At 02:55 07/31/01, Julie Scorer wrote:
>I'm afraid I don't understand Cas or Density so would
>appreciate an explanation of these.


Hi Julie

Three parts to this message. (Comments on using both SIMMs
and DIMMS together; what CAS means; what density means.)

***

It may not be a good idea to use both SIMMs and DIMMs on your
motherboard at the same time.

The reason is that SIMMs run at 5 volts and DIMMs run at 3.3 volts.
If they're both present at the same time, voltage can "leak"
from the higher voltage SIMMs to the lower voltage DIMM and damage
the DIMM over time. (All the memory is connected electrically
through the memory circuitry.) Using both types of memory at the
same time will probably work...for a while. How long before the
DIMM is damaged? It may take days or weeks or even months, but it
probably will happen sooner or later.

A lot of the motherboard makers come right out and tell you (in
the motherboard manual) not to mix both types of modules. I'd go
so far as to say that you should only use both types together if
your motherboard manual explicitly says it's okay. If you don't
have a manual and this isn't stated on the manufacturer website...
maybe you shouldn't do it.

Besides, using the SIMMs should slow down your computer...say by
10% or 20% (guessing here)...because EDO SIMMs are slower than
SDRAM DIMMs.

***

CAS stands for Column Access Strobe. I know...that doesn't
explain what it is.

The easy answer is that "CAS Latency" is the amount of time it
takes for your memory to respond to a command. Specifically, it's
the length of time between memory receiving a command to read data,
and the first piece of data being output from memory. Latency is
measured in terms of clock cycles and is often noted as CL2 (two
clock cycles) or CL3 (three clock cycles).

     (If you have a 200 megahertz computer, there are 200 million
     clock cycles each second. Think of these cycles as ticks of
     your computer's clock. A 1 Gigahertz computer ticks a
     billion times each second. The computer is designed so it
     does things on clock ticks.)

So CAS 3 means the computer is set to wait 3 clock cycles each time
it must read memory from a different "column". CAS 2 is better than
CAS3 because there aren't so many wasted clock cycles. However,
after the first piece of data in a column is processed, the rest of
the data in that column is processed at equal speeds whether CAS
is 2 or 3.

     (Data is located at addresses in memory that are at by the
     intersections of rows and columns...sort of like how city
     people might live on the corners of streets and avenues.
     Columns doesn't refer to where the DIMMs are plugged into
     your motherboard.)

***

I can think of three meanings for "density" as it applies to memory
modules (SIMMs or DIMMs).

(1) Density used to refer to the amount of memory on a memory stick.
     For example, a 128 MB DIMM was a higher density DIMM than a
     64 MB DIMM. Some older computers would only recognize DIMMs up
     to some limit. For example, early Pentium II motherboards didn't
     work with 256 MB or higher DIMMs.

(2) Density might refer to "single sided" or "double sided". That is,
     whether the little circuit board that we call a DIMM has black
     memory chips mounted on only one side or on both sides. Some
     motherboards (but not too many) don't like double sided DIMMs
     (black chips on both sides of the DIMM).

(3) Density might refer to the number of black memory chips mounted
     on the memory module. As memory technology develops, manufacturers
     can fit more and more memory into one of those little clack chips.
     So a module with *less* chips must use more dense memory chips. If
     a DIMM has 8 chips mounted onto its circuit board, the memory is
     not as "dense" as if it only has 4 chips mounted on the board.
     (Each chip on the DIMM with less chips must hold twice as much
     memory.) Note that more dense wouldn't refer to more black chips.

Take your pick... Someone else?

Regards,
Bill

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