Enter/Leave Protected Mode

stub.asm

org 100h
bits 16

section .text
	global __start
	jmp __start

Unsupported	db 	'This application requires at least an 80386 processor.',	0x0d, 0x0a, '$'   ; $-terminated message
AlreadyInit	db 	'Already in Protected Mode!',					0x0d, 0x0a, '$'   ; $-terminated message
A20LineFail	db 	'A20 Line is Disabled or Not Present.',				0x0d, 0x0a, '$'   ; $-terminated message
SuccessMesg	db 	'Successfully returned to real mode.',				0x0d, 0x0a, '$'   ; $-terminated message

IsSupportedCPU:	pushf   						; save flags
		cli     						; make sure no ISR will interfere now
									; 
		pushf							; 1. make sure have at least 386
		pop     ax						; AX=flags
		xor     ah,	0x40					; toggle NT
		push    ax          					; stack: modified_flags,original_flags
		popf							; 
		pushf							; stack: modified_flags_from_cpu,original_flags
		pop	dx						; DX=flags passed via CPU
		mov	al,	dh					; 
		xor	al,	ah					; 
		jnz 		UnsupportedCPU   			; improper CPU
									; 
		smsw	ax						; 2. make sure are in Real Mode
		and	al,	1					; 
		jnz     	UnsupportedCPU				; if already in PM (maybe V8086)
									; 
		push    ds          	    				; 3. make sure A20 is enabled
		push    es						; 
		push    bx						; 
		xor     bx,	bx					; 
		mov     ds,	bx					; 
		mov     ax,	-1					; 
		mov     es,	ax					; 
		xor     [bx],	ah					; change byte[000000h]
		mov     al,	[es:bx + 0x10]				; get byte[100000h]
		xor     [bx],	ah					; change byte[000000h]
		xor     al,	[es:bx + 0x10]				; compare byte[100000h] with its previous value
									; 0 if unchanged, -1 means [000000h]==[100000h]
		pop     bx						; 
		pop     es						; 
		pop     ds						; 
									; 
UnsupportedCPU:	popf							; restore flags
		test    al,	al					; test error code
		ret

ExitWithError:	lea	dx,	[A20LineFail]
		jc		WriteAndExit
		lea	dx,	[Unsupported]
		js		WriteAndExit
		lea	dx,	[AlreadyInit]
WriteAndExit:	mov	ah,	9					; 
		int	21h						; 
		mov	ax,	4cffh					; exit, status=-1
		int	21h						; 

__start:	call		IsSupportedCPU
		add	al,	al
		jnz		ExitWithError
		
		mov 	dx,	SuccessMesg
		call		WriteAndExit