Unit I — CPU: ALU, Control Unit & Registers

The Central Processing Unit (CPU) is the brain of the computer. It is responsible for fetching, decoding, and executing instructions. The CPU consists of three core components: the Arithmetic Logic Unit (ALU), the Control Unit (CU), and Registers.

---

Arithmetic Logic Unit (ALU)

The ALU performs all computation inside the CPU.

Arithmetic Operations:

• Addition, Subtraction, Multiplication, Division
• Increment (add 1), Decrement (subtract 1)

Logical Operations:

• Comparison: Greater than, Less than, Equal to
• Boolean: AND, OR, NOT, XOR, NAND, NOR

The ALU receives operands from registers, performs the operation, and stores the result back in a register or memory.

---

Control Unit (CU)

The Control Unit is the manager of the CPU. It does not perform calculations but directs all other components.

Functions of the Control Unit:

1. Fetch — Retrieves the next instruction from memory.
2. Decode — Interprets the instruction to determine what operation is needed.
3. Execute — Signals the ALU, memory, or I/O to carry out the operation.
4. Store — Writes the result back to memory or a register.

This sequence is called the Fetch-Decode-Execute (FDE) cycle or instruction cycle.

---

Registers

Registers are the fastest and smallest memory units, located inside the CPU itself.

---

CPU Clock Speed

• The CPU operates in synchronisation with a clock signal.
• Speed is measured in GHz (gigahertz) — billions of cycles per second.
• Higher clock speed → more instructions executed per second.

---

Instruction Cycle (FDE Cycle)

1. Fetch: PC → MAR → Memory sends instruction → MDR → IR
2. Decode: CU interprets the opcode in IR
3. Execute: ALU or memory operation is carried out
4. Increment PC: PC points to next instruction

Key Takeaway: The CPU's ALU handles all calculations, the Control Unit orchestrates every operation, and registers provide ultra-fast temporary storage. The Fetch-Decode-Execute cycle is the heartbeat of every computer — one cycle per instruction.