[HECnet] DU11 vs. DUV11

[Johnny Billquist]

On 2013-01-06 03:40, Johnny Billquist wrote:
On 2013-01-06 03:17, Brian Schenkenberger, VAXman- wrote:
Johnny Billquist <bqt at softjar.se> writes:

{...snip...}
Well, this code runs on any Qbus, so all Qbus machines "benifit" from
the code, not just the PDT-11. It's proably that on the Unibus, the
speed penalty for a read-modify-write is less than reading a byte and
pushing it on the stack, and then popping the stack and writing the word
to the bus. Also, the DU-11 might respond much faster than the DUV-11.
The buses are asynchronous, so the time for the execution is totally
dependent on the speed of the device to respond to the bus cycles.

OK.   That wasn't clear from the choice of the symbol in the conditional.

I know that the selection whether to call the driver DU or DUV is based
on the L$$SI1 symbol. Sorry I probably didn't point that out enough.
That tells me that all Qbus machines get that variant. (The choice of
symbol name is probably just simply because the LSI-11 was the first
Qbus machine.)

I assume the PDT-11 mentioned is the one with a Qbus, so it's the same
controller as all other Qbus machines. But that part is really a guess.
It might be that I'm wrong on that, and that the PDT-11 have something
else, which looks and works just the same way as any other Qbus machine,
but DEC didn't have any convenient way of setting up a specific
variation for the PDT-11, so they took the small hit on Qbus machines in
order to save the PDT-11, but could avoid even the small hit on Unibus
machines, since the PDT-11 isn't even in the picture there.

And based on John's response, it seems as if it actually is this latter guess that is right.

The cost on any other Qbus machine is pretty minimal, even if it is slower to go through the stack.

	Johnny

-- 
Johnny Billquist                                   || "I'm on a bus
                                                                  ||   on a psychedelic trip
email: bqt at softjar.se                         ||   Reading murder books
pdp is alive!                                         ||   tryin' to stay hip" - B. Idol