引导分析之一： bootsect.S

/*
 *    bootsect.S        Copyright (C) 1991, 1992 Linus Torvalds
 *
 *    modified by Drew Eckhardt
 *    modified by Bruce Evans (bde)
 *    modified by Chris Noe (May 1999) (as86 -> gas)
 *    gutted by H. Peter Anvin (Jan 2003)
 *
 * BIG FAT NOTE: We're in real mode using 64k segments.  Therefore segment
 * addresses must be multiplied by 16 to obtain their respective linear
 * addresses. To avoid confusion, linear addresses are written using leading
 * hex while segment addresses are written as segment:offset.
 *
 */

#include

SETUPSECTS    = 4            /* default nr of setup-sectors */
BOOTSEG        = 0x07C0        /* original address of boot-sector */
INITSEG        = DEF_INITSEG        /* we move boot here - out of the way */
SETUPSEG    = DEF_SETUPSEG        /* setup starts here */
SYSSEG        = DEF_SYSSEG        /* system loaded at 0x10000 (65536) */
SYSSIZE        = DEF_SYSSIZE        /* system size: # of 16-byte clicks */
                    /* to be loaded */
ROOT_DEV    = 0             /* ROOT_DEV is now written by "build" */
SWAP_DEV    = 0            /* SWAP_DEV is now written by "build" */

#ifndef SVGA_MODE
#define SVGA_MODE ASK_VGA
#endif

#ifndef RAMDISK
#define RAMDISK 0
#endif

#ifndef ROOT_RDONLY
#define ROOT_RDONLY 1
#endif

.code16
.text

.global _start
_start:

    # Normalize the start address
    jmpl    $BOOTSEG, $start2

start2:
    movw    %cs, %ax
    movw    %ax, %ds
    movw    %ax, %es
    movw    %ax, %ss
    movw    $0x7c00, %sp
    sti
    cld

    movw    $bugger_off_msg, %si

msg_loop:
    lodsb
    andb    %al, %al
    jz    die
    movb    $0xe, %ah
    movw    $7, %bx
    int    $0x10
    jmp    msg_loop

die:
    # Allow the user to press a key, then reboot
    xorw    %ax, %ax
    int    $0x16
    int    $0x19

    # int 0x19 should never return.  In case it does anyway,
    # invoke the BIOS reset code...
    ljmp    $0xf000,$0xfff0


bugger_off_msg:
    .ascii    "Direct booting from floppy is no longer supported.\r\n"
    .ascii    "Please use a boot loader program instead.\r\n"
    .ascii    "\n"
    .ascii    "Remove disk and press any key to reboot . . .\r\n"
    .byte    0
    

    # Kernel attributes; used by setup

    .org 497
setup_sects:    .byte SETUPSECTS
root_flags:    .word ROOT_RDONLY
syssize:    .word SYSSIZE
swap_dev:    .word SWAP_DEV
ram_size:    .word RAMDISK
vid_mode:    .word SVGA_MODE
root_dev:    .word ROOT_DEV
boot_flag:    .word 0xAA55