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;; boot.stage2.a20.asm
_enable_a20:
call _check_a20
test ax, ax
jnz .end
call _enable_a20_bios
call _check_a20
test ax, ax
jnz .end
call _enable_a20_keyb
call _check_a20
test ax, ax
jnz .end
call _enable_a20_io92
call _check_a20
test ax, ax
jnz .end
.halt: hlt
jmp .halt
.end:
ret
_check_a20:
pushf
push ds
push es
push di
push si
cli ; Clear Interrupts
xor ax, ax
mov es, ax
not ax
mov ds, ax
;; 0500 and 0510 are guaranteed to be free
mov di, 0x0500
mov si, 0x0510
;; Save the original values
mov dl, byte [es:di]
push dx
mov dl, byte [ds:si]
push dx
mov byte [es:di], 0x00
mov byte [ds:si], 0xFF
cmp byte [es:di], 0xFF
mov ax, 0 ; The A20 is disabled
je .a20_disabled
mov ax, 1 ; The A20 is enabled
.a20_disabled:
pop dx
mov byte [ds:si], dl
pop dx
mov byte [es:di], dl
pop si
pop di
pop es
pop ds
popf
sti ; Enable interrupts again
ret
_enable_a20_bios:
mov ax, 0x2401
int 0x15
jc .fast_gate
test ah, ah
jnz .failed
call _check_a20
test ax, ax
jnz .done
.fast_gate:
in al, 0x92
test al, 2
jnz .done
or al, 2
and al, 0xfe
out 0x92, al
call _check_a20
test ax, ax
jnz .done
.done:
mov ax, 1
ret
.failed:
mov ax, 0
ret
_enable_a20_keyb:
cli
call .wait_io1
mov al, 0xad
out 0x64, al
call .wait_io1
mov al, 0x0d0
out 0x64, al
call .wait_io2
in al, 0x60
push eax
call .wait_io1
mov al, 0xd1
out 0x64, al
call .wait_io1
mov al, 0xae
out 0x64, al
sti
ret
.wait_io1:
in al, 0x64
test al, 2
jnz .wait_io1
ret
.wait_io2:
in al, 0x64
test al, 1
jz .wait_io2
ret
_enable_a20_io92:
in al, 0x92 ; Read from port 0x92
test al, 2 ; Check if bit 1
jnz .end
or al, 2
and al, 0xfe
out 0x92, al
.end:
ret
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