pdp11/84 PMI memory: What is the problem with Q bus?

Chris Zach cz at alembic.crystel.com
Thu Apr 23 20:35:17 CDT 2020

Well I turned on block mode DMA on my MTI controller, booted RT11 5.5, 
did an init on VM0: and tried to copy a .dsk file of 2500 blocks in size 
with 2mb of PMI memory from an 11/84 (CA, bad memory for Q Bus).

First time was not good:
.copy dungeo.dsk vm:
  Files copied:
?PIP-F-Input error DK:DUNGEO.DSK

.dir dungeo.dsk

  1 Files, 2500 Blocks
  34488 Free blocks

Second time was better, took ~4 seconds.

.copy dungeo.dsk vm:
  Files copied:

Switched off block mode and tried again:

Takes 5 seconds.

Changed burst length from 16 words to 2:
Took 25 seconds.
4: 23 seconds
16: 5 seconds

Turning off the throttle mode (which requires Block mode and 16 word 
transfers) and it took 4 seconds. No speed increase.

Then I decided to try using 2mb of crummy Q bus memory:
16: 5 seconds
8: 15 seconds
2: 26 seconds
Didn't do 4.

So it looks like what really speeds things up is having a large burst of 
Q busy transfers, going from 2 word transfers to 16 speeds things up by 
a factor of five. Makes sense, controller is spending less time setting 
up the transfer. Somewhat interesting that PMI doesn't really help, I 
understand that the controller is talking to PMI at normal Q bus speeds, 
but I would expect something for the CPU to manage the VM: driver. 
Unless the CPU just says "Oh, VM:? Point your data right into memory and 
be done with it".

Moral: PMI memory does not really speed up disk to memory performance. 
Good to know. Anyone know a good benchmark to see if PMI is worth 
anything in terms of performance? Maybe the unibus map interfacing 
directly to PMI speeds things up a lot by allowing full hog mode DMA 
(which in theory could mean that an 11/84 could run a RH11-C controller 
and keep up with disks like the RM02 and RM05)

Thinking along those lines, was the 11/70's MASSBUS channels nothing 
more than RH11-C's attached to the old FASTBUS on the 11/45 cpu core 
(which is what an 11/70 really is, with cache) or did they port right to 
the memory box?


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