Re: aliased & packed?

[kilgallen@eisner.decus.org (Larry Kilgallen)]

In article <EA4nII.803@world.std.com>, bobduff@world.std.com (Robert A Duff) writes:

> Is this really true?  I mean, if you access one byte at the start of a
> word (using byte instructions on the 21164), then won't the next seven
> be sitting there in the cache waiting for you?

Good point.  I don't know what they have in there, but probably they
have that.  Of course registers are still faster than cache.

> And it seems like more than a couple of instructions to shift and mask
> an 8-byte word (I mean, using 21064 instructions only), in the case
> where the byte you want is dynamic (i.e. you're indexing randomly into
> an array of bytes -- on the other hand, most array accesses are probably
> sequential).

From what I have seen (several years ago), the register-to-register
instructions on Alpha look quite weird but many of them have clever
uses in such operations.  The instruction set was designed with heavy
input from compiler writers.

Larry Kilgallen