Computing GCSE's Revision - System Architecture

Purpose of the CPU

The purpose of the CPU is to execute instructions and perform calculations. It also helps to control the tasks and operations in a computer system.

What are the processes carried out in: Fetch

• CU copies memory address from the Program Counter to the MAR

• CPU copies the instruction stored in the MAR address to the MDR

• Increment(increase) the program counter to point to the address for the next instruction, ready for the next cycle

What are the processes carried out in: Decode

The instruction in the MDR is decoded by the CU (Control Unit). The CU may then prepare for the next step

What are the processes carried out in: Execute

The instruction is performed/repeated

CPU Components:

CU (CONTROL UNIT)

The CU is overall in control of the CPU. Its main task is to manage the fetching, decoding and execution of program instructions by following the fetch-execute cycle

It also controls the flow of data inside the CPU (in registers, ALU, Cache) and outside the CPU (E.G. RAM).

ALU (ARITHMETIC LOGIC UNIT)

• Does all the calculations

• completes simple addition, subtraction, multiplying, division, compares the size of numbers etc

• It performs logic operations for example - AND, OR, NOT + binary shifts

• Contains the accumulators register

CACHE

• Small amounts of high speed memory that helps to control instructions e.g the data in the CPU

• Stores regularly used data so that the CPU can access it quicker the next time it may be needed

REGISTERS

Tiny, super fast pieces of onboard memory inside the CPU, each with a specific purpose.

Registers in the CPU -

• MAR(Memory Address Register) - holds any memory address about to be used by the CPU. The address might point to data or a CPU instruction

• MDR(Memory Data Register) - Holds the actual data or instruction. This may have been fetched from memory, or be waiting to be written to memory.

• ACCUMULATOR - Stores/ holds the results of the calculations solved by the ALU.

• PC(PROGRAM COUNTER) - Holds the memory address of the next instruction.

How common characteristic of CPU’s affect their performance?

Clock Speed - Clock speed affects a CPU's performance by determining how many instructions it can process per second; a higher clock speed generally allows the CPU to execute more instructions, improving performance.

Cache Size - Cache size affects a CPU's performance by allowing it to store more frequently accessed data close to the processor, reducing the time needed to retrieve it from main memory and improving overall speed.

Number of Cores - The number of cores in a CPU affects performance by allowing it to handle more tasks simultaneously; more cores enable better multitasking and parallel processing, improving overall efficiency and speed.

Embedded systems -

What is an embedded system?

An embedded system is a small computer that forms part of a larger system, device or machine. They are dedicated to a specific task

What are the properties of embedded systems compared to general purpose computers:

They are -

• Lower in power consumption

• smaller in size

• rugged operating systems

• lower in costs per unit

Examples of embedded systems -

• traffic light

• factory equipment

• hospital equipment

• digital watches

• domestic appliances e.g dishwashers