Date: Sun, 2 Feb 1997 17:23:22 -0600 (CST)
From: Steve DiCostanzo <steverd AT jcccnet DOT johnco DOT cc DOT ks DOT us>
To: djgpp AT delorie DOT com
Subject: Re: Programming Graphics
In-Reply-To: <5cumje$n0n@sjx-ixn10.ix.netcom.com>
Message-Id: <Pine.A32.3.91.970202162016.31300D-100000@jcccnet.johnco.cc.ks.us>
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On 1 Feb 1997, Max wrote:

> 	I've found that what seems to be culprits in the code involve the use 
> of the _inp, _outp, and _fmemset commands.  *sigh*

	Max baby, many things can go wrong with graphics, and I still get
lots of lockups, but I know a few bytes about video i/o.

	Problems:
		garbage on screen: data put in the wrong place
		lockups: data put in the wrong place

	Writing to a register at a port is theoretically a two step process
	{
		1. select register
		2. output to register
	}

	But it's really a three step process!
	{
		0. Read from the port to make sure it's on step 1 and hasn't
			been left waiting for step two by some other events.
		1. *Promptly* write your register number to the port to prevent
			other events from interfering.
		2. *Promptly* write the data you want to go to the register
	}

	Here's a piece of "trivial" code to change border color on the
PCjr, which contained an early, if not the earliest, Video Gate Array.
(source Robert Jourdain's _The Programmer's Problem Solver_ (c) 1986 Brady
Books Div., Simon & Schuster), because I don't have Michael Abrash's _Zen
of Graphics Programming_ with me: 

	asm{
		mov	dx,3dah	;address of video gate array chip
		in	al,dx	;dummy read to ready the chip (step 0)
		mov	al,2	;register number
		out	dx,al	;send the request
		mov	al,4	;turn on only bit 2 (red)
		out	dx,al	;set the border color
	}

	Other VGA ports also require reads.
	In general, I'd say graphics programming is painful if you don't 
test code more or less a line at a time, and lockups slow you way down!  
I'm going to build a FAQ from my book collection, and I hate rebooting so 
much I may be designing a boot ROM card (public domain design) so I can 
experiment freely.