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Instruction timings for the VAX
- Thread starter vol.litwr
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AK6DN
Veteran Member
The complexity of the instruction set.
PDP-8 has a very simple instruction set and it is very easy to determine the exact instruction cycle count per instruction.
Similar for PDP-11. It has a bit more complexity in the various address modes, but they straightforward to analyze with respect to timing and memory cycles.
The VAX instruction set, one of the ultimate CISC implementations, has so many address modes and options per instruction that make it a very difficult analysis.
And ultimately one must ask what is the point?
PDP-8 has a very simple instruction set and it is very easy to determine the exact instruction cycle count per instruction.
Similar for PDP-11. It has a bit more complexity in the various address modes, but they straightforward to analyze with respect to timing and memory cycles.
The VAX instruction set, one of the ultimate CISC implementations, has so many address modes and options per instruction that make it a very difficult analysis.
And ultimately one must ask what is the point?
Ian Hammond
Member
Coming from a real-time environment I recall asking that question back in the 1980s, and while I agree with the complexity argument, there could have been at least some baseline measurements for the simpler addressing mode cases.
I think we can be certain that DEC did measure performance, and that if the 780 had been significantly faster than its competitors then I seriously doubt that the marketing people wouldn't have advertised that.
IIRC the early versions of Unix for the VAX first used the CALL instruction to invoke functions, but then dumped it in favor of JSB because of the performance overhead that CALL entailed.
They certainly wouldn't have wanted to publish performance data for the 730. I had one and it was no match for an 11/73.
I think we can be certain that DEC did measure performance, and that if the 780 had been significantly faster than its competitors then I seriously doubt that the marketing people wouldn't have advertised that.
IIRC the early versions of Unix for the VAX first used the CALL instruction to invoke functions, but then dumped it in favor of JSB because of the performance overhead that CALL entailed.
They certainly wouldn't have wanted to publish performance data for the 730. I had one and it was no match for an 11/73.
Chuck(G)
25k Member
It was no match for an IBM PC. I was offered one for free and turned it down. Too much trouble for too little return.