64-bit CPU

[Lennart Sorensen]

On Thu, Sep 16, 2004 at 11:30:08AM -0400, Andrej Marjan wrote:
> Hmm.. interesting. I had always assumed that MMX allowed you to do 64 
> bit integer arithmetic directly, but based on a bit of googling, I can' 
> find any arithmetic support for "quadwords", only for "dwords". SSE 
> doesn't add this support either. Does SSE2?

As far as I remember, MMX was mostly about doing the same operation (one
instruction) on a chunk of data at a time, so doing a bitshift on each 8
bit pieece in a 64bit register, or each 16bit piece in a 64bit register,
or even 32bit in a 64bit register.  so that you could process more than
one audio sample or pixel in one instruction.  I do not believe it did
anything to improve 64bit integer operations, which I am not actually
sure if are available in hardware on x86.  I would be surprised if the
386 had added those, and I don't know how many new instructions have
really been added (outside MMX, SSE, etc) since.  Of course an
application using MMX can't use floating point.  It has to switch back
and forth between the mode to do that.  Probably no big deal since they
are probably mostly used in different applications.

> I haven't thought about it much, but just off the cuff, it seems to me 
> that given equivalent OOO infrastructure supporting an ISA with few 
> architectural registers and an ISA with many architectural registers, 
> the latter should still perform better, even though the former performs 
> better than a register starved ISA without the OOO bits would.
> 
> That's more or less what you get with AMD64: there are still rename 
> registers and the works in 64 bit mode, but there are also 8 more GPRs, 
> meaning compilers can be less hackish, the cpu needs to do less work 
> rewriting all those load/store requests, etc. So the cpu can waste less 
> effort working around the ISA.

Well x86-64 does add quite a few new instructions when in 64bit mode.
Being a new mode it also makes sense to add more registers (why SSE and
such didn't do that while they were at it, I can't imagine.)

Lennart Sorensen
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