From: u2d92@seq1.keele.ac.uk (MJ Gumbley)
Newsgroups: comp.sys.amiga.advocacy
Subject: Screens Unique to Amiga? (Was Re: CBM mention on 12/11)
Date: 23 Dec 92 11:05:33 GMT
Organization: Keele University, England

From article <1992Dec22.145116.29159@sbcs.sunysb.edu>, by dtiberio@engws10.ic.sunysb.edu (David Tiberio):
> In article <1992Dec21.193851.25752@sol.ctr.columbia.edu> jerry@msi.com (Jerry Shekhel) writes:
>>Aric.Caley@ofa123.fidonet.org wrote:
>>:
>>: He's  talking  HARDWARE.   The  Amiga  hardware  allows  for true, multiple
>>: screens,  in  a way thats different from any computer I have ever seen.
>>:
>>
>>Yeah, yeah.  The Atari 400 could do similar things, although, of course, it
>>didn't have a multitasking OS to take advantage of it.  The video display
>>was constructed from a list of instructions, each of which could specify
>>the resolution and pixel data address of a single line of graphics (or text).
>>Add horizontal blank interrupts, and you can modify the palette after each
>>scan line.  Interesting, but really nothing new.
> 			  ^^^^^^^^^^^^^^^^^^^^^^
> 
> 	Wrong, the Atari 400 and the C64, with all their screen interrupts,
> can not display multiple screens like the Amiga can...an Atari ST can do a 
> 
>>| JERRY J. SHEKHEL  | Molecular Simulations Inc. | Time just fades the pages |
> 
>       Amiga Graphics Association BBS (516) 473-6351  24 HRS 14.4 v.32bis
> Thanks Gwen. Like I said, I had faith in #3. At least it answers the phone now.
> 	

  Well, I don't want to start a cascade here, but...

  Just let's clear things up. The Amiga's graphics co-processor processes a
PROGRAM called a COPPER LIST. This is a list of instructions (effectively)
telling the video hardware what graphics mode to use for each scanline,
(and probably sub-division of a scanline). You tell the video hardware
where the video memory is, and what resolution to display it in, then issue
a WAIT command, telling the co-processor to keep displaying in the current
mode until you reach a certain position on the screen, where you can change
resolution etc., again. 

  This is how SCREENS work on the Amiga. You change the X and Y co-ordinate
of the WAIT command. (There's a bit more to it....)

  The 8-bit Atari COULD do this as well; the ANTIC chip operated in a very
similar way to the Amiga's co-processor. The co-processor program was
called a DISPLAY LIST. The work involved in rewriting the display list
would be more complex, but it could be done.

  The C-64 could do it too, I assume, but it didn't have a co-processor:
you just interrupted the CPU at a predefined place on the screen. A RASTER
INTERRUPT, the same as the 8-bit Atari's DLI (Display List Interrupt). LOTS
MORE WORK.

  Perhaps someone can fill me in with the real story regarding the IBM
world. AS FAR AS I KNOW, AND I MAY BE WRONG, the VGA does not use a copper
list/display list, cannot change resolution on the fly, and doesn't have
display list interrupts. Therefore, (again, I may be wrong here) you will
not see screen handling capabilities similar to an Amiga, on a PC.

>>scan line.  Interesting, but really nothing new.
                           ^^^^^^^^^^^^^^^^^^^^^^^

  No, nothing new, but if it's such a useful concept (IMHO it is one of the
Amiga's finest points - I really love those WinDoze screen updates when you
switch from app to app - the Amiga does it INSTANTANEOUSLY, because there's
NO SOFTWARE UPDATE necessary)..... Why doesn't the PC have it, as it's the
world's most popular micro computer? ;-) Multiple video pages don't quite
do the same job...

  (Note: POPULAR does not equate with BEST in my book. Ten million flies
may eat horsedung, but that doesn't mean *I* have to. Each to their own.)

  Merry Christmas, all, from Matt.

--
Matt J. Gumbley              || Still needs a good signature, not liking
u2d92@seq1.cc.keele.ac.uk    || any of his recent feeble improvements on:
University of Keele, England || Why can't _I_ think of a good signature?
Newsgroups: comp.sys.amiga.advocacy,comp.sys.amiga.misc