Re: format difference between a 4040 and a 1541

Curious what the 1540 Vic disk drive would be
B

On Sat, Sep 27, 2025 at 3:01 PM Francesco Messineo <
francesco.messineo_at_gmail.com> wrote:

> Hi all,
> I am playing a bit with greaseweazle dumps (in raw format, scp file)
> then convert one track to "text bytes" with g64conv.
> For now I'm looking at track 18 on floppies formatted by the two drives.
>
> Headers have almost the same length (including the end gap):
>
> 4040:
> ; header
>    gcr 08
>    begin-checksum
>       checksum 12
>       ; sector
>       gcr 00
>       ; track
>       gcr 12
>       ; id2
>       gcr 30
>       ; id1
>       gcr 30
>    end-checksum
>    gcr 00
>    gcr 00
>    ; Trk 18 Sec 0
>    bytes 52 94 a5 29 4a 52 94 a5 10 de ff
>    bits 1
>
> notice 10 (raw I assume, this conversion program is quite in "beta"
> revision) bytes as gap. It seems the 4040 writes "00" bytes as gap and
> they result in that gcr 10 bytes sequence, then the data sync starts
> (ff...)
>
> 1541:
>  ; header
>    gcr 08
>    begin-checksum
>       checksum 16
>       ; sector
>       gcr 00
>       ; track
>       gcr 12
>       ; id2
>       gcr 46
>       ; id1
>       gcr 42
>    end-checksum
>    gcr 0f
>    gcr 0f
>    ; Trk 18 Sec 0
>    bytes 55 55 55 55 55 55 55 55 52 bf
>    bits 111
>
> seems like 9+1/2 raw bytes of gap (55, 52, bf) might be a flux
> decoding artifact.
> However the 10 bytes on 4040 and 9 on 1541 are very consistent on all
> header's gap
>
> However, gap after the data part of a sector has a very different
> length, here's the end of 18,0 data of both disks:
>
> 4040:
>       checksum 74
>    end-checksum
>    gcr 74
>    gcr 12
>    bytes b7 97 a5 29 4a 52 94 a5 29 4a 52 94 a5 29 4a 52 94 af
>    bits 11111
>
>
> 1541:
>       checksum 9c
>    end-checksum
>    gcr 00
>    bits 0101001100
>    bytes aa aa aa aa aa aa aa aa aa aa aa aa ab
>    bits 1111111
>
> seems like 19 raw byes of padding before the next sync on 4040 and 15
> (not sure how to count on this dump anyway) on 1541.
> If I just dump the raw data without the gcr decoding, the data part is
> larger by  4 bytes on the 4040.
> I assume the difference isn't really preventing "cross writing"
> floppies on the two drives, but I'd like if someone could clarify why
> the data padding difference seems so large between the two drives.
>
> Thanks in advance
> Frank IZ8DWF
>
>