; Assembly example
 
; nasm boot.asm -o boot.bin
; partcopy boot.bin 0 200 -f0
 
[ORG 0x7c00]              ; add to offsets
 
start:   xor ax, ax       ; make it zero
   mov ds, ax             ; DS=0
   mov ss, ax             ; stack starts at seg 0
   mov sp, 0x9c00         ; 2000h past code start, 
                          ; making the stack 7.5k in size
 
   cli                    ; no interrupts
   push ds                ; save real mode
 
   lgdt [gdtinfo]         ; load gdt register
 
   mov  eax, cr0          ; switch to pmode by
   or al,1                ; set pmode bit
   mov  cr0, eax
 
   jmp $+2                ; tell 386/486 to not crash
 
   mov  bx, 0x08          ; select descriptor 1
   mov  ds, bx            ; 8h = 1000b
 
   and al,0xFE            ; back to realmode
   mov  cr0, eax          ; by toggling bit again
 
   pop ds                 ; get back old segment
   sti
 
   mov bx, 0x0f01         ; attrib/char of smiley
   mov eax, 0x0b8000      ; note 32 bit offset
   mov word [ds:eax], bx
 
   jmp $                  ; loop forever
 
gdtinfo:
   dw gdt_end - gdt - 1   ;last byte in table
   dd gdt                 ;start of table
 
gdt         dd 0,0        ; entry 0 is always unused
flatdesc    db 0xff, 0xff, 0, 0, 0, 10010010b, 11001111b, 0
gdt_end:
 
   times 510-($-$$) db 0  ; fill sector w/ 0's
   dw 0xAA55              ; Required by some BIOSes