WD1793 read-track weirdness

From: Tom Jennings <tomj_at_wps.com>
Date: Tue Mar 30 14:53:44 2004

On Thu, 25 Mar 2004, Vintage Computer Festival wrote:

> > Has anybody out there written drivers for the WD1793 floppy
> > disk controller chip? I've been doing that for fun, and have
> > hit on a very strange problem with the 'read track' command.
> > It seems to work just fine, returning a stream of bytes that
> > includes the data for each sector, until it finds a byte with
> > a value of $29 (or 29H, if you prefer). From that point up
> > to the next address mark, it gets garbled data. No other data
> > value seems to have any odd effect. 'Read sector' returns >
> the right data with no weirdness.

The data isn't necesarily aligned on byte boundaries. It's
a bit stream. Back when I used to disassemble track data by
hand, I'd simply do multiple track reads until what I wanted
was byte-aligned, and view with SID or the PDOS equiv.

You could code up a bit stream and look for the sync patterns
and all that and byte align it yourself.

Write current turn off creates glitches in the inter-sector
gaps. It's normal.

If I recall, dumped tracks that were never written often
byte-align. It's been a long time...
Received on Tue Mar 30 2004 - 14:53:44 BST

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