PC2700 memory chips - bus speed question

[Big Mac]

I have a newer 2.2 GHz Athalon computer. It came with a 512 MB PC2700
chip. The computer maker told me it had a Bus Speed of
166 MHz. I wanted to have 1 gig of memory, so I bought another 512 MB
PC2700 memory chip. It has a 333 MHz bus speed. The manufacturer
said that was OK., and it is, as I now have 1 gig of memory.

My question is about Bus speed - how important is it as far as
determining the overall processing speed of everything? I understand
the new faster chip is slowed down to the slower chips speed of 166
MHz. Just how much slower are we talking 333 MHz Bus vs 166 MHz bus,
in an overall concept?

 

[RobD]

First, you should know PC2700 = DDR333 = 166.66 MHz memory bus speed. The
bus runs at ~167 MHz clock rate and the memory transfers data twice every
clock cycle (on leading and trailing edges of the clock). That's why it is
called double-data-rate memory. This is only a rating; it is not telling
you how fast the memory *will* run. You should also know that even if you
put in PC3200 memory (DDR400), you would not automatically see the memory
run faster. The FSB speed controls how fast the memory system runs, not the
individual modules. The memory modules could define the various memory
timings like RAS, CAS, RCD, etc., but only if your bios has "by spd"
enabled. Otherwise, you decide those settings. The module in bank 0 is
typically used to decide timings for "by spd" control. If you ever install
different rated memory modules, you should put the slower module in bank 0,
otherwise the wrong set of timings may be used. If you want to learn more
about cpu/memory bus speeds, use google and search for topics like "memory
speeds" or "bus speeds" or whatever. You'll find lots of info... within
nanoseconds.

- Robert -