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; functions to handle A20 address line status and switching
; returns 0 if not A20_ENABLED, 1 if A20_ENABLED in AX
check_A20_enabled:
pushf
push ds
push es
push di
push si
cli
xor ax, ax
mov es, ax
mov di, 0x0500 ; es:di = 0000:0500
mov ax, 0xffff
mov ds, ax
mov si, 0x0510 ; ds:si = ffff:0510
mov al, byte [es:di] ; preserve values in memory
push ax ; on stack
mov al, byte [ds:si]
push ax
mov byte [es:di], 0x00 ; now the test: write 0x00 to 0000:0500
mov byte [ds:si], 0xFF ; write 0xff to ffff:0510
cmp byte [es:di], 0xFF ; memory wrap? A20 notA20_ENABLED
pop ax ; restore original memory contents
mov byte [ds:si], al
pop ax
mov byte [es:di], al
je .enabled
jmp .disabled
.enabled:
mov al, '+'
call print_char
mov ax, 1 ; not wrapped around
jmp .exit
.disabled:
mov al, '-'
call print_char
mov ax, 0 ; wrapped around (last cmp)
jmp .exit
.exit:
pop si
pop di
pop es
pop ds
popf
ret
check_and_enable_A20:
call check_A20_enabled
cmp ax, 1
je A20_ENABLED
A20_ENABLE_KBD_PORT:
mov al, 'K'
call print_char
mov al, 0xdd
out 0x64, al
call check_A20_enabled
cmp ax, 1
je A20_ENABLED
A20_ENABLE_KBD_OUT:
mov al, 'k'
call print_char
cli ; disable interrupts, we talk directly to the keyboard ports
call .wait_input
mov al,0xAD
out 0x64,al ; disable keyboard
call .wait_input
mov al,0xD0
out 0x64,al ; tell controller to read output port
call .wait_output
in al,0x60
push eax ; get output port data and store it
call .wait_input
mov al,0xD1
out 0x64,al ; tell controller to write output port
call .wait_input
pop eax
or al,2 ; set bit 1 (enable a20)
out 0x60,al ; write out data back to the output port
call .wait_input
mov al,0xAE ; enable keyboard
out 0x64,al
call .wait_input
sti
jmp .retest
; wait for input buffer to be clear
.wait_input:
in al,0x64
test al,2
jnz .wait_input
ret
; wait for output buffer to be clear
.wait_output:
in al,0x64
test al,1
jz .wait_output
ret
.retest:
call check_A20_enabled
cmp ax, 1
je A20_ENABLED
A20_FAST_SPECIAL_PORT:
mov al, 'F'
call print_char
in al, 0x92
or al, 2
out 0x92, al
call check_A20_enabled
cmp ax, 1
je A20_ENABLED
ret
A20_ENABLE_VIA_BIOS:
mov al, 'B'
call print_char
mov ax, 0x2401
int 0x15
call check_A20_enabled
cmp ax, 1
je A20_ENABLED
A20_ENABLED:
ret
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