From: Paul Koning
But the sector format is a different matter. If
it's designed for
PDP-11 and friends, presumably it has a 512 byte sector size. For
PDP-10 or -20 use you'd presumably want a sector size consisting of
some round number of 36 bit words.
Actually, the -10/-20 MASSBUS situation is even more complicated than that.
The MASSBUS can operate in 16 or 18 bit data width (for everyone else; this
is totally different from the Q16/Q18/Q22 of the QBUS, which is _address_
width), so it can support 36-bit words directly, using two extra data lines.
So for the RP04 and other disks, and their 'controllers' (at least, the part
that's in the device), they have to be able to turn the bit-stream from the
mass storage device into 18-bit wide words. (And they actually have different
sector formats depending on whether they are in 16- or 18-bit mode.)
What the KM11 does, I don't know (I'm too lazy to go look at the TM); I would
not at all be suprised to find that it can _only_ operate in 16-bit mode
(i.e. the array of memory chips is 16 bits wide, and it just ships a line at a
time from that out in parallel, so there's no way to even produce 18-bit wide
words). The name of the device (KM11) adds weight to that supposition.