; Public Domain

	cpu 8086
	org 256
start:
	int3

before:
	call test
	mov dx, msg.offbefore
	je .off
.on:
	mov dx, msg.onbefore
.off:
	mov ah, 09h
	int 21h

after:
	call test
	mov dx, msg.offafter
	je .off
.on:
	mov dx, msg.onafter
.off:
	mov ah, 09h
	int 21h

switchoff:
	mov ax, 4300h
	int 2Fh
	cmp al, 80h
	jne noxms
	mov dx, msg.tryingxms
	mov ah, 09h
	int 21h
	mov ax, 4310h
	int 2Fh
	push es
	push bx
	mov bp, sp
	mov ah, 04h
	call far [bp]
	mov ah, 06h
	call far [bp]
	jmp end

noxms:
	mov dx, msg.tryingrombios
	mov ah, 09h
	int 21h
	mov ax, 2400h
	int 15h
end:

testfirst:
	call test
	mov dx, msg.off
	je .off
.on:
	mov dx, msg.on
.off:
	mov ah, 09h
	int 21h

testagain:
	call test
	mov dx, msg.offagain
	je .off
.on:
	mov dx, msg.onagain
.off:
	mov ah, 09h
	int 21h

	mov ax, 4C00h
	int 21h


test:
	les di, [cs:high]
	lds si, [cs:low]
	mov cx, 16
	repe cmpsw
	push cs
	pop ds
	retn


align 2
high:	dd 0FFFF_0010h
low:	dd 0

msg:
.offbefore:
.off:	db "A20 was off. ",36
.onbefore:
.on:	db "A20 was on. ",36
.offafter:
.offagain:	db "After int 21h A20 is off.",13,10,36
.onafter:
.onagain:	db "After int 21h A20 is on.",13,10,36
.tryingxms:	db "XMS detected, trying to disable A20 on XMS.",13,10,36
.tryingrombios:	db "XMS not detected, trying to disable A20 using ROM-BIOS.",13,10,36