Electronic Control Module (ECM): Memories

The 3 types of memory in the ECM are: Read Only Memory (ROM), Random Access Memory (RAM) and the Memory-Calibration (MEM-CAL) unit. 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 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 in this memory is lost.
• MEM-CAL
  
  Contains calibration information for each specific engine, transmission and rear axle ratio combination. Unit also contains basic fuel calibration information in case ECM should malfunction. It also carries communications information necessary for the ECM and BCM to exchange information. If battery voltage is removed, MEM-CAL information is not lost.