26 Apr
2018
26 Apr
'18
12:46 a.m.
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.
Noel