;//---------------------------------------------------------------
;//	refresh.s
;//
;//	This program visually depicts the memory-refresh cycle,
;//	by monitoring the PIT's OUT1 signal (port 0x61, bit 4).
;//
;//	   assemble with:  $ as86 refresh.s -b refresh.b 
;//	   install using:  $ dd if=refresh.b of=/dev/fd0 seek=1 
;//
;//	NOTE: This code begins executing with CS:IP = 1000:0002. 
;//
;//	programmer: ALLAN CRUSE
;//	date begun: 01 MAY 2004
;//---------------------------------------------------------------



	.SECT	.TEXT
;-----------------------------------------------------------------
	.WORD	0xABCD			; programming signature 
;-----------------------------------------------------------------
main:	mov	ax, cs			; address this segment 
	mov	ds, ax			;   with DS register  
	pop	dword return_address	; store return-address 
	mov	ss, ax			;  adjust SS register 
	lea	esp, tos0		; and set new stacktop 

	call	reset_video_textmode
	call	execute_program_demo

	push	dword return_address	; recover our exit-address 
	retf				; exit to program launcher 
;-----------------------------------------------------------------
return_address:	.LONG	0		; to store exit-address 
;-----------------------------------------------------------------
;-----------------------------------------------------------------
table:	.WORD	0x171F, 0x1F1E		; lookup-table for output
;-----------------------------------------------------------------
execute_program_demo:

	mov	ax, #0xB800		; address video memory
	mov	es, ax			;   with ES register
	mov	di, #480		; start on fourth line

	mov	cx, #18			; number of output lines
	imul	cx, #80			;  times chars-per-line
again:
	in	al, #0x61		; read PORT_B status
	and	eax, #0x10		; isolate OUT1 state
	shr	eax, #4			; shift OUT1 to low bit
	mov	ax, table[eax*2]	; fetch output-character
	stosw				; write output-character
	loop	again			; check OUT1 state again

	ret				; back to main routine 
;-----------------------------------------------------------------
;-----------------------------------------------------------------
msg:	.ASCII	"A visualization for the PC memory-refresh cycle "
len:	.WORD	* - msg			; length of the message
att:	.BYTE	0x0E
;-----------------------------------------------------------------
reset_video_textmode:

	mov	ax, cs			; address this segment
	mov	es, ax			;   with ES register

	; reset the video display-mode
	mov	ax, #0x0003		; standard 80x25 textmode
	int	0x10			; request BIOS service

	; draw the explanatory title
	lea	bp, msg			; point ES:BP to message
	mov	cx, len			; setup message's length 
	mov	bl, att			; specify attribute byte
	mov	bh, #0			; display-page is zero
	mov	dx, #0x0110		; row=1, col=16
	mov	ax, #0x1300		; write_string service
	int	0x10			; request BIOS service

	; move the cursor beneath our output-window
	mov	dx, #0x1600		; row=22, col=0
	mov	ah, #0x02		; set_cursor_position
	int	0x10			; request BIOS service

	ret
;-----------------------------------------------------------------
	.ALIGN	16			; alignment at paragraph 
	.SPACE	512			; reserved for stack use 
tos0:					; label fop top-of-stack 
;-----------------------------------------------------------------
	END