A user observed that the TS11 would not run XXDP+, even though it ran
fine with the PDP11 operating systems, VMS, and XXDP V2. I traced this
back to a conceptual error in the implementation of some magtapes,
specifically the TS11, RH11/TM02-3, and the PDP10 TU45.
The issues is that beginning of tape, and being positioned in front of
the first record, are not necessarily the same. Following BOT, tape
drives record a ID burst If high density and an inter-record gap before
the first record. When the first record is read backwards or backspaced
over, the tape ends up at position 0 but should not show BOT. Most
simulated tape drives did this correctly, but a few used sim_tape_bot()
as a shortcut for BOT, and it's simply not correct.
BOT should be set at ATTACH, by a successful rewind, and by any reverse
operation when the tape is positioned in front of the first record.
BOT should be cleared by any successful movement operation, except
rewind.
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