Yokemate of Keyboards
Posts: 11445 from 2003/5/22
> CPUs compliant to the same ISA don't understand the same instructions?
Exactly, the Power ISA revision usually tells nothing about the supported instructions of the microarchitecture/core declared compliant with the respective Power ISA revision. With each new Power ISA revision, new optional features/instructions were added (exception: Power ISA v3.0 which killed entire Book III-E and Book VLE). Every new Power core was simply declared compliant with the ISA revision that was recent at time of release, no matter if it supported the new optional instructions or not. Usually, the big IBM POWER processors were the ones that supported the new instructions and that the new ISA revision was released for in the first place. This way of assigning ISA revisions to cores has been legit as new instructions have always been optional. That's why a core declared compliant with a certain ISA revision is automatically compliant with all follow-up revisions (exception again: Power ISA v3.0).
This way, the very same ISA revisions are shared by
- cores with FPU and without
- cores with MMU and without
- Book III-S cores and Book III-E cores
- Book III-E cores with SPE and Book III-E cores without
- cores with VLE and cores without
- VLE cores with Book I-III instructions and VLE cores without
- cores with SIMD and cores without
- SIMD cores with VSX and SIMD cores without
- cores with AltiVec/VMX in both endians and cores with AltiVec/VMX only in big-endian
- 32 bit-only cores, 64 bit-only cores and 32/64-bit cores
- cores with decimal FP and cores without
- cores with transactional memory and cores without
These are just the most obvious instruction set differences. And to give you the most extreme example, both the e200z0 and the POWER5+ implement Power ISA v2.03, yet they do not have even one single instruction in common.
> Seems a bit more complicated
than I thought
This graph is quite okay, but it has a major flaw: it implies that Power ISA v3.0 and above still contain Book III-E and Book VLE, which they do not. (As minor flaws, it falsely implies that Power ISA v3.0 and above have been released by Power.org, and it's missing Power ISA v3.0C.)