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. 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