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From: adamd@rhi.hi.is (Adam David)
Newsgroups: comp.os.minix
Subject: Larger / non-std fd formats (ST).
Message-ID: <3143@krafla.rhi.hi.is>
Date: 14 May 91 15:57:15 GMT
Article-I.D.: krafla.3143
References: <3115@krafla.rhi.hi.is>
Organization: University of Iceland
Lines: 34

In a previous article with subject 'Cannot open /dev/rfd? (ST 1.5:3)'
I wrote:

>So, why is it that /dev/rfd? fails during open()?    (I didn't break it).
>Has anybody already tackled the issue of formatting different floppy disk
>parameters under minix? There are several issues involved, most notably
>which interleave and skew factors to use and reducing the inter-sector gap
>to add another 10% to disk capacity.

The error was found in stfloppy.c and was causing open() to fail for all
/dev/*fd? devices (except /dev/fmtfd? which was disabled). Actually it would
handle SS disks correctly but barfed on DS disks. 
simple fix: multiply nr_sectors by nr_heads in the sanity checks at the end.

I have heavily modified format.c and it now succesfully formats DD disks to
924k and HD disks to 1848k using 512-byte sectors. Changes to support ED disks
up to 3696k are trivial. An ED format of 3864k is possible using 1024-byte
sectors but will require further modification to the system, because the
512-byte sector size seems to be fairly extensively used as a constant.
stfloppy.c could handle the deblocking internally, and no other part of the
system would have to know about it - but there might be better ways since
the minix system works mainly with 1024-byte disk blocks.

The formatter supports complete flexibility in specifying interleave and skew
factors, also various sector and track combinations. Slight squeezing of sector
gaps is used for the 21-sector HD format, and tight squeezing is used for the
11-sector DD and 22-sector HD format. When they are implemented the ED formats
above 42-sector will also need to use squeezing.

If there is reasonable interest I shall post the stfloppy.c patches and the
modified version of format.c, otherwise I will make them available by email. 
--
Adam David.
(adamd@rhi.hi.is)