Brief Introduction to Alpha Systems and Processors: 21064 performance vs 21066 performance

4. 21064 performance vs 21066 performance

The 21064 and the 21066 have the same (EV4) CPU core. If the same program is run on a 21064 and a 21066, at the same CPU speed, then the difference in performance comes only as a result of system Bcache/memory bandwidth. Any code thread that has a high hit-rate on the internal caches will perform the same. There are 2 big performance killers:

Code that is write-intensive. Even though the 21064 and the 21066 have write buffers to swallow some of the delays, code that is write-intensive will be throttled by write bandwidth at the system bus. This arises because the on-chip caches are write-through.
Code that wants to treat floats as integers. The Alpha architecture does not allow register-register transfers from integer registers to floating point registers. Such a conversion has to be done via memory (And therefore, because the on-chip caches are write-through, via the Bcache). (Editor's note: it seems that both the EV4 and EV45 can perform the conversion through the primary data cache (Dcache), provided that the memory is cached already. In such a case, the store in the conversion sequence will update the Dcache and the subsequent load is, under certain circumstances, able to read the updated d-cache value, thus avoiding a costly roundtrip to the Bcache. In particular, it seems best to execute the stq/ldt or stt/ldq instructions back-to-back, which is somewhat counter-intuitive.)

If you make the same comparison between a 21064A and a 21066A, there is an additional factor due to the different Icache and Dcache sizes between the two chips.

Now, the 21164 solves both these problems: it achieve much higher system bus bandwidths (despite having the same number of signal pins - yes, I know it's got about twice as many pins as a 21064, but all those extra ones are power and ground! (yes, really!!)) and it has write-back caches. The only remaining problem is the answer to the question "how much does it cost?"

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