25 Feb
2021
25 Feb
'21
10:38 a.m.
On Feb 25, 2021, at 1:30 PM, Noel Chiappa via cctalk
<cctalk at classiccmp.org> wrote:
Yes, I meant the RH70. The point is that, without Massbus, the only I/O bus you have in
PDP-11 systems is the Unibus (pre-Qbus that is) with all its limitations. The Massbus
enabled the creation of the RH70 with its direct memory connection both for more bandwidth
and more address bits. So, for example, you can use an RM03 rather than being limited to
an RM02 because of the bus performance. And your disks and tapes don't have to fight
all the other I/O devices for unibus map slots.
paul
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.