A DIMM MUST have an SPD (Serial Presence Detect) chip if it is PC133 compliant.
The SPD simply contains information pertaining to the DIMM and RAM itself: the capacity of the DIMM, the RAM type, and speed, including the CAS and RAS information. The motherboard will read the information in the SPD and configure the RAM timings appropriately.
The SPD is located in a tiny chip in the upper corner of the DIMM.
Most PC66 SDRAM does not have an SPD.