrd_addr_max:
        /* We don't use an initrd but some bootloaders (e.g. SYSLINUX) have
         * been known to require this field.  Set the value to 2 GB.  This
         * value is also used by the Linux kernel. */
        .long   0x7fffffff
kernel_alignment:
        .long   0
relocatable_kernel:
        .byte   0
pad2:
        .byte   0, 0, 0
cmdline_size:
        .long   0
hardware_subarch:
        .long   0
hardware_subarch_data:
        .byte   0, 0, 0, 0, 0, 0, 0, 0

/*
        We don't need to do too much setup.

        This code gets loaded at SETUPSEG:0.  It wants to start
        executing the image that's loaded at SYSSEG:0 and
        whose entry point is SYSSEG:0.
*/
setup_code:
        /* We expect to be contiguous in memory once loaded.  The Linux image
         * boot process requires that setup code is loaded separately from
         * "non-real code".  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 image *would* be.
         */
        ljmp    $(SYSSEG-(PREFIXSIZE/16)), $run_gpxe


        .org    PREFIXSIZE
/*
        We're now at the beginning of the kernel proper.
 */
run_gpxe:
        /* Set up stack just below 0x7c00 */
        xorw    %ax, %ax
        movw    %ax, %ss
        movw    $0x7c00, %sp

        /* Install gPXE */
        call    install

        /* Set up real-mode stack */
        movw    %bx, %ss
        movw    $_estack16, %sp

        /* Jump to .text16 segment */
        pushw   %ax
        pushw   $1f
        lret
        .section ".text16", "awx", @progbits
1:
        pushl   $main
        pushw   %cs
        call    prot_call
        popl    %ecx /* discard */

        /* Uninstall gPXE */
        call    uninstall

        /* Boot next device */
        int $0x18