Created on: April 29, 2013
Last updated: April 19, 2022
Legacy SDRAM modules for older Macs utilize a small programming chip called the Serial Presence Detect (SPD). This chip tells the computer what it is, and how it should be used. It has Primary, Secondary, and Tertiary programming for CLx settings. In most cases, PC133 memory will be set to run as CL3 at PC133 speed, CL2, at PC100, and CL3 at PC100. If the computer is capable of PC133 speeds, it will use the Primary programming, CL3. If the computer is only capable of PC100, it will use the Secondary programming, CL2. If that is not possible, it will use the Tertiary setting of PC100 CL3.
