System Arch AQA Lesson 1

GCSE Computer Science

Computer Architecture

Systems Architecture

Hardware (concept)

Hardware is the physical parts that
makes up your computer system
like the printer, CPU,
motherboard, monitors and
printers.

The Von Neumann architecture describes a system where the CPU
runs programs stored in memory (instructions and data together).

Von Neumann architecture

Processor (concept)

The Central Processing Unit is
often called the brain of the
computer.

The purpose of the CPU is to
process data.

It is where all the searching,
sorting, calculating and decision
making takes place

Also known as Immediate Access Store (IAS), this is used to store data
and instructions before being sent to the processor. The word main is
used to distinguish it from external mass storage devices such as disk

drives.

Examples of main memory (or primary memory) are ROM and RAM.

Main memory

Multiple Choice

What are the physical components of a computer system called?

• Hardware

• Software

Multiple Choice

CPU stands for central ____ unit.

• Protection

• Packet

• Plasma

• Processing

Multiple Choice

What does RAM stand for?

• Read All Memory
• Random Always Memory
• Read Access Memory
• Random Access Memory

Multiple Choice

What does the CPU do?

• Provides electricity
• Executes Instructions
• Stores Files
• Connects everything together

Multiple Choice

What is this component called?

• CPU

• Hard Drive

• RAM

• Motherboard

Open Ended

What does Von Neumann Architecture state?

CPU components

Control Unit

PC

MAR

MDR

CIR

SR

ACC

ALU

The CPU is made up
of lots of different
components. You
need to know what

each one is for . . . . so
let's look at them one

at a time.

IR

CPU components

Control Unit

PC

MAR

MDR

CIR

SR

ACC

ALU

Program Counter

Holds the memory
address for the next

instruction to be

executed

IR

CPU components

Control Unit

PC

MAR

MDR

CIR

SR

ACC

ALU

Memory Address

Register

Holds any memory
addresses which are
about to be used by

the CPU

IR

CPU components

Control Unit

PC

MAR

MDR

CIR

SR

ACC

ALU

Memory Data

Register

Holds the actual data
or instruction that has

been fetched from

memory OR a

something that is

waiting to be written

to memory

IR

CPU components

Control Unit

PC

MAR

MDR

CIR

SR

ACC

ALU

Current

Instruction

Register

Holds a copy of the
instruction that is
currently being

executed
IR

CPU components

Control Unit

PC

MAR

MDR

CIR

SR

ACC

ALU

Status Register

Is an 8-bit number
where each bit is a

flag to the CPU about
its current operation
e.g. positive, negative

numbers, carry

IR

CPU components

Control Unit

PC

MAR

MDR

CIR

SR

ACC

ALU

Interrupt Register

It is a type of status
register which stores
details of any signals

that have been
received by the

processor from other
components attached
to it e.g. the printer

IR

CPU components

Control Unit

PC

MAR

MDR

CIR

SR

ACC

ALU

Control Unit

This directs the
operations of the
processor and tells

the other

components in the
CPU how to respond

IR

CPU components

Control Unit

PC

MAR

MDR

CIR

SR

ACC

ALU

Accumulator

This stores the

intermediate results
of calculations in the

ALU

IR

Memory

Fetch, decode, execute cycle (detailed)

1LOAD 5

2ADD 6

3STO 7

4

512

68

7

8

9

10

Control Unit

PC

1

MAR

1

MDR

CIR

SR

ACC

ALU

Buses

Internal

Address

Data

Control

Step 2
The contents of the
MAR are sent to
memory via the
address bus

There is also a read
command sent to
memory via the
control bus

MAR → Cache

Read

1

IR

Memory

Fetch, decode, execute cycle (detailed)

1LOAD 5

2ADD 6

3STO 7

4

512

68

7

8

9

10

Control Unit

PC

1

MAR

1

MDR

LOAD 5

CIR

LOAD 5

LOAD 5

SR

ACC

ALU

Buses

Internal

Address

Data

Control

LOAD 5

Step 7

The instruction might say LOAD 5 to us on this example but in reality it would actually be a machine

code instruction e.g.

0010101010101010101010100101010101010101010101010010101010101010101010101010101010

The control unit would split this instruction into three parts:

The CU would then lookup the opcode in the CPU’s instruction set to find out what type of instruction it

is. In our example we have said the code: 001010101010 must mean LOAD

It would then look at the addressing mode to find out if the operand is data or a memory address.

Immediate addressing: This is where the operand is actually data

Direct addressing: This is where the operand is another memory address

Opcode

Addressing Mode

Operand

CPU components

Control Unit

PC

MAR

MDR

CIR

SR

ACC

ALU

Arithmetic Logic

Unit

It is an electronic

circuit (combination
of logic gates) which

can perform

arithmetic on binary

numbers

IR

Cache & registers

Registers are either dedicated or
general purpose small areas of
memory on the CPU. These are the
fastest areas of memory in your
computer but they are also the
smallest.

Cache are again areas of memory
on the CPU that are used to store
the next instructions that need to
be executed.

Fetch, decode, execute cycle (basic)

The basic operation of a computer
is called the ‘fetch-execute’ cycle.

The computer fetches the
instruction from its memory and
then executes it.

This is done repeatedly from when
the computer is booted up to when
it is shut down.

Fetch

Decode

Execute

The first step the fetch-execute
cycle carries out is fetching the
instruction.

The CPU fetches an instruction
from the main memory and stores
it in the CPU temporary memory.

Fetch

Decode

Execute

Fetch, decode, execute cycle (basic)

Once the instruction has been
fetched, the CPU will need to
understand the instruction to
action it.

This is called decoding.

Fetch

Decode

Execute

Fetch, decode, execute cycle (basic)

When the instruction has been
decoded, the CPU can carry out the
action that is needed. This is called
executing the instruction.

The CPU is designed to understand
a set of instructions

Fetch

Decode

Execute

Fetch, decode, execute cycle (basic)

Open Ended

What is the name of the cycle that a computer performs when getting instructions?

Open Ended

Explain in detail the FDE cycle

CPU performance (concept)

There are 3 factors that can affect
the performance of the CPU;

● number of cores
● clock speed
● cache size

​https://student.craigndave.org/videos/gcse-aqa-slr1-the-performance-of-cpus

Multiple Choice

The purpose of the CPU is to ____. 

• Process data and instructions

• Store data

Multiple Choice

A CPU can contain one or more processing units. Each unit is called a ___.This is one of the  key factors about CPU architecture that affect its performance.

• Core
• Intel
• AMD
• Megabite

Multiple Choice

This term refers to the number of F-D-E cycles PER SECOND carried out by the CPU

• Cache size

• Core

• Intel

• Clock Speed

Multiple Choice

____ is a small amount of memory which is a part of the CPU - closer to the CPU than RAM.

• Cache
• Core
• Intel
• Clock speed

Multiple Choice

Describe the purpose of the ALU within the CPU

• Carrying out arithmetic i.e. calculations and logic comparisons i.e. is one number larger than another

• Controls the flow of data around a computer. Fetches and decodes using system buses

• Stores frequently accessed data

• Stores programs, data and instructions currently in use

Multiple Choice

Describe the purpose of the Control Unit within the CPU

• Fetches the data/instructions, decodes it and sends the signals via buses to the hardware components.

• Carries out calculations and comparisons, to see if one value is larger than another.

Open Ended

Explain how the number of cores affects the CPU

Open Ended

Explain how the amount of cache affects the CPU

Open Ended

Explain how the clock speed affects the CPU

Embedded systems are computers built into other devices like TVs,
microwaves and dishwashers.

These systems are often used as control systems for hardware e.g. a
dishwasher controls the water pumps, cycles, release mechanisms
based on the time and temperature readings.

Embedded system

You need to be able to recognise the two types of system and also
know some key attributes for each.

Embedded vs non-embedded

system

Embedded

Non-embedded

Programs typically written in

Assembly language

High-level language

Usually dedicated task amont

1

Many

Complexity of design

Medium

Hard

Long term efficiency

Good

Medium

Flexibility

Poor

Good

Multiple Choice

True or False, Embedded Systems use Microprocessors to control specific functions relating to the device they are installed on?

• True

• False

Multiple Select

How many of the following are examples of Embedded Systems?

• Mobile Phone

• Washing Machine

• Microwave

• Fridge

• Laptop

Multiple Select

How many of the following are not examples of Embedded Systems?

• Mobile Phone

• Washing Machine

• Microwave

• Fridge

• Laptop

Multiple Choice

True or False, all embedded systems need an operating system to enable them to function correctly?

• True

• False