More VGA tests

tests/pc/vga/L29-4.ASM

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+; Program to demonstrate screen blanking via bit 5 of the
+; Attribute Controller Index register.
+;
+AC_INDEX	equ	3c0h	;Attribute Controller Index register
+INPUT_STATUS_1	equ	3dah	;color-mode address of the Input
+				; Status 1 register
+;
+; Macro to wait for and clear the next keypress.
+;
+WAIT_KEY macro
+	mov	ah,8		;DOS input without echo function
+	int	21h
+	endm
+;
+stack	segment para stack 'STACK'
+	db	512 dup (?)
+stack	ends
+;
+Data	segment word 'DATA'
+SampleText db	'This is bit-mapped text, drawn in hi-res '
+	db	'EGA graphics mode 10h.', 0dh, 0ah, 0ah
+	db	'Press any key to blank the screen, then '
+	db	'any key to unblank it,', 0dh, 0ah
+	db	'then any key to end.$'
+Data	ends
+;
+Code	segment
+	assume	cs:Code, ds:Data
+Start	proc	near
+	mov	ax,Data
+	mov	ds,ax
+;
+; Go to hi-res graphics mode.
+;
+	mov	ax,10h		;AH = 0 means mode set, AL = 10h selects
+				; hi-res graphics mode
+	int	10h		;BIOS video interrupt
+;
+; Put up some text, so the screen isn't empty.
+;
+	mov	ah,9		;DOS print string function
+	mov	dx,offset SampleText
+	int	21h
+;
+	WAIT_KEY
+;
+; Blank the screen.
+;
+	mov	dx,INPUT_STATUS_1
+	in	al,dx		;reset port 3c0h to index (rather than data)
+				; mode
+	mov	dx,AC_INDEX
+	sub	al,al		;make bit 5 zero...
+	out	dx,al		;...which blanks the screen
+;
+	WAIT_KEY
+;
+; Unblank the screen.
+;
+	mov	dx,INPUT_STATUS_1
+	in	al,dx		;reset port 3c0h to Index (rather than data)
+				; mode
+	mov	dx,AC_INDEX
+	mov	al,20h		;make bit 5 one...
+	out	dx,al		;...which unblanks the screen
+;
+	WAIT_KEY
+;
+; Restore text mode.
+;
+	mov	ax,2
+	int	10h
+;
+; Done.
+;
+Done:
+	mov	ah,4ch		;DOS terminate function
+	int	21h
+Start	endp
+Code	ends
+	end	Start