Electronic Control Module (ECM) Operation: Memories

The 3 types of memory in the ECM are: Read Only Memory (ROM), Random Access Memory (RAM) and Programmable Read Only Memory (PROM). Function of each memory is as follows:

• Read Only Memory (ROM) - The ROM is programmed information that can only be read by the ECM. The ROM program cannot be changed. If battery voltage is removed, ROM information will be retained.
• Random Access Memory (RAM) - This memory is the scratch pad for the CPU. Information can be read into or out of RAM memory, similar to a calculator. Data sensor information, diagnostic codes and results of calculations are temporarily stored in RAM memory. If battery voltage is removed, all information stored in this memory is lost.
• Programmable Read Only Memory (PROM) - This memory contains calibration information about each engine, transmission, body and rear axle ratio combination. If battery voltage is lost, PROM information is retained. PROM may be easily replaced as needed.
• CALPAK - Contains basic fuel calibration information in case of ECM or PROM malfunction. It also carries communications information necessary for the ECM and BCM to exchange information. If battery voltage is removed, CALPAK memory is not lost.