25 Feb
2021
25 Feb
'21
10:30 a.m.
From: Paul Koning
There's a good reason why the big disks on many
DEC machines were Massbus
devices until MSCP arrived. It's quite clear on Unibus PDP-11s, which
needed Massbus both for speed and for a cleaner answer to more-than-18
bit addressing.
I follow the first sentence, but I'm confused by the second, especially "a
cleaner answer to more-than-18 bit addressing". The UNIBUS MASSBUS
controller/adapter, the RH11, only has 18-bit addressing on the main memory
side. It does have more than 18-bit addressing on the device side, but so does
the RP11 (sort of). Are you thinking of the RH70? That does have access to
more than 2^18 bytes of main memory, but that's because it connects to the
-11/70 memory bus (as well as the UNIBUS, which is only used for control, not
data).
Similar questions about the speed point; passing data through an RH11 doesn't
increase the speed of the UNIBUS? Yes, the RH70 is faster, but that's because
of its connection to the -11/70 memory bus.
Noel