The operation to be performed is specified by signals from the control unit . The data upon which operations are performed can come from memory or an external input. The data may be combined in some way with the contents of the accumulator and the results are typically placed in the accumulator. From there they may be transferred to memory or to an output unit. The accumulator is the principal register of the arithmetic logic unit of a microprocessor.
Registers are sets of flip-flops which can hold data.
The accumulator typically holds the first piece of data for a calculation. If a number from memory is added to that date, the sum replaces the original data in the accumulator. It is the repository for successive results of arithmetic operations, which may then be transferred to memory, to an output device, etc. The control unit of a microprocessor directs the operation of the other units by providing timing and control signals. It is the function of the microcomputer to execute programs which are stored in memory in the form of instructions and data.