Re: Disk ][ Launcher (was: Late night musings...)

[dempson@actrix.gen.nz (David Empson)]

Nathan Mates <nathan@visi.com> wrote:

>    The fact that emulators work one way does not mean that the real
> computer has a chance of duplicating some functionality in software,
> especially when hardware is involved.

...unless the real computer performs an emulation of part of itself!

This is not impossible - it would basically be a matter of porting one
of the existing Apple II+ or IIe emulators to the IIgs.  The emulator
would even have the benefit of having compatible hardware, so it could
do things for real where possible.

Some operations would be tricky under GS/OS, due to memory restrictions.
For example, it might not be possible to use hi-res graphics, because
some other software may have allocated memory in the $E0/2000-$5FFF area
(not very likely).

It would still boil down to checking each executed instruction to work
out what was being accessed, then executing that instruction.  It would
probably be easiest to emulate the CPU, rather than trying to execute
the real instruction in place.

We are therefore talking about emulating a 65C02 in software on the
IIgs.  This is a rather CPU intensive operation - you need a reasonably
fast Mac or PC just to get real-time emulation of an Apple ][+, let
alone a IIe (which has more complicated memory and I/O).  On a IIgs, you
wouldn't get anything like reasonable execution speeds, even with a very
fast ZIP GS accelerator.

-- 
David Empson
dempson@actrix.gen.nz
Snail Mail: P.O. Box 27-103, Wellington, New Zealand