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. The PROM is easily removed from ECM. If battery voltage is removed, PROM information will be retained.