Re: 1541 question (was Re: Catweasel)

From: Nicolas Welte (
Date: 2002-07-10 18:02:37

Bryan Pope wrote:
> And thusly Nicolas Welte spake:
> >
> > track drives, not the old 40 track ones (because the 1541 also has 80 tracks,
> > even if it uses the head from a 40 track drive).
> >
> So is it possible to move the 1541's head as if there were 80 tracks?

Yes. The tracks between the 40 normal tracks (numbered 1 to 40) are called
half-tracks, and are numbered 1.5 to 40.5. The DOS of a 1541 steps onto these
half tracks when a read error occurs on the full track position. Half tracks
are also said to be used as a copy protection (note that it is impossible to
use both a full track and one or two of its adjacent half tracks, the
previously written track would get overwritten because of the 40 track r/w
head). But you can use e.g. track 23, track 24.5 and track 25.5, and test the
copy for correct positions of the tracks. A simple copy program would
probably move the half tracks back to the full track position. An advanced
protection scheme could just use a single sector or only a few bytes on the
half track, at a position that has only gaps on the full track. This would
also need track synchronisation, which the 1541 cannot do easily, it lacks an
index hole sensor. 


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