It is used for storing programs and data. The memory locations of memory unit is uniquely specified by the memory address of the location. M(X) is used to indicate the location of the memory unit M with address X.
The data transfer between memory unit and CPU takes place with the help of data register DR. When CPU wants to read some information from memory unit, the information first brings to DR, and after that it goes to appropriate position.Similarly, data to be stored to memory must put into DR first, and then it is stored to appropriate location in the memory unit.
The address of the memory location that is used during memory read and memory write operations are stored in the memory register AR.
The information fetched from the memory is a operand of an instruction, then it is moved from DR to data processing unit (either to AC or MQ). If it is an operand, then it is moved to program control unit (either to IR or IBR).
Two additional registers for the temporary storage of operands and results are included in data processing units: the accumulator AC and the multiplier-quotient register MQ.