How to Run PC100 RAM on a PC133 Machine

An SDRAM memory module

SDRAM (Synchronous Dynamic Random Access Memory) has three recognized speeds within its specification: PC66, PC100, and PC133. Higher speed modules are always backward-compatible with slower modules. Slower modules, on the other hand, generally cannot be made to run at higher speeds unless they are comprised of high quality memory chips, and even then, there is no guarantee. A "PC133 machine" is simply a computer that currently has PC133 RAM installed. Using PC100 in such a machine (to save money, as an example) is possible, but will require forcing all RAM to run at PC100 speed.

Things You'll Need

• #2 Phillips Screwdriver

Instructions

1. Turn the PC off and unplug it. Remove the side access panel using the Phillips screwdriver.

2. Locate the 168-pin memory expansion slots on the motherboard. These slots are most often black in color, with white retaining clips at each end, and will be found in the upper right quadrant of the motherboard. They will have two notches--one close to the middle, one roughly 1 inch from one end. The memory module will have indented notches in the exact same positions so that they cannot be plugged in backwards.

   Install your module into an open slot, preferably next to the existing module. Press the retaining clips outward, then carefully line the module up with the expansion slot matching the notches in the slot with the notches in the module, and press firmly at both ends of the module until you hear it pop into place. Both retaining arms should clamp onto the module loosely. Press them firmly against the module until they click into place.

3. Replace the side panel and plug the computer back in. Access the BIOS setup screen by pressing the "Delete" key, or whichever key the motherboard manufacturer has specified. Access the page set by the manufacturer to adjust memory timings. This is different between all motherboard vendors, so you may need to use the manual that came with your specific brand and model to locate the proper BIOS settings. Usually they will be named "CPU Settings," "Advanced Settings," or "Advanced CPU Settings". Look for a setting that is titled "Detect DRAM timing by SPD (Serial Presence Detect)" and set it to "Disabled" or "Off." Set the memory bus speed to 100MHz by locating the "DRAM FSB" or equivalent setting, and manually changing the number in it to 100. In most cases this is done by highlighting the field and pressing "Enter," then typing in the number and pressing "Enter" again.

4. Exit the BIOS, saving the changes, and reboot the PC.

Tips & Warnings

• It is possible to force PC100 to run at PC133 speed, but overclocking your system can have unwanted effects, and will most likely void the warranty on your RAM, CPU, and motherboard. Running memory at speeds higher than it is rated can lead to data corruption and crashing of applications or the operating system. If you notice any system instability due to overclocking, remove the installed modules, or force all memory modules to run at the slower 100MHz bus speed as directed.

References

• William Bellisle-Pio; Network Systems Administrator; Auburn, Washington