A20-test

a20test.asm

[bits 16]

org 07c00h

    mov ax, cs
    mov ds, ax
    mov es, ax

    call check_a20
    test ax, ax
    mov ax, A20On
    jnz Print           ; Enabled
    mov ax, A20Off

Print:
    mov bp, ax
    mov cx, 16
    mov ax, 01301h
    mov bx, 000ch
    mov dl, 0
    int 10h
    
    cli                 ; Shutdown
    hlt



check_a20:
    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
 
    ret

A20On:
    db "A20 is On        "
A20Off:
    db "A20 is Off       "

times 510-($-$$) db 0

db 0x55
db 0xaa