RE: format difference between a 4040 and a 1541

From: Bill Degnan <billdegnan_at_gmail.com>
Sent: Saturday, September 27, 2025 12:04 PM
To: cbm-hackers_at_musoftware.de
Subject: 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<mailto: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