Electronic Control Module (ECM): Memories

The 3 types of memories 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 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 is factory-programmed information containing engine calibration data for each engine, transmission, body and rear axle ratio application. If battery voltage is removed, PROM information will be retained.