Who could forget memory specs?

Oops. I learn the hard way.

I wanted to upgrade the memory of one system, and take the old memory from that system to upgrade another. Seems that at some point, the number of pins on the SIMMS changed. I've got so many old systems laying around, I didn't realize there was such a divergence of desktop memory.

It was easy back in the 168 pin days of the turn of the century. In the '90s, we had PC33, PC66, and PC100. After Y2K, we got PC133. For the most part, any chip could run a grade lower. Thus, PC133 could run in a PC100 system. In the case of Celeron and Pentium III CPUs, it was even possible to use PC133 and PC66 interchangeably... Not that it was a good idea.

Then we moved to the 184 pins. This was for the radically faster front side bus speeds that were being inovated by AMD. Soon the industry standard was 400Mhz, rather than 133Mhz. The 184's were also called DDR followed by a speed indicator of 266, 333, 400, or 500. In keeping with tradition, each had a PC moniker, unfortunately there is no logical relationship between the DDR and PC values.

Next came DDR2. This is the one that burned me, as it now has 240 pins. Speeds range from 400Mhz to about 1.2Ghz. Beyand the gigahertz barrier, it was necessary to move to DDR3, which starts at 1Ghz and is passing the 2Ghz mark. Oddly, it uses the same 240 pins and DDR2.

Here it is in easy to digest table form:

168 pin
  PC33, PC66, PC100, PC133
184 pin
  DDR-266, DDR-333, DDR-400, DDR-500
240 pin
  DDR2-
      400, 533, 667, 675, 800, 900, 1000,
      1066, 1100, 1142, 1150, 1200
  DDR3-
      1066, 1333, 1375, 1600, 1625,
      1800, 1866, 2000, 2133

This list is, of course, no where near complete.