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From: scotty@l5comp.UUCP (Scott Turner)
Newsgroups: comp.sys.amiga
Subject: Re: Reading MacDisks on Amiga
Message-ID: <112@l5comp.UUCP>
Date: Sat, 9-May-87 21:23:21 EDT
Article-I.D.: l5comp.112
Posted: Sat May  9 21:23:21 1987
Date-Received: Sun, 10-May-87 11:43:20 EDT
References: <8705080543.AA18112@cory.Berkeley.EDU>
Reply-To: scotty@l5comp.UUCP (Scott Turner)
Organization: L5 Computing, Edmonds, WA
Lines: 65
Summary: Facts about reading Mac disks.

In article <8705080543.AA18112@cory.Berkeley.EDU> dillon@CORY.BERKELEY.EDU (Matt Dillon) writes:
><I've read several people who stated that, since the Mac drives spin
><at variable rates and the Amiga drives at fixed rates, that it was not
><possible to read Mac Disks with the Amiga drive.
><   Well...  one way to get around this is to read in data faster or
><slower, depending on what track you are on.  This is how the Commodore
><1541 drives manage to squeeze 170k on a single density 5 1/4" disk, putting
><more sectors on the outer tracks of a disk and reading the bits at a faster
><rate (the drive spins at a constant 300 rpm).
><   So, a program to read Mac disks on an Amiga seems not-impossible-- I
><would try to write such a beast, but don't yet even have a C-compiler
><for my 'miga.
>
><   Am I missing something?
>
>	Yes.  The 1541's firmware has access to a hardware IO register which 
>allows it to select the speed.  No such register exists on the Amiga.  
>Still... has anyone *tried* raw-reading mac disks??? I assume the Mac drive 
>doesn't spin any faster than then 300rpm, which means you *might* be able
>to decipher the data the Amiga would spew at you.
>					-Matt
I apologize for including the whole thing again folks but I'll be addressing
most all the points above so I wanted yall to have 'em in front of ya.

First lets correct that statement about selecting speed, the Amiga DOES have
such a register! Go read yer hardware manual again Matt.

Yes, the Mac has variable speed drives. But that doesn't imply they vary based
on 300 rpm! We aren't the center of the universe folks! :-)

Back when the 3.5" drive was first floated by Sony they wanted it to rip around
at 600 rpm. The industry though decided that being able to plug them into
existing systems meant they had to rotate at 300 rpm. So Sony lost, almost.
The Mac people didn't give a rats behind about the rest of the industry, after
all their disk format was already incompatible. Apple had just come off the
embarrasment of the Twiggy drive. Analysis showed that the variable speed
concept was fine, it was the drive and media that nailed them. So they kept the
variable speed part and switched to "stock" drives and media. As a result Mac
disks vary their speed from 400 to 600 rpm. Alot of people have stated that the
Mac system is "brain damaged" because of it's use of variable speed and software
decoded GCR. Well that extra rpm cancels out quite a bit.

I have read the slowest moving pieces of Mac disks, back in Nov 1985. The Amiga
can decode Apple's GCR. I would have pursued this to the end but the trackdisk.
device and floppy treatment in general was rather err brain damaged back then.
It still hasn't gotten much better... :-( Anyway, is there hope? YES! Now that
all the facts are out I'm sure a few of you will get right to work, :-).

Note, a C Compiler IS NOT REQUIRED to work on the Amiga. I've NEVER used any of
the C "compilers" sent to me by C-A. When working on ground that no one else has
trompled it helps to know EXACTLY what is going on. With a C compiler in
between you and the hardware that gets tricky at best. After all, if something
goes bump you don't know if it was Manx tripping or you. And all to often
it seems people find it was Manx and not themselves. PD assemblers are available
that will let you get right to work for the cost of a download.

Are you missing anything? YES! The Amiga 3.5" disk drive spins from 0 to 300+
rpm...

Scott Turner

L5 Computing, the home of Merlin, Arthur, Excalibur and the CRAM.
GEnie: JST | UUCP: stride!l5comp!scotty | 12311 Maplewood Ave; Edmonds WA 98020
If Motorola had wanted us to use BPTR's they'd have built in shifts on A regs
[ BCPL? Just say *NO*! ] (I don't smoke, send flames to /dev/null)