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Politics : Formerly About Advanced Micro Devices
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To: Scumbria who wrote (98716)3/16/2000 3:03:00 PM
From: Hans de Vries  Read Replies (2) of 1571055
 
Scumbria: RE: It is possible that 1/2 cycle ALU is out of balance with the rest of the pipeline, but I doubt it. However, if the ALU is the critical path, it will indeed be the limiting factor for MHz.

Clock skew is not an additional issue with the ALU, because it runs off the same clock as the rest of the design. It simply completes it's operation in one phase of the clock.

The Willamette has more and shorter pipeline stages as Coppermine which could do the slowest ALU operations in 1 cycle. This is not possible anymore. The slowest ALU operation takes about 1.5 cycle (not 0.5) The latter would mean that the ALU would be 3 times faster with the same process... Which is not the case.

The Willemette is the first processor where the pipeline stages are so short that the ALU can not perform it's task anymore within a single cycle.

One of the biggest bottlenecks in an x86 processor is the ALU in case of instructions which depend on the result of the previous operation like C=A+B; E=C+D; G=E+F; et cetera.

A 1 Ghz Coppermine can execute these instructions at 1 GHz.
A 1.5 GHz Willemette without 1/2 clock tricks would execute these instructions at 750 MHz (feedback after 2 cycles)
A 1.5 GHz Willemette which can feedback the result after 1.5 cycle can however reach 1 GHz for these instructions and uses the ALU at it's maximum performance.

Any processor which shortens the pipeline stage to 67%..99% of the ALU just has to use half clock tricks if it does not want to lose on it's IPC performance.

Regards Hans.
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