Re: Commodore 15 Second disk format routine disassembled

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.
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.

IZ8DWF