Lesson 4 of 7
The CPU
Step inside the brain of the computer and discover how the Central Processing Unit executes instructions.
Learn it
The Central Processing Unit, or CPU, is often called the 'brain' of the computer. It is a tiny silicon chip that carries out billions of instructions every single second!
The CPU does its job using a never-ending cycle called the Fetch-Decode-Execute cycle. First, it fetches an instruction from memory. Next, it decodes what the instruction means. Finally, it executes the instruction.
Inside the CPU, you will find specialised parts: the ALU (which does all the math and logic), the Control Unit (which acts like a traffic cop directing data), and Cache (super-fast memory).
Key terms
- CPU
- Central Processing Unit; the primary silicon microchip that fetches, decodes, and executes program instructions.
- Fetch-Decode-Execute Cycle
- The fundamental operational cycle of the CPU: retrieving an instruction from RAM, interpreting it, and carrying it out.
- Arithmetic Logic Unit (ALU)
- The CPU sub-component that carries out arithmetic calculations (+, -, *, /) and logical comparisons (AND, OR, NOT).
- Control Unit (CU)
- The CPU sub-component that directs the flow of data and instructions between CPU components and system peripherals.
- Clock Speed
- The frequency at which a CPU executes cycles, measured in Hertz (Hz) or Gigahertz (GHz).
The Fetch-Decode-Execute Cycle
Every computer program breaks down into machine instructions processed in this continuous loop.
- 11. Fetch: The CPU reads the address in the Program Counter and fetches the next instruction from RAM into the instruction register.
- 22. Decode: The Control Unit splits the instruction into an opcode (the operation) and operand (the data), determining what action is required.
- 33. Execute: The ALU performs calculations or data transfers take place between registers and system memory.
- 44. Repeat: The Program Counter increments by one to point to the subsequent instruction, and the cycle repeats billions of times a second.
Simulating the Fetch-Decode-Execute Cycle
python# A simplified virtual CPU instruction emulator
memory = [
('LOAD', 5), # Load value 5 into register A
('ADD', 12), # Add 12 to register A
('PRINT', None) # Display register A
]
register_a = 0
pc = 0 # Program Counter
while pc < len(memory):
# 1. FETCH
instruction, operand = memory[pc]
# 2. DECODE & 3. EXECUTE
if instruction == 'LOAD':
register_a = operand
elif instruction == 'ADD':
register_a += operand
elif instruction == 'PRINT':
print('CPU Accumulator Output:', register_a)
# Advance Program Counter
pc += 1This Python simulation reflects how a real processor steps sequentially through instructions, decoding opcodes and updating internal registers.
Try it
Predict what this CPU emulation script will output to the terminal when executed.
pythonaccumulator = 10
instructions = [('ADD', 5), ('SUB', 3), ('MUL', 2)]
for opcode, val in instructions:
if opcode == 'ADD':
accumulator += val
elif opcode == 'SUB':
accumulator -= val
elif opcode == 'MUL':
accumulator *= val
print(accumulator)Challenge
Explain why a 3.5 GHz quad-core CPU might perform faster on video editing than a 4.0 GHz dual-core CPU.
Pick whichever way suits you — every mode earns the same bonus XP.
Write at least 40 more characters to submit.
Mark your own work
Guided walkthrough — 0/5 clues revealed
- Clue 1 locked — reveal it only if you get stuck.
- Clue 2 locked — reveal it only if you get stuck.
- Clue 3 locked — reveal it only if you get stuck.
- Clue 4 locked — reveal it only if you get stuck.
- Clue 5 locked — reveal it only if you get stuck.
Each clue costs 4 XP (never below 19 XP). You'd earn 38 XP right now.
Extension: Research CPU cache levels (L1, L2, and L3) and explain why cache is built directly onto the CPU die instead of just using standard RAM.