source: bootcd/isolinux/syslinux-6.03/dos/crt0.S

        .code16

#ifndef REGPARM
# error "This file assumes -mregparm=3 -DREGPARM=3"
#endif

        .section ".text","ax"
        .globl _start
        .type _start,@function
_start:
        # Align the stack and make sure the high half is zero
        andl $0xfffc,%esp

        # DS, ES points to the PSP at this point
        pushw %es               # Save PSP pointer
        movw %cs,%ax
        movw %ax,%ds
        movw %ax,%es

        # Clear the .bss
        cld
        xorl %eax,%eax
        movw $__bss_start,%di
        movw $__bss_end+3,%cx
        subw %di,%cx
        shrw $2,%cx
        rep ; stosl

        # Copy the PSP into our own segment
        popw %fs                # FS -> PSP
        movw $_PSP,%di
        xorw %si,%si
        movw $0x40,%cx
        fs ; rep ; movsl

        # Verify that this is a supportable DOS version
        movw $0x3001,%ax
        int $0x21
        xchgb %ah,%al
        movw %ax,dos_version
        cmpw $0x0314,%ax        # DOS >= 3.20?
        jae 1f                  # If so, okay
        movw $bad_dos,%dx       # Print error message
        movb $0x09,%ah
        int $0x21
        int $0x20               # Die

1:
        # Compute argc and argv (assumes REGPARM)
        pushl %eax              # Make space for argv
        movl %esp,%eax
        calll __parse_argv
        pushl %eax              # argc

        # Initialize malloc
        calll __init_memory_arena

        # Now call main
        popl %eax               # argc
        popl %edx               # argv
        calll main

        # Here %eax is the exit code, fall through into exit

        .size _start,.-_start

        .globl exit
        .type exit,@function
exit:
        # Exit code already in %eax
        movb $0x4c,%ah          # Terminate program
        int $0x21
1:      hlt
        jmp 1b
        .size exit,.-exit

        .section ".rodata","a"
bad_dos:
        .ascii "Unsupported DOS version\r\n$"
        .size bad_dos,.-bad_dos

        .section ".bss","aw"
        .balign 16
        .globl _PSP
_PSP:
        .space 256
        .size _PSP, .-_PSP

        /* Purely for sanity */
        .section ".null","a"
        .long 0,0,0,0