The SDRAM memories are synchronized by the bus clock (clock external to processor), and as such, are classified according to the maximum operation frequency they can operate. Every SDRAM memory has a mark of this frequency, declared in nanoseconds. Many folks call this mark access time, which is not true, as the access time for SDRAM memories is a characteristic called CAS latency, but that is not written in it. The marking can be checked in the table

Marking	Maximum Operating Frequency
-15	66 MHz
-12	83 MHz
-10	100 MHz
-8	125 MHz
-7	133 MHz

It happens that when the earlier processors using the external 100 MHz bus appeared (Pentium II higher than 350 MHz and K6-2 higher than 300 MHz), the manufacturers verified that 100 MHz memories couldn't operate correctly at 100 MHz as computer kept crashing randomly. To solve this problem, a PC-100 specification was created.

All memories -15 and -12 were classified as PC66, as they can operate with processors externally clocked at 66 MHz. All memories -8 and -7 were classified as PC100, as they can operate with processors externally clocked at 100 MHz (as well as in the old 66 MHz processors). The problem stays with the -10 memories, as there are PC66 and PC100 memories using this same marking.

Many memories are delivered with a label written PC100. It happens that we've received many reports from readers some fraudulent people were sticking this label in any type of SDRAM - even the 66 MHz - to sell them as if they were PC100. If you install a PC66 in a computer with a 100 MHz bus, it will keep crashing randomly. Then comes the question: how to recognize properly the PC-100 modules?

All SDRAM memories have a small configuration memory called SPD (Serial Presence Detect). Inside this small memory chip all information related to the memory chip are registered - including whether it is a PC66 or a PC100 part.

Figure 1: Location of the SPD chip, which holds all memory parameters.

Figure 2: Close-up of the SPD chip (the smaller one).

With hardware identification utilities, like Hwinfo, Everest and Sandra (all available on our download section) you can read the SPD chip contents and see what kind of memory chips your memory modules really use, as you can see on Figure 3.

Figure 3: Screenshot of Hwinfo program. The computer on this example has 128 MB of RAM, using two 64 MB memory modules, one is PC100 and the other one is PC133.

With this you can see if your memory module is really PC100 or is a relabeled PC66. If it is a relabeled PC66 and your CPU runs externally at 100 MHz, replace it with a PC100 part.