Powertrain Control Module (Pcm): Memories

PCM uses 3 types of memories: Read Only Memory (ROM), Random Access Memory (RAM) and Programmable Read Only Memory (PROM).

Read Only Memory (ROM)
ROM is programmed information that can only be read by PCM. The ROM program cannot be changed. If battery voltage is removed, ROM information will be retained.
Random Access Memory (RAM)
RAM is the scratch pad for the CPU. Data input, diagnostic codes and results of calculations are constantly updated and temporarily stored in RAM. If battery voltage is removed from PCM, all information stored in RAM is lost.
Programmable Read Only Memory (PROM)
PROM is factory programmed engine calibration data which "tailors" PCM for specific transmission, engine, emission, vehicle weight and rear axle ratio applications. The PROM can be removed from PCM. If battery voltage is removed, PROM information will be retained. An Electronically Erasable Programmable Read Only Memory (EEPROM) is used on some models. This is the same as a PROM except it can be electronically reprogrammed by the manufacturer using special equipment.
Electronically Erasable Programmable Read Only Memory (EEPROM)
Some models may use an EEPROM. This is the same as a PROM except it can be electronically reprogrammed by the manufacturer using special equipment.

NOTE: Components are grouped into 2 categories. The first category covers INPUT DEVICES, which control or produce voltage signals monitored by the control unit. The second category covers OUTPUT SIGNALS , which are components controlled by the control unit.