Path: utzoo!utgpu!attcan!uunet!cs.utexas.edu!rutgers!ucsd!sdcc6!sdcc13!pa1017
From: pa1017@sdcc13.ucsd.edu (Nick Lenz)
Newsgroups: comp.sys.apple
Subject: Re: Problems for a new IIGS owner (II)
Message-ID: <5256@sdcc6.ucsd.edu>
Date: 20 Nov 89 02:06:37 GMT
References: <1137@godot.radonc.unc.edu> <5659@wpi.wpi.edu>
Sender: news@sdcc6.ucsd.edu
Reply-To: pa1017@sdcc13.ucsd.edu (Nick Lenz)
Organization: University of California, San Diego
Lines: 37

David I. Seah (dseah@wpi.wpi.edu) writes...

> You can't write into "the frame", but some games have done some
tricks with
> it, most notably Tomahawk GS. There is a register that contains the
number of
> the current scanline being written. One could conceivably watch this
register
> very closely and change the border color on an arbitrary scanline.
The result
> would be a two-color border split horizontally across the screen. If
you are

Well I don't remember Tomahawk doing any of those things, but
theoretically you could implement something like overscan video
given enough speed. Sure, you'd spend a LOT of processor time
keeping track of the softswitches at $C02E AND $C02F. Right now it
is possible to check the softswitch at $C02E to find out (within 2
scan lines) which line of the screen is being updated and modify the
text, border or background colors as you wish thus creating a type
of overscan video (at least vertically). My guess is that, given a
sufficiently fast GS you can also keep track of the horizontal
counter at $C02F and also know exactly which scanline is being
redrawn (one of the bits of the vertical location is stored in $C02E
so that's why you can get within two line accuracy using $C02E).
Using this info you could (theoretically, of course) get overscan
video using the 16 colors available for text, border and background.
Anyways, my point in this lengthy (and first) posting to the net is
to say that it would be possible to do overscan if you could read
the aforementioned softswitches with time to act on what is in them
before they change again. Processor-intensive? Yes, but neato-keen,
too.

Nick Lenz               "How would you feel about life if Death was
                           your older sister?" -- _Sandman_ #11
Internet: nlenz@ucsd.edu
              or