The Central Processing Unit (CPU) is the component inside every computer, games console, laptop, and phone that does the actual "thinking." Its one job — its — is to fetch an instruction, decode it (work out what it means), and execute it (actually do it).

Every device follows the same input → process → output pattern:

This is one of the most commonly confused pairs in the whole topic, so let's be really precise about it.

A microprocessor is a type of integrated circuit contained on a single chip. It's the actual physical hardware you could pick up and hold. It the CPU, and often other components too, such as:

• Memory controllers
• I/O interfaces

Proposed by mathematician John Von Neumann in the 1940s, this architecture is the blueprint that most general-purpose computers are still built on today. It describes how the memory, input/output devices, and processor all connect and communicate.

The Von Neumann architecture is built from three core parts:

• Control Unit (CU) — the "manager" that coordinates everything
• Arithmetic Logic Unit (ALU) — does the maths and logic
• Registers — tiny, super-fast storage spots inside the CPU

...plus buses, the "roads" that carry data between all these parts.

aArithmetic Logic Unit (ALU)

The ALU is the calculator of the CPU. It performs two types of operation:

• Arithmetic operations — addition, subtraction, etc.
• Logical decisions — comparisons like IF X > 5 THEN...

bControl Unit (CU)

The CU is the coordinator — it doesn't do the maths itself, but it directs traffic. It:

• Coordinates how data moves around the CPU by sending control signals
• Decodes the instructions that have been fetched from memory

Registers are extremely small, extremely fast memory locations built inside the CPU itself (not in RAM). They hold small amounts of data that the CPU needs right now, as part of the fetch-decode-execute cycle. Each register has one specific, dedicated job — this matters a lot for exams, because a common trap is describing what a register stores explaining or it's used.

Components inside the CPU (and the wider computer) are connected by buses. A bus is simply a set of parallel wires through which data or signals travel from one component to another.

This is the heart of the whole chapter. The Fetch-Decode-Execute Cycle is the process the CPU runs through billions of times per second to make a computer actually do anything. Every single thing your computer does — from moving a mouse cursor to rendering a video game — boils down to this cycle repeating over and over.

1Fetch Stage — grab the instruction

Step by step, here's exactly what happens:

1. The Program Counter (PC) holds the address of the next instruction to be fetched.
2. That address is copied into the Memory Address Register (MAR).
3. The address is sent along the address bus and waits for a signal from the control bus.
4. The Control Unit (CU) sends a signal along the control bus to main memory.
5. The instruction is retrieved and transferred to the Memory Data Register (MDR) via the data bus.
6. A copy of the instruction is stored in the Current Instruction Register (CIR).
7. The PC increments by 1, so it now points to the next instruction to be fetched.

2Decode Stage — work out what it means

The CPU now needs to work out what is required from the instruction it just fetched. Every instruction is split into two parts:

3Execute Stage — actually do it

Finally, the CPU carries out the instruction it fetched and decoded. Depending on what the instruction was, this could mean:

• Performing a calculation (using the ALU)
• Storing a result or data back into main memory (RAM)
• Going back to main memory to fetch data from a different location

Not all CPUs perform equally — three key characteristics determine how fast and powerful a CPU is:

• Clock Speed
• Cache Size
• Number of Cores

aClock Speed

The clock speed is measured in Hertz (Hz) and measures the number of fetch-decode-execute cycles that can happen in 1 second. The faster the clock speed, the more instructions can be fetched and executed per second.

Modern computers measure clock speed in Gigahertz (GHz) — where "giga" means billion.

bCache Size

Cache is very small, very fast memory located on or extremely close to the CPU. It's used as temporary storage that keeps a copy of frequently used instructions and data ready to go.

• The larger the cache, the more frequently-used items can be stored there.
• This means the CPU has to complete fewer fetch cycles from RAM — which is much slower to access than cache.
• Result: better overall performance, because reading from cache is significantly faster than reading from RAM.

cNumber of Cores

A core works like it is its own mini-CPU. A processor with multiple cores has multiple separate processing units that can each fetch, decode, and execute instructions at the same time (this is called parallel processing).

Each core in a multi-core processor has its own:

• Control Unit (CU)
• Arithmetic Logic Unit (ALU)
• Accumulator (ACC)
• Registers

An instruction set is a list of all the commands that a particular CPU can process. Each command has a corresponding binary code, called machine code.

Each instruction is also given a mnemonic — a short, human-readable label (like ADD or SUB) that indicates what the instruction does. After the CPU decodes an instruction, it matches the binary pattern to an entry in its instruction set, so it knows exactly what operation to carry out during the execute stage.

An embedded system is a computer system built to perform one dedicated function, housed inside a larger mechanical device. Unlike a laptop or phone (which can run all sorts of different programs), an embedded system is designed to do just one job, and do it well.

Classic examples that examiners expect you to know:

• Heating thermostats
• Hospital equipment
• Washing machines
• Dishwashers
• Coffee machines
• Satellite navigation systems
• Factory equipment
• Security systems
• Traffic lights

Properties of an embedded system:

• Small in size
• Use less power than a general-purpose computer
• Have a lower cost