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; 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
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