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Disk track images (aka PC99 format)


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MESS and TIImageTool have been supporting this PC99 format for some time now, but as I learned there is a little glitch in there, concerning the single-sided formats, and while I'm working on the floppy modernization, I guess this is a good time to fix that.


Is it correct that PC99 stores single-sided formats into the same image files as double-sided ones, just leaving the second half uninitialized?


This would mean we have only two valid sizes:


- FM recording (single density): 260240 bytes

- MFM recording (double density): 549760 bytes


each one for both single sided and double sided images.


Are there any other known formats for track images? (80 tracks? 36 sectors?)

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I've got a reply from Ernest Pergrem via PM, confirming that there are only two PC99 formats around (single density and double density), each one used for both single and double-sided. I guess this should be found in the PC99 manual as well.

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OK, on page 70 the manual says that there are only two sizes: 260240 or 549760, which depends on the emulated controller (1771 or 1772) that is used in PC99; single-sided formats use the same file size as the corresponding double-sided format. I suppose only the first half of the file is used, and the second half is either zeros or simply ignored; I did not see anything about that in the text.

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The bytes between the sectors.


No, this would account for the difference between the track size of 3253 bytes for single density compared to 9*256 = 2304 bytes. I'd say the extra 90K that acadiel is wondering about are simply unused, similar to a floppy disk that is recorded on one side only.


By the way, I found that the tracks of the PC99 format (which is used for the TDF in MESS) are somewhat too long. This became apparent when I was working to adapt the TI floppy emulation in MESS to the new core floppy system and tried to put one such track into an emulated floppy track.


Simple calculation: We have 300 rpm, which means that reading a complete track takes 0.2 seconds. The cell sizes for FM recording are 4 µs, so we have 50000 cells on the track, and it takes 16 cells to encode 8 bits, so this means we get 3125 bytes at max per track. For MFM the cell sizes are 2 µs, which means 6250 bytes. PC99 defined 3253 bytes (FM) and 6872 bytes (MFM).


My guess is that the track sizes are a result of reading a track with the "read track" function of the disk controller, which is known to deliver trailing bytes for some time after the track is finished (until it "interrupts out" (spec of WD177x)).

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