In some code snippets in the OSDev wiki to enable the A20 line , we have clicommand interrupts. In some others we do not have them.
eg. when configuring the A20 line through the old keyboard controller method, all code is surrounded by a combination of cliand sti. I can imagine that this should happen because we use keyboard communication through the ports, and interrupting the keyboard can also change data on the ports. But it's true? I only guessed ...
enable_A20:
cli
call a20wait
mov al,0xAD
out 0x64,al
call a20wait
mov al,0xD0
out 0x64,al
call a20wait2
in al,0x60
push eax
call a20wait
mov al,0xD1
out 0x64,al
call a20wait
pop eax
or al,2
out 0x60,al
call a20wait
mov al,0xAE
out 0x64,al
call a20wait
sti
ret
a20wait:
in al,0x64
test al,2
jnz a20wait
ret
a20wait2:
in al,0x64
test al,1
jz a20wait2
ret
A20 ( ) , . , ? , , , , ?
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
, A20- . ( ). , , , 0x92 , , ? , .
fast_a20_gate:
in al, 0x92
test al, 2
jnz after
or al, 2
and al, 0xFE
out 0x92, al
after:
- , , cli, ? , .