Question about HP-9114 disc drive


I am just finishing HP-IL emulation for my HP-71X MicroCode Emulator for HP-48GX and HP-49G (some details are on and now I have a question about HP-9114 disc drive:

What is the real capacity of this device? From the manual, it has 2 surfaces, 80 tracks per surface, 16 sectors per track and 256 bytes per sector. This gives 2*80*16*256=655360 bytes as the total capacity. But LIF files extracted from the HP-9114 drive are 630784 bytes in size. What happened with the missing 24576 bytes ?!?

Best regards.


HP says in the 9114B manual (page B-1 in the manual on the Museum CD-ROMs #3) that they reserve some tracks for spares - actually it says all HP computers do. The stated capacity works out to be 77 cylinders so I guess they reserve the remaining 3 cylinders (6 tracks) for spares.

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