Memory Access Times & CAS

When the Intel 440x BX chipset was released a few years ago now we first started with 10ns memory which was rated at PC66 and operated at 66MHz - thus the name PC66. From there in memory has continued to get faster and faster. Since the original Intel Celeron which had a FSB of 66MHz, PC66 memory was more than enough to work in-conjunction with the processor, however if you wanted to overclock the processor to 100MHz FSB you’d need 8ns PC100 memory which could cope with the increased FSB unlike poor quality PC66 memory (some are lucky and can get PC66 operating at 100MHz). Then we move onto the Intel Pentium III which ran at 100MHz FSB. If you wanted to overclock this puppies FSB you’d either need top quality PC100 memory or 7ns PC133 to increase the FSB of the processor effectively. Enter "PC150" memory. Currently PC150 memory is the highest quality ram of all and is manufactured and shipped as PC150 memory, when in fact the actual memory is overclocked at 150MHz and promoted as PC150. Mushkin is one such company which does this. The Mushkin PC150 HSDRAM (High Speed Dynamic Random Access Memory) is one of the fastest memory types on the market at time of writing this article, the access time of this memory is incredibly fast at 4.5ns. Kingmax also produce PC150 "TinyBGA" memory, but with a difference, it is actually PC150 memory and not overclocked PC133 memory. So if you are intending to set your FSB to say 166MHz, you may want to buy some Kingmax memory. Because if you want to set the FSB to 166MHz with Mushkin "PC150" memory you are in fact overclocking already overclocked memory.

Now that memory access time has been briefly explained, let’s take a look at memory CAS (column access select) settings. Memory either comes in CAS3 or CAS2, CAS2 being the fastest. CAS refers to the number of latency cycles that the module itself needs to synchronize correctly with the system clock. The lower the number the better. PC100 modules come in two CAS latency flavors, CAS2 and CAS3. A CAS3 latency module running at 100MHz requires one extra cycle during its input and output cycles effectively slowing its performance compared to CAS2. A CAS2 module is able to perform both at higher speeds and with more stability because of this design enhancement. All you really need to remember is CAS2 is faster than CAS3.

To adjust the CAS setting you will need to access your computer’s BIOS (Basic Input Output System) area. To do this, reboot your computer and press DEL while your computer is checking the memory. You may have to search for the CAS delay option because chances are it will not be in the exact section in every motherboard’s BIOS. When you find the CAS delay setting, it will most likely default at 3. For optimal performance change this setting to 2. If your computer refuses to boot or crashes, change the CAS setting back down to 3. The reason your computer may crash is because you are forcing the memory to performance faster at CAS2. However, if your memory is rated at CAS2 you should have no worries with the CAS delay setting of 2.