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CPU, RAM, Cache and Speed
Name: Tyler R.
Status: Student
Age: 14
Location: N/A
Date: 10/2/2004
Question:
Well, I'm doing a research project on CPU speed and
RAM. I know RAM has a big effect on CPU speed. Can you give me a
realistic idea on how much effect RAM has on CPU speed? Also, what
else affects CPU speed? Cache? Type of RAM?
Replies:
All depends on whether RAM is responding quickly enough that the CPU
does not have to wait for it. If RAM is not fast enough, the CPU must
wait several nanoseconds every time it reads a new datum. If RAM is
fast enough that this does not happen, any additional quickness will
not speed anything up, because the CPU is already going as fast as it
can.
These days, CPU's are so fast that it would be unreasonable to make
RAM fast enough to keep up. So system designers have made complicated
arrangements in which small amounts of very fast RAM (i.e., cache),
located close to the CPU, are filled with data the CPU is expected to
want soon. If the algorithm guesses right, the CPU can get what it
needs rapidly from this cache, because the cache already read it from
main RAM.
There are also various arrangements in all kinds of RAM to speed up
certain kinds of accesses. Normally, to access a memory word, you
have to write the address of that word, wait for the address lines to
settle, tell the RAM to fetch the data at the address, wait for that
to happen, wait while the RAM puts the data on the bus, wait for the
data lines to settle, and then read. If you know you want a bunch of
consecutive memory words, you could just say "start at this address",
do the whole wait/read thing for the first word, and then just say
"next", read, say "next", read, etc.
--
Tim Mooney
Beamline Controls & Data Acquisition Group
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