Subject: Re: Apple Computer, What Are You Up To?
Newsgroups: comp.sys.apple2
From: dempson@actrix.gen.nz (David Empson)
Date: Tue, 16 Mar 1999 00:14:40 +1300
Message-ID: <1doqf46.1kbcvxk1abzb3oN@dempson.actrix.gen.nz>
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Rubywand <rubywand@swbell.net> wrote:

>      Various versions of System add patches to in-ROM code. Until 1989, the
> patches were exclusively for the ROM-01 ROM.

System 1.0 and 1.1 only support ROM 0.  Systems 2.0 through 3.2 have
patches for ROM 0 and ROM 01.

System 4.0 only officially supports ROM 01.  System 5.0 and later
support ROM 01 and 3.

> Evidently, the ROM 3 code is different enough from ROM-01 code so that
> System versions up through 3 or 4 can not boot on the ROM 3.

ROM 3 _is_ able to boot System 4.0.  Apple went to some pains in the ROM
3 firmware to ensure this.

I have also succeeded in booting System 3.2 on a ROM 3.  I don't have
System 2.0, and don't recall what happened with 1.1 or 1.0.  I think
that 1.0 doesn't work (for native software) on anything other than a ROM
0.

A potential problem is that if you boot older system software on a ROM
3, the application ends up using the newer ROM-based versions of the
toolsets, not the ones that would be loaded from disk.  This could cause
compatibility problems due to the application depending on specific
features of older toolset versions, which changed somewhat by System 5.0
(which matches the toolset versions in ROM 3, for the most part).

Most other compatibility issues would be due to dependency on
undocumented firmware entry points or firmware RAM usage.

Personally, I've encountered a grand total of two pieces of software
that didn't work properly on the ROM 3.

One of them was the "Screen Dimmer" screen blanker from Roger Wagner
Publishing (written by Dave Lyons), which was depending on the mouse
firmware implementation in the ROM 1.  I was able to disassemble and
modify it so that it worked properly on the ROM 3.  (The only symptom
was that mouse clicks were not resetting the blanking timeout, so the
screen would go blank if you were only using the mouse for several
minutes.)

The second was an INIT whose name now escapes me, but a later version of
it was compatible.

I should add at this point that I don't play many games on the IIgs, but
all the ones I was still playing at the time worked OK.  I recall a
large number of compatibility problems with games going from ROM 0 to
ROM 1.

Oh - one more problem: Tunnels of Armageddon didn't cope with the mouse
speed setting I was using on the ROM 3.  The mouse had to be set to its
default value for movement to work properly.

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