30 May
2018
30 May
'18
3:32 a.m.
Long ago, at Caltech, I ran a VAX which had a mix of DZ11s and
Able DH/DMs. The latter were indeed much more pleasant, both
because of their DMA output and their fuller modem control.
For the DZ11s we used a scheme that originated somewhere in
the USG-UNIX world: output was handled through a KMC11.
Output interrupts were disablled on the DZ; a program
running in the KMC fetched output data with DMA, then
spoon-fed it into the DZ, polling the status register
to see when it was OK to send more, then sending an
interrupt to the host when the entire data block had
been sent.
The KMC, for those fortunate enough not to have programmed
it, has a very simple processor sort of on the level of
microcode. It has a few specialized registers and a
simple instruction set which I don't think had all four
of the usual arithmetic ops. I had to debug the KMC
program so I had to learn about it.
When I moved to Bell Labs a few years later, we didn't
need the KMC to drive serial lines--Dennis's stream I/O
code was able to do that smoothly enough, even with DZ11s
on a VAX-11/750 (and without any assembly-language help
either!). But my experience with the KMC was useful
anyway: we used them as protocol-offload processors for
Datakit, and that code needed occasional debugging and
tweaking too. Bill Marshall had looked after that stuff
in the past, but was happy to have someone else who
wasn't scared of it.
Norman Wilson
Toronto ON