RAM (Random Access Memory)

Random Access Memory is the working memory. All sensor values and values that the CPU has calculated and will use during travel are stored here. The memory is volatile and is thus erased if there is a loss of power. RAM is like a notebook for the CPU.

The values read using the Read Values menu of the diagnostic tool are from RAM. The CPU retrieves these values from I/O and updates them (overwrites the old value) at specific time intervals. For the tank sensor, the update is once/second, for the coolant temperature sensor 10 times/second and for the mass air flow sensor 1000 times/second. For a crankshaft position sensor, update is done with the highest priority as soon as the value changes.

Thus, RAM always has the latest sensor data. Not only does the control module update sensor values at specific time intervals, it must also perform a task, such as inject fuel or lock a door. The program for the different functions retrieves the necessary sensor values from RAM and makes its calculation. The result of the calculation is the output value delivered to I/O, which then activates the relevant output. RAM in a home computer currently averages between 64 and 256 MB. In a car computer, the size is about 2 KB.