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%ifndef __MEM_ASM
%define __MEM_ASM
check_a20:
pushf ; preserve flags
push ds ; data segment
push es ; extra segment
push di ; destination index
push si ; source index
cli ; disable interrupts
xor ax, ax ; zero out ax
mov es, ax ; zero out extra segment
not ax ; flip all bits in ax
mov ds, ax
mov di, 0x0500 ; destination address
mov si, 0x0510 ; source address
mov al, byte [es:di] ; copy byte from destination (FFFF:0500)
push ax
mov al, byte [ds:si] ; copy byte from source (FFFF:0510)
push ax
mov byte [es:di], 0x00 ; set es:di to 0
mov byte [ds:si], 0xFF ; set es:si to 255
cmp byte [es:di], 0xFF ; did it wrap around?
pop ax
mov byte [ds:si], al
pop ax
mov byte [es:di], al
mov ax, 0
je check_a20__exit
mov ax, 1
check_a20__exit:
sti ; enable interrupts
pop si ; restore registers
pop di
pop es
pop ds
popf ; restore flags
ret
enable_a20: ; fast A20
cli ; disble interrupts
call check_a20 ; is A20 already enabled?
cmp al, 1
je .no_a20
in al, 0x92 ; enable A20
test al, 2
jnz .return
or al, 2
and al, 0xFE
out 0x92, al
jmp .return
.no_a20:
stc ; set carry on failure
.return:
sti ; enable interrupts
clc ; clear carry
ret
%endif
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