From: s047@SAND.SICS.BU.OZ.AU (Jeremy Lee)
Newsgroups: comp.sys.acorn
Subject: Re: A5000 disk format recognising
Date: 10 Apr 92 07:52:03 GMT

In article <ARM200-920408131404-37018D55*@MHS> you write:
>From the collected works of ssurawls@susssys1.reading.ac.uk (Mister E):
>
>>Does the A5000 recognise its disc types automatically and can you override
>>them? What I mean is if you put a 2 meg disc in and select format does it
>>default to 1.6 meg or 800K, and could you force it to format a 2 meg disc to
>>800K?
>
>No, it doesn't recognise the disc coating.  Think about it for a minute.

	I think that means yes, you can format it to whatever density
you want.

>>And also, does anyone know how flexible the A5000 disc controller is, ie is
>>mac and amiga format reading a possibility? I have heard great things about
>>it....
>
>No again, unfortunately.  Mac discs require a variable speed *drive*, and I
>remember the relevant Acorn people turning green at the mere mention of
>Amiga format :-)

	I can't see any reason why the Arc cannot read Mac disks. The
drive is varaible, yes, but only on a track by track basis. I'm sure
that you could tell a flexible drive controller chip that the data
density is different for the current track. I thin that there are 8
different speeds that they use. Slow for the outer tracks, and slow for
the inner tracks. The reson is simple, to keep the liner velocity the
same (and therefore the R/W density). this makes s more reliable and
efficient disk management approach. On all other systems, the only track
that is working at it's most efficient is one of the central tracks. the
innermost tracks have a terribly high data density. The outer tracks
have a low data density. The outer tracks are twice as long as the inner
track, but they are forced to contain the same amount of data. It's a
very sensible approach, except it makes drive manufacture difficult. On
the Arc, all you need to do is tell the drive controller that the track
you are currently working on has a density of X, and twiddle a few other
numbers, as assuming you drive can handle the high data rate that would
result from reading the outer edges, then you should be alright.
	Actually, I believe (but connot confirm) that the new
superdrives actually don't vary the speed. (ie, the 1.4 meg format.)
I'm not sure though.
	The Amiga is impossible to read on the Arc because the drive
(I believe) uses some special hardware tricks (like the rest of the
machine) that are difficult to achieve in software. I think it's
something like a switch that tells when the disk has rotated to a
certain position, but don't quote me of that. (Anyone know?)

>Rhodri James                   *  Disclaimer:  These are my opinions, not
>rmj10@phx.cam.ac.uk            *


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*    . Jeremy Lee s047@sand.sics.bu.oz.au    Student of Everything    *
*   /|            "Where the naked spotless intelect is without       *
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* /  |rchimedes      Leland, look to the light" - Dale Cooper         *
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