Types of Registers in Computer Organization

Summary Table

Registers are small, high-speed storage units in a computer processor that temporarily hold data, instructions, or addresses during execution. They play a crucial role in the functioning of the CPU. Here are the major types of registers used in computer organization:

1. Accumulator (AC)

• Purpose: Temporarily stores intermediate results of arithmetic and logical operations.
• Function: Acts as a default register for the Arithmetic Logic Unit (ALU) during calculations.
• Example: Used to store the result of an addition before moving it to memory.

2. Program Counter (PC)

• Purpose: Holds the address of the next instruction to be executed.
• Function: Automatically increments after fetching an instruction, ensuring sequential execution.
• Example: If the current instruction is at memory address 1000, the PC points to 1001.

3. Memory Address Register (MAR)

• Purpose: Stores the address of a memory location that is to be accessed.
• Function: Acts as a bridge between the CPU and memory during read/write operations.
• Example: If the CPU needs to fetch data from address 2000, the MAR holds “2000.”

4. Memory Data Register (MDR) / Data Register (DR)

• Purpose: Temporarily holds data being transferred to/from memory.
• Function: Acts as a buffer between the CPU and memory.
• Example: Stores the data fetched from memory before it’s processed by the CPU.

5. Instruction Register (IR)

• Purpose: Holds the current instruction being executed.
• Function: Decodes and prepares the instruction for execution.
• Example: If the fetched instruction is “ADD A, B,” the IR temporarily holds this command.

6. General Purpose Registers (GPRs)

• Purpose: Used for temporary storage of data during program execution.
• Function: Can store variables, intermediate results, or addresses.
• Example: Registers labeled as R0, R1, R2 in some CPUs.

7. Index Register (IX)

• Purpose: Holds an index value for memory addressing.
• Function: Supports indexed addressing modes, where the memory address is calculated by adding a constant value to the index register.
• Example: Accessing array elements in a loop.

8. Stack Pointer (SP)

• Purpose: Points to the top of the stack in memory.
• Function: Tracks the address of the last accessed stack location during function calls or interrupts.
• Example: When a function is called, the return address is pushed onto the stack.

9. Status Register / Flags Register

• Purpose: Indicates the current state of the processor and outcomes of operations.
• Function: Holds flags for conditions like zero (Z), carry (C), sign (S), or overflow (O).
• Example: After a subtraction, the zero flag is set if the result is zero.

10. Base Register

• Purpose: Holds the starting address of a memory segment.
• Function: Used in segmented memory addressing.
• Example: When a program is loaded, the base register points to the starting address of its memory block.

11. Input/Output Registers (I/O Registers)

• Purpose: Temporarily store data exchanged between the CPU and I/O devices.
• Function: Facilitates communication between the processor and peripherals.
• Example: Data from a keyboard or to a display.

12. Floating-Point Registers

• Purpose: Store floating-point numbers during arithmetic operations.
• Function: Used in processors that support floating-point operations (e.g., scientific calculations).
• Example: Registers in a GPU or a math co-processor.

Conclusion

Registers are essential components of a CPU, ensuring high-speed data access and smooth program execution. Each register has a specific function, contributing to the overall efficiency of the computer system.