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; The following code is public domain licensed
[bits 16]
; Function: check_a20
;
; Purpose: to check the status of the a20 line in a completely self-contained state-preserving way.
; The function can be modified as necessary by removing push's at the beginning and their
; respective pop's at the end if complete self-containment is not required.
;
; Returns: 0 in ax if the a20 line is disabled (memory wraps around)
; 1 in ax if the a20 line is enabled (memory does not wrap around)
check_a20:
pushf
push ds
push es
push di
push si
cli
xor ax, ax ; ax = 0
mov es, ax
not ax ; ax = 0xFFFF
mov ds, ax
mov di, 0x0500
mov si, 0x0510
mov al, byte [es:di]
push ax
mov al, byte [ds:si]
push ax
mov byte [es:di], 0x00
mov byte [ds:si], 0xFF
cmp byte [es:di], 0xFF
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:
pop si
pop di
pop es
pop ds
popf
ret
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