Problems in Modernizing the IIgs, con't

[bjblackbear@ibm.net (B.J. Major)]

Subject:     Re: Fwd: Solutions in Modernizing GS (Re: Problems in
"Moderniz
Sent:        10/29/97 3:01 AM
Received:    10/28/97 6:03 PM
From:        Lewin A.R.W. Edwards, sysadm@zws.com
To:          Scott G., comp.sys.apple2


>I talked to an friend of mine that uses and programs an Amiga 500
>(a computer that uses the 68000) and asked him about these supposed
>problems in upgrading the GS.  Here's what he has to say:

Let me preface my comments here by saying that I am very familiar with 
the Amiga from a developer perspective, having performed a large number
of hardware and software hacks on the revs 5 and 6a A500, so if you were
hoping to choose an example about which I knew nothing in order to 
perform a little smoke-and-mirrors magic, you've been unlucky.

>Some Mac users have spammed csa2 and claim that the GS could never
>be upgraded.

Whilst I can't comment on what other people have been posting to csa2, 
since this message is at least partly directed at me, I feel it
necessary to point out that I did not say the GS could never be
upgraded, the original thread was about someone thinking of spending
money realizing the 65832 design, and I pointed out some of the
technical problems and mentioned that it wasn't really worth the effort,
except from the purely hackerish point of view that "it would be cool to
do". Which is, of course, a legitimate reason to do something. The
argument has now moved on to whether or not it makes sense to develop
the 65xxx series MPUs beyond their current state.

>They doubt the derived 65C816 32-bit chips could have been made.

I never said that either. What I said was that it would have been 
commercially stupid for Apple to go on developing the 65xxx series MPUs,
it would be stupid for them to do so now, and that I regard the idea of
a hot-rodded IIgs as being fun and cool in a theoretical sense, but of
no commercial value whatsoever.

>"That figures, we've been trying to convice them that they don't
>have pre-emtive multitasking for ages and they don't get it.

Ah, someone having a nice hearty bitch. So what about PEM? Your Amiga
500 example lacks an MMU and doesn't have virtual memory or protected
memory areas either. And the preemptive multitasking is not a hardware
feature anyway, so it's completely irrelevant to the discussion. In
AmigaDOS, the multitasking is driven by the vertical retrace interrupt.
The Mac 512K and upwards have a vertical retrace interrupt service which
could similarly be used to implement a fully preemptively multitasking
OS, if someone could be bothered.

>could justify killing it.  This is not saying that even Mac
>Classic prices were reasonable when you consider that Amiga 500
>systems were selling for less that $800 fully equipped, but
>the Classic was deliberately made a steal compared to the GS."

$800 was not fully equipped. The A590 hard drive unit (which was 
incidentally based around an impossible-to-source XT-IDE hard disk mech)
was nearly that cost by itself. The baseline A500 was 512K chip-mem and
optionally 512K slow-mem, single 880K floppy drive (Classics had FDHD 
drives). To expand beyond that point required quite substantial outlay.
The A500's RAM is 16 x 41256 (256K x 1) in the older motherboard 
revisions, 4 x 44256 (256K x 4) DRAMs in the rev 6a and onwards units,
it 
doesn't use SIMMs or other variable-density memory devices.

The A500 also has serious performance problems due to the fact that
Agnus and the CPU fight for all the $00000-$7FFFF/$100000 and
$C00000-$C7FFFF areas.

>"Clock for clock?  No way is the 68000 more efficient at all
>tasks.  At the equivalent frequency, the 65C816 will beat a
>68000 in many tasks.

In artificial tests of non-real-programming-task instruction mixes only.

>The Amiga deals with this by using custom hardware graphics
>chips, which is why our graphics are so much better than the
>equivalent Macs and don't slow down the computer."

That indicates a serious lack of understanding of the Amiga's 
architecture, too. The baseline A500 does not outperform any color 
Macintosh in terms of graphic performance because "the custom chips
don't slow down the computer". The reason that the blitter in Agnus
makes animation tasks smoother and faster on the Amiga is because this
device is essentially a custom MPU optimized for graphics operations.
Buss contention slows the 68000 to a crawl when you're using
high-resolution graphics on the stock Amiga 500. Audio, disk and blitter
DMA slow it down even more.

Basically, doing a complex series of graphic ops on the Amiga involves 
setting up a list of blitter ops to be performed, installing a blitter 
DMA finished ISR which will fetch the next item pending from the list, 
and starting the first op manually. While that is running, you have a 
*little* CPU timeslice in the foreground, but that's really borrowed 
time. Agnus is chewing most of the RAM bandwidth.

>by Gateway 2000, so it can be done.  The market for computers
>priced at $600 is large and left by the void formerly filled

You can't be serious. GW2K is into interactive CATV. The personal 
computer market as personal computers is driven essentially by 
availability of Microsoft Office.

If this fantasy was even remotely feasible, then Be would still be
making BeBoxen.

=== From the Apple PowerBook of ======================
Lewin A.R.W. Edwards [Team OS/2] <http://www.zws.com/>
Programmer + Hardware Eng     Certified Apple Engineer
Need a programmer?    <http://www.zws.com/resume.html>
Tel  +61-412809805                   Fax +61-392281335
"No non-trivial programming task is atomic" - LARWE 97
=== (C) Copyright 1997 by Lewin A.R.W. Edwards =======