code for testing A20

%macro change_es 1
    push ax
    mov ax, %1
    mov es, ax
    pop ax
%endmacro


TestA20:
    push es

    change_es 0
    mov ax, word [es:0x7DFE]
    change_es 0xFFFF
    mov bx, word [es:0x7E0E]
    cmp ax, bx
    jne .not_enabled

    change_es 0
    ror byte [es:0x7DFE], 8
    mov ax, word [es:0x7DFE]
    change_es 0xFFFF
    mov bx, word [es:0x7E0E]

    ; restore boot signature
    change_es 0
    rol word [es:0x7DFE], 8

    cmp ax, bx
    jne .not_enabled

    .enabled:
    mov ax, 0
    jmp .done


    .not_enabled:
    mov ax, 1

    .done:
    pop es
    ret