Running earlier XXDP tests revealed that a technique of concurrent command
initiation and continued housekeeping for the command completion was used in
the old code.
For example, code could initiate a SEEK command for a drive, and knowing that
CS_DONE (and thus, an interrupt) is coming in about 16us, it would then go
ahead and clear a flag, which registers that the interrupt has occurred
(expected to be set to 1 by the ISR). If CS_DONE is set by the implementation
at the function initiation immediately, that would mean that the interrupt
could be triggered before the next instruction, and the flag would be set by
the ISR right away. The main code, however, would proceed with the the flag
clear as the following instruction, thus, never detecting the interrupt down
the road.
Since this technique was in existence, it is better to introduce a delay for
setting CS_DONE in the "fast" initiation commands like SEEK and HOME, to
accommodate the software that was relying on it.
So far, however, no issues were encountered in testing (except one), where
this delay mattered, but it's hard to tell if it would not be needed at all.
All I/O commands always delay CS_DONE already because they were never supposed
to be immediate.
Since the time for CS_DONE in initiation commands was documented at 16us, the
introduced delay is set to 10 instructions, which usually took more than that
to execute. But the interrupt flag clear case would be covered, as well as
the counted waits, which used some 25+-iteration tight loops for "drive ready",
before flagging a time-out (so the delay cannot be longer, either).
It also looks like more modern code never used any such tricks, so for it, it
should not matter if CS_DONE was slightly delayed or not.
This is the codebase of SIMH, a framework and collection of computer system simulators.
SIMH was created by Bob Supnik, originally at Digital Equipment Corporation, and extended by contributions of many other people. It is now an open source project, licensed under an MIT open source license (see LICENSE.txt for the specific wording). The project gatekeepers are the members of the SIMH Steering Group. We welcome and encourage contributions from all. Contributions will be covered by the project license.
The Open SIMH code base was taken from a code base maintained by Mark Pizzolato as of 12 May 2022. From that point onward there is no connection between that source and the Open SIMH code base. A detailed listing of features as of that point may be found in SIMH-V4-status.
PLEASE NOTE
Do not contribute material taken from github.com/simh/simh unless you are the author of the material in question.
eeebbed273