----- Original Message -----
From: Peter Pachla <peter.pachla@xxxxxxxxxxxxxxxxx>
To: <stella@xxxxxxxxxxx>
Sent: Thursday, April 27, 2000 11:52 AM
Subject: Re: [stella] Hardware comparisons
> Hi Clay,
>
> >> OK ANTIC *IS* a CPU, albeit one with a very simple instruction
set....
> >
> > Actually ANTIC is a coprocessor....
>
> I think we're arguing about semantics here.
Yes, we are. And semantics are very important in the computer field.
> It's being USED as a graphics co-processor, but it is a very simple
> processor in it's own right.
VERY simple. It has no loops, conditionals, etc... It could never be
anything but a coprocessor.
> >....A system can only have one *central* processing unit, after all.
>
> Again, we're heading down a slippery slope here.
>
> It depends on how you define "CPU". The CPU in simple systems may consist
of
> a single (micro)processor, in more complex systems (like IBM RS/6000
systems
> for example) the CPU may be composed of several individual processor
units.
You mean several individual processors, which together make up a single
<cough> central processing unit? ;-)
Take the math coprocessor that used to be a separate chip in PCs. Nobody
ever called that a CPU either.
> I think the problem is that I, like most people, tend to use the word
"CPU"
> when I actually mean "microprocessor", IYSWIM?
And then you've got those nitwits who call the entire computer a CPU. Argh!
> >> AFAIR, without intervention from the 6502, all ANTIC can change on a
> >>scanline by scanline basis is the display mode.
> >
> > ANTIC can also change where display data is being fetched from....
>
> Errm, AFAIR that is defined by the instructions in the display list along
> with the contents of the HSCROL/VSCROL registers. ANTIC cannot change
these
> itself.
Read what you just said. Of course it's defined in the display list. That's
ANTIC's program! Anyway, the specific instruction is LMS (Load Memory Scan),
and is usually the first thing in the display list after the blank lines.
THAT is how an Atari knows where its display memory is, and you can have as
many of them in a display list as you want (in fact, games which do
horizontal scrolling have one on every row).
> >....where display list data is fetched from....
>
> But that is controlled by the DLISTH/L registers. Again I don't know of
any
> way ANTIC can alter this register itself.
My memory is a bit foggy on this one, but I think those are shadow registers
(560/561, right?). The last instruction in any properly constructed display
list is a jump back to the beginning of the list. However, that doesn't
necessarily have to be the same list! You could actually have, say, two
display lists that link to each other, swapping back and forth every 1/60th
second.
> Yes, you can perform "jump" instructions within the display list but these
> only allow you to jump over a 1K boundary and to jump back to the start of
> the "program".
Actually it's a 4K boundary that needs jumping over. But there's nothing
stopping you from jumping wherever in memory you want.
--
Clay Halliwell | ATARI XL/XE DO + THE
clay.FAKEMEAT.h@xxxxxxx | 8 - B I T S --------
e.halliwell@xxxxxxxxx | /////////// M A T H
Keeper of the Atari Jaguar Cheats and Codes FAQ. Request your copy today!
Editor, Jaguar Explorer Online - More Jaguar News Than Should Be Allowed!
http://www.atarihq.com/jeo/ "Where do you want to play Atari today?"
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