[rc5] Pentium vs. Pentium Pro (renamed)

Sanford Olson sanford at msn.fullfeed.com
Thu Jul 3 00:55:05 EDT 1997


At 10:21 PM 7/2/97 -0400, you wrote:
>Oscar Chang drunkenly mumbled...
>> 
>>   From what I understand, the PPro is faster, but nothing changed in the
>> way it pipelines data and instructions, basically it's faster because of
>> it's bigger cache and higher running speed. No real need to have a
>> seperate binary for that.
>
>my limited understanding of the whole PC thing aside, this is what i know.
>
>PPros are faster because:
>
>the internal (L1) cache is larger)
>the external (L2) cache runs at a much higher speed since it is on the chip
>it does not deal well with 16-bit code as it is a "true" 32-bit CPU.
>the higher speed.
>
>i think that is all.
>
>-brian
>
>-- 
>** Brian Hechinger ** wonko at mail.arkham.net ** http://www.arkham.net **

I'm not an expert either, but I believe that the Pentium and Pentium Pros
are quite different internally, too.  The Pentium has dual integer
instruction pipelines (U & V) plus a FPU.  Something like 70% of the time a
Pentium executes two instructions at once.  It also has some limited branch
prediction logic.  The Pentium Pro actually has RISC-type execution units
(I think 3 of them).  The X86 instructions are translated into RISC-type
sub-instructions which can be executed out of order, with register renaming
and better branch prediction.  The problem is that the segment registers
are not aliased and renamed, so that changing a segment register (which
happens in 16-bit code constantly) causes practically the entire CPU
context to be dumped and reloaded.  The Pentium II chips handle segment
register changes better and don't suffer as much with poor 16-bit
performance.  Of course, 512K of sub-10ns L2 cache on a Pentium Pro doesn't
hurt performance either. :)  So, a .EXE that was optimized for a Pentium
Pro would help performance (many compilers have a P6 optimizing setting).
I don't know anything about RC5 algorithms to even have a clue about
whether it would make a noticeable difference.  Generally, a P6-optimized
.EXE runs quite well on a regular Pentium, so you wouldn't need separate
.EXE's.

- Sanford Olson

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