On 2013-01-06 03:07, Brian Schenkenberger, VAXman- wrote: Johnny Billquist <bqt at softjar.se> writes: {...snip...} But it seems very likely that it's the byte write that is the problem. That becomes a read-modify-write cycle on the bus, since everything on the bus is accessed as words. And accessing the DUV-11 on a PDT-11 is extremely slow. Just guessing, mind you... But wouldn't all benefit from that? Why conditionalize it? 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. 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