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Aucbvax.6888 fa.info-vax utzoo!decvax!ucbvax!info-vax Fri Apr 23 07:13:13 1982 instruction timings >From Bruce.C.Wright@Duke.University Fri Apr 23 07:03:35 1982 We tried to get DEC to give us a good idea what timings to expect when we got our VAX-11/780 in the Medical Center here. What we got was the line that the precise numbers were company confidential [!], but they did let us look at (but not copy) a set of documentation which gave some samples. There *are* some problems with the simpleminded methods of calculating speed which work on the lower-end PDP-11's (like a specific factor for each memory reference which works pretty well for 11/20 through 11/45 but breaks down on the 11/70 because of cache), because there is a lot of cache, pipelining, and so forth going on. The figures did have some interesting information, of which what I remember is something as follows:

o For doing heavy floating-point number crunching, the 780 should be run with the floating point accelerator if at all possible. As I remember the floating point times were comparable to a PDP-11/34 [!!] without the unit (i. e., about 10-40 us) and a bit faster than the PDP-11/70 with the unit (0.5-1.5 us).

o For purely number-crunching work (if you don't need decimal or character instructions and if you don't need a lot of memory for efficient execution), the 780 is perhaps slightly faster than the PDP-11/70. This does not count the fact that large programs will be easier to deal with and will run faster on the 780 because overlays or forks will no longer be needed, nor does it count the fact that the 780 has a higher i/o bandwidth.

o DEC claims that the 780's processing power is roughly that of the IBM-370/155. This seems to be a reasonable guess from what I've seen.

This was before the 750 (or 730). Benchmarks which we have run on the 780 and the PDP-11/70 seem to confirm this: for pure number crunching (assuming the FPA on the VAX and the FPP on the PDP-11/70), the two machines are roughly equivalent (within the restrictions given above). I don't have any of the numbers in front of me at the moment, but they are within about 5-15% of each other, sometimes one is faster and sometimes the other (perhaps the vax is usually faster, but you know how hard it is to get reliable benchmark results).

The 11/750 *is* faster for some types of instructions, e. g., if you have the G/H option for the 750 this obviously will be faster than software emulation for the 780. It will be interesting to see if this holds when the G/H microcode option for the 780 comes out later this spring or early summer. The 750 is also faster on shift instructions (ASHL, ROTL) and perhaps some other instructions because of the superrotator in the microengine, but I don't know what instructions use its full possibilities (not having dealt with the 750).

I know very little about the 730 except that it exists and is "somewhat slower than a 750." I assume that similar differences will exist between the 730 and the other members of the family as between the 750 and the 780 -- i. e., some instructions will take disproportionate amounts of time on that machine compared to others in the family.

Bruce C. Wright @ Duke University

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