CPU and Microprocessor Architecture: ALU, Control Unit, Registers, Cache, Clock Speed and Buses for SSC Exams

The Central Processing Unit (CPU) is the brain of the computer, and questions on it are asked in every computer awareness section. This guide on CPU and microprocessor for SSC explains the parts of the CPU, the registers, cache, buses, clock speed, the instruction cycle and the difference between RISC and CISC, with practice questions and FAQs.

What Is a CPU?

The CPU (Central Processing Unit) is the part of the computer that executes instructions and controls all other parts. A CPU built on a single chip is called a microprocessor. The first microprocessor was the Intel 4004 (1971).

Main Parts of the CPU

Important Registers

Instruction Cycle (Fetch–Decode–Execute)

1. Fetch: the control unit gets the next instruction from memory using the program counter.
2. Decode: the instruction is decoded to find what operation is required.
3. Execute: the ALU or another unit carries out the operation.
4. Store: the result is written back to a register or memory.

Buses

Clock Speed and Performance

• Clock speed is the number of cycles per second, measured in hertz (Hz), megahertz (MHz) or gigahertz (GHz). A higher clock speed means faster processing.
• Word length is the number of bits the CPU can process at one time (for example 32-bit or 64-bit).
• MIPS measures millions of instructions per second.
• Multi-core processors have two or more cores (dual-core, quad-core) on one chip.
• GPU (Graphics Processing Unit) is specialised for graphics and parallel processing.

Cache Memory Levels

RISC and CISC

Memory Hierarchy (Fastest to Slowest)

Registers → Cache → RAM (main memory) → Secondary storage (hard disk, SSD) → Tertiary storage.

Important Facts

• The CPU is also called the processor or the brain of the computer.
• The speed of the CPU is measured in GHz.
• The ALU and CU together form the processing part of the CPU.
• Moore’s law says that the number of transistors on a chip doubles about every two years.
• Intel and AMD are the two main makers of CPUs for personal computers.

Quick Tips

• Learn the three parts of the CPU: ALU, CU and registers.
• Learn the function of each register.
• Remember the instruction cycle: fetch, decode, execute.
• Remember the memory hierarchy order.

Quick Revision Points

• CPU = ALU + CU + registers.
• The program counter holds the address of the next instruction.
• Clock speed is measured in GHz.
• The address bus is unidirectional; the data bus is bidirectional.
• RISC has simple instructions; CISC has complex ones.

Practice Questions With Answers

1. What is the full form of CPU?
   Answer: Central Processing Unit
2. Which unit performs arithmetic and logical operations?
   Answer: ALU
3. Which register holds the address of the next instruction?
   Answer: Program Counter
4. Which register stores the results of arithmetic operations?
   Answer: Accumulator
5. In which unit is clock speed measured?
   Answer: Hertz (GHz)
6. Which bus is bidirectional?
   Answer: Data bus
7. What is the fastest memory in a computer?
   Answer: Registers
8. What is the full form of RISC?
   Answer: Reduced Instruction Set Computer
9. Which was the first microprocessor?
   Answer: Intel 4004
10. Which unit controls the other parts of the CPU?
    Answer: Control Unit

How to Practise This Chapter

Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Computer Knowledge notes page, read the previous topic History of Computers and the next topic Booting Process and BIOS.

Frequently Asked Questions

What is the CPU?

The central processing unit, the brain of the computer that executes instructions.

What are the main parts of the CPU?

The arithmetic logic unit, the control unit and the registers.

What does the ALU do?

It performs arithmetic and logical operations.

What does the control unit do?

It controls and coordinates all operations of the computer.

What is a register?

A very fast storage inside the CPU.

What is cache memory?

A small, fast memory between the CPU and RAM that stores frequently used data.

How is the speed of the CPU measured?

In hertz, usually gigahertz.

What is the instruction cycle?

The fetch, decode and execute cycle.

What are the three types of buses?

Data bus, address bus and control bus.

What is a microprocessor?

A CPU built on a single chip.

What is the difference between RISC and CISC?

RISC has a small set of simple instructions; CISC has a large set of complex instructions.

Is this topic asked in SSC?

Yes. CPU, ALU and registers are common questions in the computer awareness section.