Path: utzoo!mnetor!uunet!mcvax!ukc!mupsy!liv-cs!unpowell
From: unpowell@csvax.liv.ac.uk
Newsgroups: comp.sys.atari.st
Subject: Re: TRAP handler question (ST hardware scrolling)
Message-ID: <1283@csvax.liv.ac.uk>
Date: 2 May 88 22:01:13 GMT
References: <544@csvax.liv.ac.uk> <1204@usl-pc.UUCP>
Lines: 32
Organisation: Computer Science CSVAX (VAX1), Liverpool University


In article <1204@usl-pc.UUCP>, jpdres13@usl-pc.UUCP (John Joubert) writes:
> -----------------------------------
> Mark,
> 
> 	How did you get the machine to scroll from a non-512 byte boundary?
> 
> In the past, when I changed the address of the logical screen I had to make
> sure that I placed it on a 512 byte boundary.  When I did not I got a really
> screwy screen if I did not crash.  How did you get around that?

	I'm not quite sure what your problem is. If you do displace the screen
by, say, 256 bytes while the desktop is being displayed you do get a "really
screwy screen". This is due to each raster line of the screen requiring
160 bytes in colour and 80 bytes in monochrome. So this displacment of the
screen puts the display somewhere in the "middle" of a "display line". The
first number that is a multiple of 160, 80 and 256 is 1280, which is
incidentally the amount of memory each character row (on a 25 row display)
occupies. Thus moving the screen by this amount, $500, will displace the
screen by an entire character row. With a careful bit of manipulation this
effect can be turned into a hardware screen scrolling routine.
	I don't know about the crashing, the STs not "that" bad.

	Mark Powell

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 "...I hate the white	JANET unpowell@uk.ac.liv.csvax
  man, and the man	UUCP  {backbone}!mcvax!ukc!mupsy!liv-cs!unpowell
  who turned you all	ARPA  unpowell%csvax.liv.ac.uk@nss.cs.ucl.ac.uk
  loose..." R. Harper

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