[TUHS] /dev/drum

Noel Chiappa jnc at mercury.lcs.mit.edu
Thu Apr 26 08:46:46 AEST 2018

    > From: Johnny Billquist

    > I don't know exactly why DEC left the last three tracks unused. Might
    > have been for diagnostic tools to have a scratch area to play with.
    > Might have been that those tracks were found to be less reliable. Or
    > maybe something completely different. But it was not for bad block
    > replacement, as DEC didn't even do that on RK05

The "pdp11 peripherals handbook" (1975 edition at least, I can't be bothered
to check them all) says, for the RK11:

  "Tracks/surface: 200+3 spare"

and for the RP11:

  "Tracks/surface: 400 (plus 6 spares)"

which sounds like it could be for bad block replacement, but the RP11-C
Maintenance Manual says (pg. 3-10) "the inner-most cylinders 400-405 are only
used for maintenance".

Unix blithely ignored all that, and used every block available on both the
RK11 and RP11.


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