4.4 Systems Architecture/4.5 The CPU and Fetch-Decode-Execute cycle

Central Processing Unit (CPU)

Processes and executes instructions

What is stored in the RAM?

Program instructions and data

Registers

Stores small amounts of data used when executing/processing instructions to be quickly accessed

Arithmetic Logic Unit (ALU)

Performs arithmetic, logical and shift operations and compares values stored in the registers

Control Unit (CU)

Controls and coordinates the execution of instructions (per second) in the CPU by decoding instructions

Clock

Controls and synchronises all CPU operations/number of executed instructions by sending regular electronic pulses

Cache

Stores frequently used data and instructions in memory for fast access

Clock frequency (unit)

Number of clock cycles per second (Hz)

How does clock speed affect CPU performance

The greater the clock speed, the more FDE cycles/executed instructions per second, increasing peformacne

How does cache size affect CPU performance

A larger cache reduces the need for the CPU to access the RAM, which is slower to access, improving the speed of performance and allowing more data to be held

How does the number of processors affect CPU performance?

It allows more instructions to be executed simultaneously per second, and different cores can deal with different types of instructions

Rank types of memory/storage by capacity (largest first)

Secondary storage, RAM, cache, registers

Buses

Used to transfer data/instructions between components in the CPU

Address bus

Stores the address of the memory/device controller location

Data bus

Carries data throughout the computer system

Control bus

Carries control signals (from the CU) to control all CPU activities

How is main memory used

It stores data/instructions whilst a program is being executed. Every memory location has a unique address so it can be found later when needed

2 Disadvantages of cache

Expensive, has a low capacity

What happens in the fetch part of the cycle

The instruction is fetched from the main memory

What happens in the decode part of the cycle

The control unit decodes the instruction to work out what it is

What happens in the execute part of the cycle

The instruction is carried out

What could occur in the execute part of the cycle

Data could be fetched from or a value may be stored in the main memory

Disadvantage of a higher clock speed

More power consumption, making it less efficient so the CPU may run hotter