Sys Arch L1. The purpose of the CPU: FDE and CPU performance

The course

Unit

Assessment

Weighting

Computer systems

Exam

80 marks

1 hour 30 minutes

(no calculators)

50%

Computational thinking,

algorithms and
programming

Exam

80 marks

1 hour 30 minutes

(no calculators)

50%

Practical programming

During the course, all students must be given the
opportunity to undertake a programming task, either
to a specification or to solve a problem.

OCR GCSE

(J277)

SLR1.1 Systems architecture

Learning Intention

To know the purpose of the CPU
=The fetch-decode-execute cycle

AND

The factors that affect CPU

performance:

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

What is a computer?

INPUT

An electronic device that takes input…

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

INPUT

PROCESS

An electronic device that takes input…processes data…

What is a computer?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

INPUT

PROCESS

OUTPUT

An electronic device that takes input…processes data…and delivers output.

What is a computer?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

INPUT

5
PROCESS

OUTPUT

An electronic device that takes input…processes data…and delivers output.

What is a computer?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

INPUT

5

PROCESS

Multiply by 2
OUTPUT

An electronic device that takes input…processes data…and delivers output.

What is a computer?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

INPUT

5

PROCESS

Multiply by 2

OUTPUT

10

An electronic device that takes input…processes data…and delivers output.

What is a computer?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

INPUT

PROCESS

OUTPUT

What is a computer?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

INPUT

PROCESS

OUTPUT

What is a computer?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

INPUT

PROCESS

OUTPUT

What is a computer?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

INPUT

PROCESS

OUTPUT

To do this, computers follow a set of instructions known as a program.

What is a computer?

Open Ended

What is the definition of a computer? Describe and give examples each of the three parts of the definition.

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

INPUT

5

PROCESS

Multiply by 2

OUTPUT

10

An electronic device that takes input…processes data…and delivers output.

What is a computer?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

INPUT

PROCESS

OUTPUT

What is a computer?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

Print out image of computer

Put up image on white boards.

A)
In groups label the image with each components name.
Explain what they do

You can add additional information about any of them,

1 person/group are Researchers. They stay at the computer/s
and tell scribe(s) what do write
I person/group acts as Scribes
Also 1 person will be Speaker to describe what you have written
up when asked

B)
Vote on who has best the best mind map but you must
explain why
C)

Improve you own mindmap.

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

Print Out.

Put up on
white
boards.

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

MEMORY (RAM)

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

MEMORY

CPU

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

MEMORY (RAM)

CPU
(+CPU
FAN)

POWER
SUPLY

EXPANSION
CARD SLOTS

COOLING

FANS

HARD DRIVES
(SECONDARY

STORAGE)

POWER
LEADS
DATA
CABLES

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

Fetch

Decode

Execute

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

The fetch-decode-execute cycle

Fetch

Decode

Execute

Fetch stage

• Fetch the next instruction from

main memory (RAM).

• Bring it back into the CPU.

Open Ended

What happens in the fetch stage?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

The fetch-decode-execute cycle

Fetch

Decode

Execute

Fetch stage

• Fetch the next instruction from

main memory (RAM).

• Bring it back into the CPU.

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

The fetch-decode-execute cycle

Fetch

Decode

Execute

Decode stage

• Inspect the instruction to find

out what needs to be done.

• This is carried out by a CPU

component Control Unit

Open Ended

What happens in the decode stage?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

The fetch-decode-execute cycle

Fetch

Decode

Execute

Decode stage

• Inspect the instruction to find

out what needs to be done.

• This is carried out by a CPU

component Control Unit

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

The fetch-decode-execute cycle

Fetch

Decode

Execute

Execute stage

• Carry out the instruction,

which could be:

o Performing a calculation.

o Or other things

o Going back to main

memory and fetching data
e.g. a variable in a program

o Storing information back in

main memory e.g. a
variable in a program

Open Ended

What happens in the execute stage?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

The fetch-decode-execute cycle

Fetch

Decode

Execute

Execute stage

• Carry out the instruction,

which could be:

o Performing a calculation.

o Or other things

o Going back to main

memory and fetching data
e.g. a variable in a program

o Storing information back in

main memory e.g. a
variable in a program

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

The fetch-decode-execute cycle

Fetch

Decode

Execute

How often do you
think this happens?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

The fetch-decode-execute cycle

Fetch

Decode

Execute

On a modern CPU, the fetch-
decode-execute cycle occurs
billions of times per second.

Clock speed

3Ghz processor = 3 billion cycles
per second.

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

Print out image of computer

Put up image on white boards.

A)
In groups mindmap
Fetch , Decode , Execute Cycle

1 person/group are Researchers. They stay at the computer/s
and tell scribe(s) what do write
I person/group acts as Scribes
Also 1 person will be Speaker to describe what you have written
up when asked

B)
Vote on who has best the best mind map but you must
explain why
C)

Improve you own mindmap.

Fetch

Decode

Execute

​

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

A computer is an electronic device that:

•takes input;

•processes data; and

•delivers output.

The purpose of the CPU is to fetch, decode and execute instructions.

Fetch

•Fetch the next instruction from main memory (RAM).

•Bring it back into the CPU.

Decode

•Inspect the instruction to find out what needs to be done. This is carried out by the control Unit

Execute

•Carry out the instruction, which could be:

o Going back to main memory and fetching data.
o Performing a calculation.
o Storing information back in main memory.

OCR GCSE

(J277)

SLR1.1 Systems architecture

Learning Intention

To know the purpose of the CPU
=The fetch-decode-execute cycle

AND

The factors that affect CPU

performance:

OCR GCSE

(J277)

SLR1.1 Systems architecture

Learning Intention

The factors that affect CPU

performance:

OCR GCSE

(J277)

What are the factors
that affect CPU
performance?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

How common characteristics of CPUs affect their performance

Clock speed

•Measured in the number cycles
per second – or hertz (Hz).

•Modern processors operate at
billions of cycles per second – or
gigahertz (GHz).

•3.2GHz clock speed = 3.2 billion
instructions fetched per second.

• How does clock speed relate

to the Fetch, Decode and
Execute Cycle?

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

Fetch

Decode

Execute

Clock speed

1 pulse from the clock
= 1 Cycle
= 1 Fetch Decode and Execute Cycle

Open Ended

How does clock speed relate to Fetch Decode and Execute Cycle? Why does clock speed affect CPU performance?

​A faster clock speed means more FDE cylce per second which means more instructions are processed per second so improving CPU performance

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

How common characteristics of CPUs affect their performance

Clock speed

•Temporary storage of data and
instructions being read from and
written to main memory (RAM).

•Located on or very near the CPU.

•Stores copies of recent data and
instructions.

•Often these instructions may
need to be carried out millions of
time (e.g. looping over each pixel
of image to make it lighter)

•Much quicker to read from than
main memory (RAM).

•Reduces the need to go and get
instructions and data from
memory, saving time.

Cache size

• Frequently used instructions are

fetched must faster from Cache than
from RAM improving CPU
performance.
A larger cache reduces how often instruction must be fetched from RAM as more instructions can be store in larger cache

Open Ended

How does cache size affect CPU perfomance?

​•Frequently used instructions are fetched must faster from Cache than from RAM improving CPU performance.

•A larger cache reduces how often instruction must be fetched from RAM as more instructions can be store in larger cache

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

How common characteristics of CPUs affect their performance

Clock speed

•A core is, in very simple terms, a
complete copy of a CPU.

•A quad-core processor has four
separate processing units.

•Each processing unit has its own:

oRegisters

oALU

oAccumulator

oControl unit, etc.

•CPUs with multiple cores have more
power to run multiple programs at
the same time.

Cache size

Number of cores

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

How common characteristics of CPUs affect their performance

Clock speed

•Doubling the number of cores
won’t double processing speed.

•Cores need to communicate with
each other, which takes time.

•Many programs are not designed
to make use of multiple cores.

•BUT these days most programs
can

•And doubling cores will nearly
double processing in some
circumstances………..

Cache size

Number of cores

Each core performs Fetch Decode Execute.
So 2 cores does double FDE, but….
Sometimes cores have to
communicate with each
other...
Other factors affect performance…
Some programs can only use 1 core

Open Ended

Why does having more cores improve CPU performance?

​More cores means more FDE cycles, so more instructions processed per second, which improves CPU performance

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

Print out image of computer

Put up image on white boards.

A)
In groups mindmap
Factors that affect performance

1 person/group are Researchers. They stay at the computer/s
and tell scribe(s) what do write
I person/group acts as Scribes
Also 1 person will be Speaker to describe what you have written
up when asked

B)
Vote on who has best the best mind map but you must
explain why
C)

Improve you own mindmap.

How common characteristics of

CPUs affect their performance

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

A CPU’s overall performance can be affected by several factors. The three most important factors are:

Clock speed

•Measured in the number of cycles per second or hertz (Hz).

•3.2GHz clock speed = 3.2 billion instructions fetched per second – in theory.

Cache size

•Temporary storage of data and instructions being read from and written to main memory (RAM).

•Stores copies of recent data and instructions.

•Much quicker to read from than main memory (RAM).

•Reduces the need to go and get instructions and data from memory, saving time.

Number of cores

•A core is a complete copy of a CPU.

•A quad-core processor has four separate processing units, each with its own registers, ALU, accumulator and CU.

•Doubling the number of cores won’t normally double processing speed but it might nearly do so in ideal
conditions. There are double the FDE cycles, just some cycles will be….

•Need to for cores to communicate with each other, which takes time

•And some programs are not designed to make use of multiple cores.

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle

OCR GCSE (J277)

SLR1.1 Systems architecture| The purpose of the CPU: The fetch-decode-execute cycle