From Crucial Tech. Just got this for my own enlightenment on Friday:
PC100 and PC133 SDRAM considerations
If you are running a PC that is capable of running a 100MHz FSB (Front Side
Bus) you will need PC100 or PC133 SDRAM. If your PC is not capable of a
100MHz FSB you will need 66MHz SDRAM. For a current list of available PC100
and PC133 SDRAM DIMMs please see our web site at http://www.crucial.com and
select "Shop by component".
Determining the right module for you:
Should I get PC100 or PC133 SDRAM? All PCs that accept PC100 SDRAM will
accept PC133 SDRAM. When you install a PC133 module in a system with a
100MHz FSB, the module will operate at 100MHz. There are currently no
motherboards nor processors designed to meet the PC133 specification, but
there are currently multiple designs in the works from manufacturers
including Intel and AMD. PC133 won't bring you any immediate benefit over
PC100 SDRAM, but it will allow you to use your current DRAM in future PCs.
The following guidelines must still be observed when choosing either a PC100
SDRAM DIMM or a PC133 SDRAM DIMM.
SDRAM modules are available in ECC or non-parity, and unbuffered or
registered options.
ECC vs. Non-ECC
ECC is indicated by a 72 in our part number, and non-parity is designated
with a x64. If you already have a PC and you are unsure which you have, you
will need to count the number of small, black, IC chips mounted on one of
your existing DIMMs, and if the number of chips is evenly divisible by 3
then you need ECC, and if the number if chips is not evenly divisible by 3,
you have non-parity memory. If you are building a PC and are deciding on
which type to use the follow guidelines should help. If plan to use your
system as a server or similar mission critical type machine it is to your
advantage to use ECC. If you plan to use your PC for regular home, office,
or gaming applications you are better off with non-parity. ECC (Error
Checking and Correcting) performs "double bit detection and single bit
correction". What this means is that if you have a single bit memory error
the chipset and memory will find and repair the error on the fly without you
even knowing that it happened, and if you have a double bit memory error it
will detect and report it. Using ECC decreases your PC's performance by
about 2%, and using current technology DRAM is very stable and memory errors
are rare, so unless you have a need for it you are better served with
non-parity SDRAM.
Unbuffered vs. Registered SDRAM
In our part number the D refers to unbuffered (S4D8B) and the R refers to
registered (S4R8B) PC100 SDRAM. Most PCs will only accept unbuffered SDRAM.
Registered SDRAM is primarily used in servers and was designed to allow very
large quantities of DRAM to be used (1GB or more). If you are unsure if
your motherboard our PC supports registered SDRAM, and it is not listed in
our configurator, please reply to this email and include this entire message
in your reply, and we would be happy to look into it for you. When you
install registered SDRAM, all of the modules installed in your PC must be
registered as unbuffered and registered modules are not interchangeable.
Crucial Technology PC100 SDRAM DIMMs each have a letter near the end of the
module's part number which designates the timings of the module. The
following chart shows the different timings for each type of memory:
CAS Latency tRP tRCD
A 3 3 3
B 3 2 3
C 3 2 2
E 2 2 2
These different options are all equally compatible when used on a 100MHz
bus, but if you use all of one of the above part type in your PC there is a
small performance difference. The performance difference between an A and a
B is 0.1% (the B being the faster of the 2). The difference between the B
and the C is 0.1%, and the difference between the C and the E is 0.5% (the
cumulative difference between the A and the E is 0.7%).
PC133 will operate with the timings of the "E" part listed above when run at
100MHz~138MHz.
The timings that are being measured are relative to setting up which row and
column are being read in a DRAM array. The timings referred to are:
CAS Latency = Column Access Strobe Latency (how many clock cycles it takes
to set which column is being read)
tRP = time RAS Precharge (how many clock cycles it takes to precharge the
Row Address Strobe)
tRCD = time RAS to CAS Delay (how many clock cycles are required between
setting RAS and CAS)
Brad Loomis
[log in to unmask]
I am trying to select RAM for a Tyan Trinity S1590. I have determined that
I want 168 pin DIMMs SDRAM, PC100, EDO, ECC (parity), unbuffered. But
what is CAS2? CAS is mentioned but not explained in the MB manual. I gather
it has something to do with timing, and may be related to what used to be
called wait states.
Phil Parker
--------------------------------------------
URL http://www.math.twsu.edu/Faculty/Parker/
Random quote for this second:
When you drop change at a vending machine, the pennies will fall nearby,
while all other coins will roll out of sight.
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