Re: format difference between a 4040 and a 1541

As far as I remember it is this.

The dual CPU drives write 8 byte of real gap data bytes, which get 
translated into 10 GCR bytes.

When they converted to the single CPU drives, they forgot that the old 
drives converted between real and GCR in hardware. So they only wrote 8 
bytes too, but which was then GCR bytes. I believe the early 1540/1541 DOS 
images should have that value. They then later moved to 10 and back to 9 IIRC.

As for the gap between end of sector data and beginning of new header sync, 
IIRC this was calculated at format time where the drive would basically 
measure the length of the sector in bytes (which depends on the speed of 
the drive as the bit frequency is fixed) and try to distribute the sectors 
evenly.

To move that out was one of the optimizations I did in my (long forgotten) 
fast format routine...

André



Am 28. September 2025 13:19:31 schrieb ruud_at_baltissen.org:

> Hallo Francesco,
>
>
> I'm afraid I don't know the details anymore. What I can tell is that
> there is a difference in gap length and IIRC the one of the 4040 is one
> byte longer. If you want more details, have a look at "Inside Commodore
> DOS", https://archive.org/details/Inside_Commodore_Dos_1984_Datamost_a .
> That's how I learned things.
>
>
> --
>
> Kind regards / Met vriendelijke groet, Ruud Baltissen
>
> www.Baltissen.org