Difference between PA-RISC and Alpha Architecture

[Manjunath Shastry]

I would like to know the difference between
PA-RISC and Alpha architectures.

Regards,
Manjunath.

[Rodney Hills]

They are both RISC (reduced instruction set computer). But thats where it ends.

PA-RISC is used by HP

ALPHA is used by digital (who got bought by Compaq, who got bought by HP).

ALPHA's claim to fame was that digital marketed this platform for their OpenVMS operating system and for Windows NT. Our site purchased some Alpha workstations with Win NT. They were very fast, but we couldn't find supporting drivers for the periperals we were attaching (for CAD/CAM).

HTH

-- Rod Hills

[Paddy_1]

: The PA-RISC instruction set includes a "load byte"
instruction. This implies that the data path between the cache and
registers must include some level of muxes to allow an arbitrary byte within
a word to be loaded into a register. By contrast, Alpha always loads
an entire word and so has a simpler data path. Therefore, since the
PA-RISC will have more gate delays on a critical path, its CPU frequency
will tend to be somewhat slower. However, a program on an Alpha that
wants to access a single byte must execute a second instruction to extract
the relevant byte. This tradeoff is one example of the features in the
Alpha instruction set which permit a faster clock but often require more
instructions to accomplish the same work.

[Paddy_1]

I have to point out that simply the architecture comparsion might not help as implementation details overrride certain stuff and also is varied by the application mix.I would also compare their SPECint/fp benchmarks.
Hope this helps

[John Bolene]

And then my question comes -> why?

These computers are as different as night and day.

The basic similarity is that they are both now owned and produced by HP.

The Alpha and PA-RISC both have a limited lifetime to be replaced by the Itanium.

Parts of the True-64 OS will be incorporated into a version of HPUX 11 which will be released next year.

Very different instruction sets.