Re: Commodore 15 Second disk format routine disassembled

On Wed, Apr 22, 2026 at 2:22 PM <silverdr_at_srebrnysen.com> wrote:

> > 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.
>
> Yes, that's purely software limitation. DolphinDos formats 40 tracks at
> will (provided the medium is a quality one) and gives 749  BLOCKS FREE
> instead of 664 we all know and remember.
>
> > 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?).
>
> That's precisely my point. At slower linear speed on the upper (inner)
> most tracks, the reliability could suffer, but since I used to avoid crappy
> media, which used to leave dirt on the head etc. I hardly encountered any.
>

I'm working on Pi1551 project (something like Pi1541 but for C16/116/Plus4)
and I have found out about "Super DOS 1551" ROM, one of two alternative DOS
ROMs for 1551 drive that I know about:
https://plus4world.powweb.com/software/Super_DOS_1551

In its extended format disk the disk will have tracks 35-41 formatted and
it reports 840(!) free blocks.

It not only formats the inner tracks, but, if I got the disassembly right,
the whole disk is formatted with the same density as tracks 1-17, with 21
sectors on each track.
I'm not able to reproduce this in VICE or YaPe. I don't have a real 1551 to
check how reliable this is either, but indeed it works on emulated Pi1551.

Best regards,
ytm