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print msg ; this pseudo not my actual print code

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[org 0x7c00]
KERNEL_OFFSET equ 0x1000 ; The same one we used when linking the kernel

    mov [BOOT_DRIVE], dl ; Remember that the BIOS sets us the boot drive in 'dl' on boot
    mov bp, 0x9000
    mov sp, bp

    mov bx, MSG_REAL_MODE 
    call print
    call print_nl

    call load_kernel ; read the kernel from disk
    call switch_to_pm ; disable interrupts, load GDT,  etc. Finally jumps to 'BEGIN_PM'
    jmp $ ; Never executed

%include "../05-bootsector-functions-strings/boot_sect_print.asm"
%include "../05-bootsector-functions-strings/boot_sect_print_hex.asm"
%include "../07-bootsector-disk/boot_sect_disk.asm"
%include "../09-32bit-gdt/32bit-gdt.asm"
%include "../08-32bit-print/32bit-print.asm"
%include "../10-32bit-enter/32bit-switch.asm"

[bits 16]
load_kernel:
    mov bx, MSG_LOAD_KERNEL
    call print
    call print_nl

    mov bx, KERNEL_OFFSET ; Read from disk and store in 0x1000
    mov dh, 2
    mov dl, [BOOT_DRIVE]
    call disk_load
    ret

[bits 32]
BEGIN_PM:
    mov ebx, MSG_PROT_MODE
    call print_string_pm
    call KERNEL_OFFSET ; Give control to the kernel
    jmp $ ; Stay here when the kernel returns control to us (if ever)


BOOT_DRIVE db 0 ; It is a good idea to store it in memory because 'dl' may get overwritten
MSG_REAL_MODE db "Started in 16-bit Real Mode", 0
MSG_PROT_MODE db "Landed in 32-bit Protected Mode", 0
MSG_LOAD_KERNEL db "Loading kernel into memory", 0

; padding
times 510 - ($-$$) db 0
dw 0xaa55

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call dword [KERNEL_OFFSET + 24]

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#include <linux/string.h>

char * strstr(const char * cs,const char * ct)
{
int	d0, d1;
register char * __res;
__asm__ __volatile__(
	"movl %6,%%edi\n\t"
	"repne\n\t"
	"scasb\n\t"
	"notl %%ecx\n\t"
	"decl %%ecx\n\t"	/* NOTE! This also sets Z if searchstring='' */
	"movl %%ecx,%%edx\n"
	"1:\tmovl %6,%%edi\n\t"
	"movl %%esi,%%eax\n\t"
	"movl %%edx,%%ecx\n\t"
	"repe\n\t"
	"cmpsb\n\t"
	"je 2f\n\t"		/* also works for empty string, see above */
	"xchgl %%eax,%%esi\n\t"
	"incl %%esi\n\t"
	"cmpb $0,-1(%%eax)\n\t"
	"jne 1b\n\t"
	"xorl %%eax,%%eax\n\t"
	"2:"
	:"=a" (__res), "=&c" (d0), "=&S" (d1)
	:"0" (0), "1" (0xffffffff), "2" (cs), "g" (ct)
	:"dx", "di");
return __res;
}

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#include <stdio.h>

char * strstr(const char * cs,const char * ct)
{
int   d0, d1;
register char * __res;
__asm__ __volatile__(
   "movl %6,%%edi\n\t"
   "repne\n\t"
   "scasb\n\t"
   "notl %%ecx\n\t"
   "decl %%ecx\n\t"   /* NOTE! This also sets Z if searchstring='' */
   "movl %%ecx,%%edx\n"
   "1:\tmovl %6,%%edi\n\t"
   "movl %%esi,%%eax\n\t"
   "movl %%edx,%%ecx\n\t"
   "repe\n\t"
   "cmpsb\n\t"
   "je 2f\n\t"      /* also works for empty string, see above */
   "xchgl %%eax,%%esi\n\t"
   "incl %%esi\n\t"
   "cmpb $0,-1(%%eax)\n\t"
   "jne 1b\n\t"
   "xorl %%eax,%%eax\n\t"
   "2:"
   :"=a" (__res), "=&c" (d0), "=&S" (d1)
   :"0" (0), "1" (0xffffffff), "2" (cs), "g" (ct)
   :"dx", "di");
return __res;
}

int main()
{
    char string[]="Hello There";
    char search[]="here";
    return (strstr(string, search) ? 1 : 0);
}

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gcc test.c -O0 -m32 -Wall
./a.out; echo $?
1

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gcc test.c -O3 -m32 -Wall
./a.out; echo $?
0

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:"dx", "di", "memory");