Re: Commodore 15 Second disk format routine disassembled

Am Freitag, 19. Juni 2026, 14:14:33 Mitteleuropäische Sommerzeit schrieb 
Francesco Messineo:
> On Fri, Jun 19, 2026 at 1:58 PM groepaz <groepaz_at_gmx.net> wrote:
> > Am Freitag, 19. Juni 2026, 09:34:14 Mitteleuropäische Sommerzeit schrieb
> > 
> > Francesco Messineo:
> > > On Fri, Jun 19, 2026 at 12:37 AM Maciej Witkowiak <ytm_at_elysium.pl> 
wrote:
> > > > 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.
> > > 
> > > and how does it configure the clock divider to allow 21 sectors on the
> > > smaller tracks?
> > 
> > The density config is independent from the track. The problem here is that
> > the faster/more dense setting becomes less reliable on the inner tracks
> > (which is why it exists in the first place)
> 
> let me change slightly the question: how does the firmware pack 21
> sectors on the smaller tracks without running out of space?
> With standard shugart/IBM controllers (so fixed clock rate independent
> from the track number), they are forced to use the lowest amount of
> sectors that fit in the smallest track in less than one rotation
> period.

Yes. And it is the same for every track.

> So I'd expect on a 1541 format that 21 sectors do overrun the track
> length well before reaching track 35.
> If you have a flux image of such a formatted floppy, I'd like to see it.

No idea what you are asking. Writing a track works exactly the same, no matter 
if Track 1 or Track 35. If you can write 21 sectors to one track, using a 
certain density, you can do the same on every other track.



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