[Simh] FW: pdp 11 timing

[Brian Knittel]

> ... there is plenty of software that assumes that an interrupt does not happen before a single instruction have been executed after the previous interrupt, from the same device, for example.

On the ibm1130 (a different machine of course) we found a case where a driver expected to execute quite a few instructions ... many hundreds ... before an interrupt could occur in real hardware, while zeroing or copying a buffer for instance. So it might be good to do what simh does and make the delay between initiating an IO operation and signaling its completion an adjustable parameter. Start with a realistically large number and see if reducing it causes failures. I would guess you would have fewer dependencies like this in drivers for hardware whose latency was random (eg disk seek and rotation) and a bigger risk that authors might  have assumed and exploited the delay time on operations that had a fixed cycle time (block to block or sector to sector times, or punched card operations)