[Simh] PDP10 RP06 parameters

[Bob Supnik]

Your original calculation is correct. There are 815 cylinders, 19 heads, 
20 sectors, and in the simulator, the sectors are 128 64b words (or 1024 
bytes).

The the reason for the discrepancy is simple: the last five tracks 
haven't been written by software. SimH data files are only as long as 
the data written; the rest is assumed to be zero.

On the PDP11, disk files for older disks usually get extended to full 
length by writing a "bad block" table at the end as part of 
initialization. The PDP10 didn't follow this practice. It used 
differently formatted disk packs, to cope with the 36b/32b word size 
difference.

/Bob