RAM Compatibility: Substituting PC-133 for PC-100

Created on: April 29, 2013
Last updated: April 19, 2022

PC-133 and PC-100 RAM modules are backwards compatible. Higher speed memory modules always default to the highest CAS latency rating at slower speeds. For example, a PC-133 CL3 (3-2-2) chip will run at either PC-100 or PC-66, CL2 (2-2-2). If you purchase a PC-100 memory module and receive a substitute PC-133 module, it is a comparable substitution without any compromise or speed degradation from a native PC-100 part. 

Occasionally, Apple System Profiler will incorrectly report the memory as having slower specs even though it is programmed correctly for the backwards compatibility. Most notably this can occur with Apple Power Macintosh G4 "AGP" "Sawtooth" logic boards and also Apple Power Macintosh G4 "Cube" systems. The reason for this is that the memory controller on the system’s logic board is not programmed to recognize PC-133 memory modules - they will function perfectly but report an incorrect speed.