CPU Registers and Their Types
What are CPU Registers?
Registers are small, high-speed storage locations inside the CPU used to hold data, addresses, and instructions temporarily while the processor is executing programs.
- Faster than main memory (RAM)
- Directly accessible by the CPU
- Used to speed up processing
Types of CPU Registers
1. Accumulator (AC)
- Stores intermediate results of arithmetic and logical operations.
- Most operations use the accumulator by default.
Example:
For A = B + C, CPU loads B into AC, then adds C, and stores the result in AC.
2. Program Counter (PC)
- Holds the address of the next instruction to be executed.
- Automatically increments after fetching an instruction.
Example:
If the current instruction is at address 2000, PC = 2000. After fetching, PC = 2001.
3. Instruction Register (IR)
- Stores the current instruction being executed.
- Retrieved from memory using the address in the Program Counter.
Example:
If instruction at 2001 is ADD B, it will be loaded into the IR.
4. Memory Address Register (MAR)
- Holds the memory address from which data will be fetched or to which data will be written.
Example:
If CPU needs data from address 3050, MAR = 3050.
5. Memory Buffer Register (MBR) / Memory Data Register (MDR)
- Holds the data being transferred to or from memory.
- Works along with MAR.
Example:
- CPU wants to write value
42hat address3050h - MAR =
3050h, MBR =42h
6. General Purpose Registers (GPRs)
- Temporarily hold data or intermediate results.
- Examples: AX, BX, CX, DX in Intel 8086.
Example:
To compute (A + B) * C,
- Store A and B in AX, BX → Add → result in AX
- Store C in CX → Multiply AX and CX
7. Status Register / Flags Register
- Stores flags indicating the status of operations:
- Zero flag (Z) – set if result is 0
- Carry flag (C) – set if there’s a carry/borrow
- Sign flag (S) – set if result is negative
- Overflow flag (O) – set on overflow
Example:
If you subtract 5 from 5 → result is 0 → Zero flag is set.
8. Stack Pointer (SP)
- Points to the top of the stack in memory.
- Stack is a LIFO (Last In First Out) memory area used for storing return addresses, local variables, etc.
Example:
During function calls, the return address is pushed to stack. SP updates accordingly.
9. Base Register and Index Register
- Used for addressing modes in memory access.
- Helps calculate effective addresses in array and structure access.
Summary Table
| Register | Purpose |
|---|---|
| AC | Intermediate results |
| PC | Next instruction address |
| IR | Current instruction |
| MAR | Address to access |
| MBR/MDR | Data to/from memory |
| GPRs | Temporary data |
| SP | Stack management |
| Flags | Operation status |
| Base/Index | Complex addressing |
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