Computer Science A1.1.1 Simple

CPU

Runs program instructions. Example: it processes the instructions needed to calculate 5 + 3.

ALU

Does calculations and logical operations. Example: it calculates 5 + 3 = 8.

CU

Controls the CPU. It decodes instructions and tells other components what to do.

Register

A tiny, very fast storage place inside the CPU.

PC

Holds the address of the next instruction. Example: PC = 500 means the next instruction is at address 500.

IR

Holds the instruction being used now. Example: IR = ADD.

MAR

Holds the memory address being accessed now. Example: MAR = 500 means the CPU is accessing memory location 500.

MDR

Holds data or an instruction moving between RAM and the CPU. Example: MDR = 120 after RAM sends the value 120.

AC

Holds a calculation result temporarily. Example: after 5 + 3, AC may hold 8.

PC vs MAR

PC = where the next instruction is. MAR = which memory location is being accessed now.

IR vs MDR

IR = instruction being processed now. MDR = data/instruction moving to or from RAM.

Address bus

Carries an address from the CPU to RAM. Think: WHERE?

Data bus

Carries data or instructions between CPU and RAM. Think: WHAT?

Control bus

Carries control signals such as READ and WRITE. Think: WHAT SHOULD HAPPEN?

Fetch order

PC → MAR → RAM → MDR → IR.

Single-core

One processing core.

Multi-core

Several processing cores that can work in parallel.

Co-processor

A specialised processor that helps the CPU. Example: a GPU handles graphics and parallel calculations.

Why is multi-core not always much faster?

The program must be able to split its work between multiple cores.