Is there a reliable source of information re RAM chips?

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Posts: 4
Joined: Sat May 24, 2003 2:46 am

Hi all,

I have quite a number of PCs now and am in the process of 'rationalising' my collection (throwing them out on the orders of 'she who must be obeyed').

I have various RAM chips (just can't bring myself to throw everything out) but I am not clear as to what chips can be used in what boards - for example: -
:?: Can a SDRAM 168 pin PC 133 be installed in any MOBO (assuming it fits of course!!),
:?: If I buy the fastest (and cheapest) DIMMS, as long as they 'fit in the slot' should this be OK
:?: What is the significance of "PC2100, PC2700" - and do I need to be concerned.

Have trawled around the Tom's Hardware site but cannot find the simple answers - I am sure that this has been covered before - can anyone help me please?

Many thanks,

BIOS Bodhisattva
Posts: 3145
Joined: Fri May 03, 2002 10:34 am
Location: Thames Valley, UK

1. Faster than required speed should not matter (with the exception of one Intel mobo I came across a while back). Chip technology does.

2. PC2100/PC2700 - these were originally terms coined to compete with RAMbus(tm). PC100 and PC133 reflected the clock speed. RAMbus started talking about transfer rate, using numbers like "800". This is in fact only as fast as PC100 - there are 8 bytes per transfer, so 8x100=800. With DDR there are 2 transfers per clock, so:
FSB 100: 8x2x100= 1600 bytes/second ("PC1600".
FSB 133: 8x2x133.3=2133 bytes/second ("PC2100").
FSB 166: 8x2x166.6 = 2666 bytes/second ("PC2700")

In fact there are complicating factors. The address in memory is put up in two chunks - row address and column address. These are loaded using the Row Address Strobe (RAS) and the Column Address Strobe (CAS). Modern memory only uses the RAS once and then the CAS multiple times for the length of the data transfer. It takes a certain time for the data to be stable after the first CAS and that depends on the speed of the RAM chips - it's called the CAS Latency (CL) and the smaller the better. You'll see this specified as CL2, CL2.5 or CL3. So for example some memory may be sold as PC133 CL2 but described by memory test programs as "PC143" because the chips should be fast enough to support a sustained transfers with a 143 MHz clock.

3. Chip technology is vital. For a discussion of this look at my sticky message in the "RAM compatibility" forum which links to an on one of the Intel chipsets. If your chipset cannot support the number of address lines used by the RAM chips, it won't work - or you will see only a fraction of the RAM, depending on how your BIOS copes with it. SDRAM including DDR carries a small chip (the SPD, Serial Presence Detect, chip) which is programmed with all sorts of information about the RAM. Different BIOSes make use of different amounts of this information - some will ignore discrepancies, some will refuse to boot.
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