Re: GS Emulator

[nraymond@k12.ucs.umass.edu (Nathan Raymond (Phillips Academy 94))]

In a previous article, barryn@NETCOM.COM (Barry Nathan) says:

>Earlier in this thread, Michael Foegelle said:
>
>>I believe the difference here is that the power PC MAC can't really be
>>considered an "emulator" in the truest sense of the word, since I believe
>>it consisted of an entire re-write/compilation of the MAC firmware on the
>>Power PC so that it runs in native mode.  (I may be totally off on this but
>>I thought that was part of the idea.)
>
>They only rewrote the often-used parts of code. They're rewriting more and
>more of it slowly, and a lot of extra emulated code is supposed to be
>coming with System 8.
>
>>Granted it still has to emulate the
>>68030 or whatever processor to run the Mac software,
>
>Close. 68020.

Although the emulator identifies itself as a 68020, it has the MOVE16
instruction implemented (the instruction that was added with the 68040)

>>but it's a much more
>>integrated system than your typical emulator.  The hardware, etc, was designed
>>to work Mac-like.
>
>True.
>
>>At any rate, I suspect a PC or PPC based 65816 emulator wouldn't be too hard
>>to accomplish, and emulation rates would be about what our realtime rates are.
>>GS video could be moderately emulated, but there wouldn't be most of the timing
>>flags and interrupts available that allow tricks like 3200 color graphics to
>>work right.  Using a Gravis Ultrasound, the GS sound shouldn't be too hard to
>>emulate, but there aren't many PC users with GUS overall.  Still, those
>>wanting to run GS software might go buy GUS's just for that purpose.  That
>>still leaves emulating all the lousy soft switches and interrupts, not to
>>mention having to copy Apple's ROMs to your emulator.
>
>There could be an option to turn off some of the softswitch/interrupt
>emulation so that the emulator could run faster. You could also try to
>intercept jumps to ROM vectors and emulate parts of the ROM through PPC
>instructions. A clean ROM that runs faster as well, but I don't know if the
>FTA demos would still work...
>
>>So unless Apple releases
>>the copyright (unlikely)
>
>Actually, Microsoft owns some of the ROMs (because of Applesoft BASIC, if I
>remember correctly), but I could be wrong.
>
>>or someone writes some clone ROMs that can withstand
>>Apple's scrutiny (even MORE unlikely) that only leaves the people owning a GS
>>already the opportunity to use the emulator.
>
>Yes, but those GSes are probably going to die sooner than those PowerPCs.

An alternative to developing "clean room" IIgs ROMs that would take MUCH
less time to implement would be to map the IIgs ToolBox calls to the Mac
Toolbox, since they are so close (exactly how close, I don't know exactly,
and there are differences such as the way scroll bars are a part of WDEFs on
a IIgs and they are seperate CDEFs on a Mac).  I'm sure these two Toolboxes
are closer than the Mac emulator they did a year or two ago for UNIX
workstations that mapped the Mac Toolbox to Motif (or a similar UNIX GUI). 
This alternative should at least be explored.

nathan
nraymond@genesis.nred.ma.us
-- 
"We don't know one millionth of one percent about anything."
- Thomas Alva Edison