[no subject]

>       The motherboard layout changed differently to produce better 
> sound, and has a bigger on board RAM (1Mb on ROM 3 vs 256k on ROM 01).

1MB fast plus 128KB slow (1.125MB), compared to 128KB+128KB on the ROM 1
(256 KB).

>       On the ROM 3, could only need to specify either Printer or Modem
> Port to be switched to AppleTalk for AppleTalk to be active, but on ROM 01
> you will require both Printer and Modem Ports to be switched to AppleTalk.

Wrong.  On a ROM 1, you must set slot 7 to AppleTalk, and either of slot
1 or 2 (depending on the port you want to use) to "Your Card".

On a ROM 3, you set the desired slot to AppleTalk.  (There is also an
AppleTalk option on slot 7, but enabling it will waste a slot.  It is
only there for compatibility with brain-dead software that assumes the
AppleTalk firmware is in slot 7 on a IIgs.)

>       Actually, I wanted to ask a LONG time, why is ROM 01 more 
> compatible ? I thought ROM 3 was just an 'enhanced' ROM 01 with more RAM,
> and a bigger ROM that incorporates the essential System Tools. Any logical
> explainations ?????

The majority of the compatibility problems are with programs that use
undocumented entry points, undocumented variables maintained by the
firmware, etc.  Several things moved around in ROM 3.  Games and demos
seem to be the worst culprits.

I encountered problems with two GS/OS programs: an early version of an
Animated Watch extension didn't work on the ROM 3, and the screen saver
I was using (the name escapes me - it came from Roger Wagner Publishing,
written by Dave Lyons if I remember right) didn't recognise mouse
movements because the undocumented variables used had changed.  (I
disassembled and patched it to work on my ROM 3.)

There was a similar set of compatibility problems with very old software
when the ROM 0 to ROM 1 upgrade occurred (the first version of Bards
Tale GS, and at least one other game).

The same thing happened with pretty much every Apple II model or
revision - some programs didn't work because of a hardware or firmware
change which conflicted with assumptions the program was making.

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