**From:** Christer Palm (*palm_at_nogui.se*)

**Date:** 2006-01-02 01:51:00

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Hi all! "Inside Commodore DOS" describes the 1541 sector gap calculation algorithm as simply measuring the actual track length, subtracting the number of raw bytes needed to store the sectors, and then divide the available space evenly among the sectors. However, they go on to say that this is not true for the gap between the last sector and sector 0, which is much larger. This obviously contradicts the first (and later) statements about the algoritm. Calculating the "optimal" sector gap assuming 300 rpm gives roughly: Zone 1 Zone 2 Zone 3 Zone 4 Track length: 7692 7142 6666 6250 No of sectors: 21 19 18 17 Data bytes: 7413 6707 6354 6001 Spare bytes: 279 435 312 249 Sector gap: 13 22 17 14 This is obviously much larger than the actual values observed by "Inside Commodore DOS" authors: Zone 1: 4 - 7 bytes Zone 2: 9 - 12 bytes Zone 3: 5 - 8 bytes Zone 4: 4 - 8 bytes Which, of course, supports the statement that the final gap is actually much larger. OTOH, the documentation for the G64 file format [http://ist.uwaterloo.ca/~schepers/formats/G64.TXT] also contains a discussion about the sector gap, arriving at the following table: Track Range Size Tail Gap MNIB (bytes) (even/odd sectors) Size ----------- ------- ------------------ ---- 1-17 7692 9/9 7692 18-24 7139 9/19 7142 25-30 6666 9/13 6666 31- 6247 9/10 6250 Now, first of all, what the hell does even/odd sectors mean? Does it suggest that the gap is actually different for odd/even-numbered sectors, and if so, is that actually so or is it a mistake by the author or a bug in MNIB? Assuming the second value is correct, this suggests that the gap is actually almost twice that of the values given by "Inside Commodore DOS", and more in line with the "optimal" values. This would also mean that the "final" gap would, in fact, not be substantially larger than the others. In other words - total confusion... I tried to "decipher" the 1541 format routine to see what's really going on, but the code is not so easy to follow and its outcome is timing dependant as well making it even harder to understand exactly how it works. Now before I try to dig deeper into that, perhaps someone has already done this already and has information to share? Cheers, -- Christer Message was sent through the cbm-hackers mailing list

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