by Joseph M. Whinery
Over the past few weeks, I have been thinking about upgrading my current system into a new leading edge screamer (what ever that is). Thinking about which components I could transfer to the new system, and which ones would stay with my current system, I finally came to the memory. Could I use my present SDRAM in my new system, after all it is 10ns DIMM memory? And everyone knows that 10ns memory should run at the new bus speed of 100MHz. Right? Well…no!
While it is true that 10ns memory will run at 100MHz, it will not be able to supply its stored data to the microprocessor reliably at that speed. The result will be a system that will not boot up, or if it does boot up, will be unreliable and could lock up on you at the most inopportune times.
In order to determine what I needed in the way of memory, I went straight to the horse's mouth (so to speak), Intel. Intel is the developer of the PC100 spec that defines the physical, electrical and timing characteristics of memory chips and the DIMM modules that will work with their chip sets at the 100MHz bus speeds. Since Intel developed the PC 100 specification and the first chip sets for the 100MHz bus, any memory you want to use in a new 100MHz bus system must be PC 100 compatible.
As the bus speeds increase, timing parameters are becoming much more critical. Remember that we are dealing with billionths of a second here. The microprocessor must be told what to expect from the memory module, and how to handle the timing. This is done with a spd (Serial Presence Detect) EEPROM. There are, among other things, three clock references that are spelled out in the PC 100 spec whose values are contained in the spd. We have to know what these are so we know what to look for, not necessarily know what they do internally.
The next question was "how can anyone tell by looking at a DIMM module whether or not it is really PC 100 memory"? Over the past few weeks I have seen several DIMM modules marked "PC 100", but were they really PC 100 compatible as defined by Intel? Not surprisingly, the answer to that question is a resounding NO!
Part of the PC-100 Spec is the marking requirements. In order to identify the three key parameters of the DIMM, the following marking convention will be used.
PC100-abc-def
Where the 100 is the speed of the buss in MHz:
a = CL value (3 is standard, 2 is better and more expensive).
b = trcd value (2 is standard).
c = trp value (2 is standard).
d = tac value (6 is standard, lower is better).
e = spd rev # (current rev is 1.2, use last digit).
f = reserved for later use.
Example: PC100-322-620 or a 7 digit convention may be used such as PC-100-6120.
Another requirement of the PC-100 spec is that all DIMM modules be backward compatible with existing memory systems. This answers my original question of whether or not I could use my present SDRAM with my new system. No, I can't, but I can use my new memory with my present system until I buy that new motherboard.
Joseph M. Whinery is a Director of HAL-PC and active in the Build or Buy a PC SIG. He can be reached at jwhinery@hal-pc.org.
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