What do you mean by processor organization? Explain various types of processor organization with suitable example.

Single accumulator organization

One special register, the accumulator (AC), holds one operand and also receives the result. The instruction has one address field for the other operand. For example, ADD X means $AC \leftarrow AC + M[X]$. To compute $Z = X + Y$ the program is LOAD X, ADD Y, STORE Z. It is simple and cheap, but every result passes through AC, so many load and store instructions are needed. Early machines such as the IBM 701 used it.

  Memory ---> [ ALU ] ---> [ AC ] ---+
                 ^                   |
                 +-------------------+

General register organization

A group of registers R1 to R7 can each hold data. Two multiplexers select the two source registers for the ALU, and a decoder selects the destination register. For example, ADD R1, R2, R3 means $R1 \leftarrow R2 + R3$. Data stays in the CPU, so fewer memory accesses are needed and the program is fast. Instructions are longer as they carry register numbers. ARM and RISC-V processors use this type. The program for $Z = X + Y$ is LOAD R1, X, LOAD R2, Y, ADD R3, R1, R2 and STORE R3, Z.

Stack organization

The CPU works with a LIFO stack, and the operations use the top items. Most instructions are zero-address. For example, ADD pops two items and pushes their sum. The program for $Z = X + Y$ is PUSH X, PUSH Y, ADD and POP Z. It suits expressions in reverse Polish notation, and the instructions are very short. But random access to the middle of the stack is not possible. The Burroughs B5500 used it.

Comparison

Most modern processors use the general register organization along with some stack support for subroutine calls.