Re: Question about writing to 1541

From: Patrycjusz R. Łogiewa (
Date: 2005-05-06 00:35:55

On 2005-05-05, at 19:59, Spiro Trikaliotis wrote:

> The formatter tries to put the sectors very balanced around the track.
> For the worst speed zones, we have an inter-sector-gap of at least 6

According to "incomdos" p. 117 "If a tail gap is not computed to be at 
least four bytes in length formatting will fail and an error will be 

> bytes (from experience). I have been told (I did not check myself) that
> approx. 5 bytes per round is the normal jitter of belt-driven 1541
> drives.
> Thus, per sector, the jitter is lower than 1 byte. Thus, we have 4 
> extra
> security bytes between each sector. Thus, I doubt your argument.

So do I.

> OTOH, the 4040 used a gap between the data block header and the data
> block itself of 10 bytes. The 2031 and 1540 (and 1541 up to $E000-ROM
> version -02) used 8 bytes, while the 1541 (-03, -05 and later) and 157x
> use 9 byte. I believe 8 bytes are critical if a disk is to be
> interchanged between a 154x and a 4040, as there might be "double SYNC
> marks", making the data block unreadable. With 9 byte, it is much more
> unlikely that this happens than with 8 byte.

But...  "... note that a header block is never rewritten after 
formatting is complete. The data block of a sector, including the sync 
character, is completely rewritten every time data is written to that 
sector. The [...] header gap is counted off by the DOS to determine 
where to start writing the data block."

> Thus, Commodore might have had some problem and tried to solve this by
> reducing the number of written bytes.

At the off-bytes then? Note also that there is enough buffer at the end 
of the track. The tail gap of the last sector is usually much longer 
than any other (up to 100+ bytes), thus if (due to one byte extended 
header gap) they had problems fitting the written sector in the space 
provided (which I still doubt) then the easiest way would be to 
increase the limit of the regular tail-gap rather than fiddle with the 
sector-write routine... at least IMHO, that is... OTOH as we all more 
or less speculate on this then who really knows?

Great spirits have always encountered violent opposition from mediocre 
minds.    -- Albert Einstein

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