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The number of registers in a processor unit may vary from just one processor register to as many as 64 registers or more.
One of the CPU registers is called as an accumulator AC or 'A' register. It is the main operand register of the ALU.
The data register (DR) acts as a buffer between the CPU and main memory. It is used as an input operand register with the accumulator.
The instruction register (IR) holds the opcode of the current instruction.
The address register (AR) holds the address of the memory in which the operand resides.
The program counter (PC) holds the address of the next instruction to be fetched for execution.
Additional addressable registers can be provided for storing operands and address. This can be viewed as replacing the single accumulator by a set of registers. If the registers are used for many purpose, the resulting computer is said to have general register organization. In the case of processor registers, a registers is selected by the multiplexers that form the buses.