Re: Commodore 15 Second disk format routine disassembled

On Thu, Apr 16, 2026 at 11:18 PM <silverdr_at_srebrnysen.com> wrote:
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> > On Thu, Apr 16, 2026 at 4:34 PM <silverdr_at_srebrnysen.com> wrote:
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> > > Technical Deep Dive: 15 second format routine for the Commodore 64 and 1541
> > > https://www.youtube.com/watch?v=74EKB7gdDv4
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> > Minor inaccuracy inherited from the previous video is the visualisation of the tracks on disk layout. 1541 is an 80 track drive but with the head it is equipped with and double-stepping it is made to be effectively 40 track drive, not 35 track one. CBM DOS simply does not use the innermost five tracks, presumably as a reliability safety margin. This means the visualisation, which shows the tracks spread evenly across the surface with no unused portion is not really correct.

all correct, however I presume the use of only 35 tacks was chosen to
remain compatible with the 4040 formatted disks. The SA-390
(effectively a SA-400 without controller) used on the 2040/3040/4040
could only use 35 tacks.
I *think* the 1541 firmware could have been tweaked to use either 35
or 40 tacks and in facts the old Speeddos plus mod allowed us to
format up to 40 tracks. However even back in the '80s I recall the
upper additional tracks weren't super reliable (maybe some gaps would
need better tuning when going over track 35?).
1541 firmware uses "half track" steps to try reading sectors in
presence of read errors.

Frank IZ8DWF