[Coco] MacOS was Re: AmigaDOS was Re: model 4 cp/m
gene.heskett at verizon.net
Fri Jan 12 05:47:48 EST 2007
On Friday 12 January 2007 03:01, Willard Goosey wrote:
>>Date: Thu, 11 Jan 2007 09:50:11 -0600
>>From: Joel Ewy <jcewy at swbell.net>
>>IIRC the Amiga drives aren't "standard" drives either. I don't
>> remember the exact details, but I do believe that you can control more
>> parameters of the Amiga drive and controller than you can on the PC.
>> Here's a quote from Wikipedia:
>Yes, but the drives themselves are almost standard. "Almost" means
>that you can't just drop a PC drive in when the old drive dies. :-(
>>So apparently getting the Amiga to read Mac disks is mainly a matter of
>>exercising the Amiga's floppy controller properly.
>Well, the board is part of the floppy daisy-chain. The floppy cable
>goes from the motherboard to the AMAX board. Then another cable goes
>from the AMAX board to the disk drive.
There are two ways of handling the amiga disk format. All methods must
understand that the amiga's floppy disk format is not sector addressed,
but the whole track. Eg the amiga reads the whole track, then searches
this buffer for the right data. Ditto for writes, changing one byte
results in the buffer being dirtied and the whole thing is then
re-written to the disk. This saved a considerable amount of intersector
housekeeping data from being needed, so what was a 720k pc disk actually
held 880k, and one format variant actually put a full decimal megabyte on
The real problem with hi-density disks for the amiga was that the Paula
chip, which contained the data pump and decoder, could not run at 1
megabaud which the HD disk expected, so commie had some two speed drives
made that only turned 150 rpm in the hi-dens mode. Toward the end, those
drives were very expen$ive cause they didn't make enough of them to go
around. I have one, but was never able to locate a 2nd one.
They 'cogged' like heck, visibly sort of stepping along as they turned.
The saving grace was that the drive pin on the hub was consistent enough
in locating the disk so that the wibbles in the data rate caused by this
during the write, were canceled by matching wibbles in the disk speed on
the read and the actual data rate was constant enough to work, only the
amplitude varied, over as much as a 3/1 range. As you can imagine, there
were interchange problems, and the heads had to be kept clean.
The catweasel board has enough buffer that a speed translation can be
done, so HD drives turn at normal speeds and the catweasel data runs at
the slower 500 kilobaud rate the amiga can use.
>Willard Goosey goosey at sdc.org
>Socorro, New Mexico, USA
>"I've never been to Contempt! Isn't that somewhere in New Mexico?"
> --- Yacko
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