At 10:17 PM +0100 06/08/2005, Neil Hughes wrote:
simple explanation of what the difference between a "proper" 64-bit
CPU (G5 I presume) and chips like the P4/Xeon with "64-bit
extensions" is.
Greatly greatly simplified (ignoring parallelisms etc):
A pure 64-bit system is 64-bits everywhere except on the peripheral
buses. IOW, it accesses memory in minimum chunks of 64-bits at a
time, over a memory bus that's at least 64-bits wide, with 64 pins
for data and 64 pins for the address going into / out of the
processor, into 64-bit wide internal processor registers. On each
cycle (clock tick) it moves at least 64 bits of data.
A non-pure system does some things in 16 or 32 bit chunks - and has
to do those operations four or two times to "create" a 64-bit
operation. eg: If the processor's package only has 32 data pins,
then it takes two cycles to read in or write out 64 bits of data.
The more data you can move in one clock tick, the faster things will
be. AND since you can move more data per tick, you can afford to use
a slower clock speed. Slower clock = less heat and less expensive
parts. :)
HTH,
- Dan.
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