bdb@cl.cam.ac.uk (Brian Brunswick) (03/22/90)
Ok folks, there has been a lot of discussion over the troubles caused by
apple because of AMAX and the software only pirated versions of it.
I have a major problem to add to this debate:
For a long time now (over two years) I have been toying off and on with
my own version of a mac emulator - it has had no influence from AMAX,
which I have never seen, and indeed was started when I had only heard of
the one for the atari.
It has the following properties:
* Runs entire mac system (multifinder optionally) as ONE amiga task in its own
screen - so rest of amiga system continues - though mac system does busy-wait
and so must be at a low priority
* Uses trick of displaying single bit plane mac screen together with itself
shifted down one line and using a pallette to blur the two together
to virtually eliminate interlace flicker.
* Runs mac filing system on ANY amiga volume that you have described by
mounting it - hard disks and ram disks included. I have also managed
to trick real macs into using the 5MB partition of my 20MB SCSI hard disk
that is currently in mac filing system format
* Also has my own special mac filing system that interfaces to amigados
and so accesses existing amigados volumes.
* Runs by copying macplus roms into ram and patching them (Is this legal?)
* Needs either: ram at 4MB point to put roms in and ram below this - mac system
seems to break if ram above rom
or: hardware to generate an address error at 4MB point - trivial
hack of a 74LS138 and a diode - and a 68010 or better for the necessary
ability to restart instructions - this is what I use.
* Comfortably gives a mac 1MB smaller in size than the amiga memory - though
I believe it could go up to only .5MB smaller.
* Reliability is about alpha right now.. it runs system 6 multifinder and
all things I've tried that are not copy protected, but crashes
spontaneously sometimes.
QUESTIONS
Is this legal?
Should I release this into the public domain - as I would like to do?
(Get your own mac roms and system should go without saying)
Would it be more legal if I avoided having the mac roms in ram - this
would be possible, but irritating, and they would have to be at 4MB point -
bending autoconfig guildlines?
Don't worry - I am not about to mangle the reputation of the amiga without
a lot of thought - but I would like your collective wisdom on this subject.
Personal mail I will periodically post summaries of UNLESS IT SAYS NOT TO.
Your opinions are invited....
Technical notes
===============
I get the low memory workspace needed for the mac by rebooting the amiga
system with execbase at $1400+. The mac system can be persuaded not to trash
the exception vectors, but best protection is afforded by using a 68010 or
better and VBR to move them out of harms way.
The mac system writes directly onto the bitmap of the amiga screen - which I
persuade intuition not to trash by whatever flag bit it was.
Screen size matches workbench - may be morerowed or whatever.
It simulates 68000 supervisor mode using 68010 user mode and a priv error
handler.
Mouse movement is stolen from intuition by a handler enabled when the mac
screen is active, disabled by using the right button, which brings back
intuition pointer for use in dragging screen/clicking it to back.
Disk access goes through trackdisk.device/harddisk.device or whatever
Makes floppies agonisingly slow, though hard disk is ok. I have not
experimented with connecting a mac expansion drive up, though that should
be possible with a home made hardware hack - I have had some experiance
with reading different disk formats.
Brian Brunswick, bdb@uk.ac.cam.cl, bdb10@uk.ac.cam.phx. Short .sig rules!