J277 1.1.1,System Architecture

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Topic 1.1 Systems architecture
Sub topic1.1.1 – Architecture of the CPU
Lesson 1 of 9 - The purpose of the CPU

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Main computer components

CPU

Main

memory

Input

Output

Secondary

Storage

Open Ended

What is the purpose of CPU?

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CPU- Central Processing Unit

The processor is like
a brain.

The processor
controls all the
processing of the
computer. The
‘brains’ of the
computer

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The Processor

• Carries out instructions (executes

instructions).

• Processes data.
• Retrieves data/instructions from main

memory (fetches).

• Stores data into main memory once

executed.

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Fetch-execute cycle

Fetch

Decode

Execute

• FETCH – Fetches the

next instruction and
associated data to be
fetched from main
memory

• DECODE – Interprets

the instruction,
decodes it.

• EXECUTE – Performs

the instruction, it
executes it.

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Topic

Allocated time

Lesson 2 - Big picture

5 minutes

Lesson 2 – Activity 1

10 minutes

Lesson 2 – Activity 2

10 minutes

Plenary

5 minutes

Talking through lesson 2 slides

30 minutes

Lesson 3 – Big picture

5 minutes

Lesson 3 – Activity 1

10 minutes

Lesson 3 – Activity 2

15 minutes

Plenary

5 minutes

Talking through lesson 3 slides

25 minutes

Content

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Learning objectives

• To be able to describe the common CPU

components and their function.
▪ ALU (Arithmetic Logic Unit)
▪ CU (Control Unit)
▪ Cache
▪ Registers

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Keywords

• Processor
• Instruction
• Arithmetic Logic

Unit

• Control Unit
• Registers
• Accumulator

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CPU components

• The main components are called:

▪ Control Unit (CU)
▪ Arithmetic Logic Unit (ALU)
▪ Registers that help carry out the above

operations.

▪ Registers are immediate memory, which is very fast

temporary memory on the CPU to speed up access.

​What is the purpose of CPU?

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ALU

• Performs Arithmetic operations such addition, subtraction, multiply and division

• Logical operations

– Basic logical operations like AND/OR calculations.

What does Control unit do?

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Control Unit (CU)

• Co-ordinates the operations carried out

by the computer using control signals:
– hardware is monitored
– input/output of devices and response of

instructions to the CPU

– data flow is controlled within the CPU.

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• Sends control signals between the different

internal components:
▪ Memory Read
▪ Memory Write
▪ Hard Disk Drive Read
▪ I/O Write

Control Unit (CU)

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Activity 1

• Complete the activity about memory.

​What is Cache memory?

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Cache

• A memory cache is a memory that

is very fast and sits between the CPU and
main memory. It is faster to access than
RAM.

CPU

Cache
Main Memory

RAM

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Processors - cache size

• Processors have ‘cache’ – This is very

fast, and usually a very small amount of
memory on the processor chip.

• As programs are executing, the cache

holds commonly used instructions.

Multiple Choice

One purpose of the control unit is to…

• Send signals to control how data moves around the CPU

• To perform calculations in the CPU

• Control the loading of data from the hard disk to RAM

• To control peripherals using device drivers

Multiple Choice

Which of these is a function of the ALU?

• Control

• Cache

• Logic

• Fetch

Multiple Choice

Which of these is a function of the ALU?

• Cache

• Control

• Fetch

• Arithmetic

Multiple Choice

Which of these is a function of the ALU?

• Binary shift

• Speeds up the processor

• Bus check

• Stores frequently used instructions

Multiple Choice

This component performs calculations and logical comparisons.

• Arithmetic Logic Unit

• Control unit

• Memory unit

• Decode unit

Multiple Choice

One purpose of the cache is to…

• Hold frequently used instructions

• Decode instructions

• Enable the computer to run more programs than it has physical RAM to hold

• Keep the internal components of the CPU synchronised

Multiple Choice

The 'CU' is a component of the processor, what does CU stand for?

• Centralised unit

• Co-processor Unit

• Control Unit

• Central Unit

Multiple Choice

One purpose of the control unit is to…

• Keep the internal components of the CPU synchronised

• Remove the need for a hard disk so an SSD can be used instead

• Enable the computer to run more programs than it has physical RAM to hold

• Hold instructions that are frequently used to speed up the computer

Multiple Choice

One purpose of the control unit is to…

• Send signals down the address bus

• Decode instructions

• Execute instructions

• Store instructions in the RAM

Multiple Choice

Why does having a small cache increase the speed of the CPU?

• Programs do not need to be executed from the hard disk

• Frequently used instructions and data are already in the CPU and are not fetched again from RAM

• The cache is another register that can be used

• There is no need to transfer binary between the accumulator and memory address register

Multiple Choice

Why does having a very large cache not give a big performance increase for the CPU?

• Instructions are not always sequential

• A large cache would need to be made of lower grade, cost effective components

• A large cache is more unreliable

• It will take more time to fetch the data in a large cache

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CPU Registers

• PC (Program Counter)
• MAR (Memory Address Register)
• MDR (Memory Data Register)
• ACC (Accumulator)

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Main Memory - EEPROMM

• EEPROM – Electronically Erasable

Programmable Read Only Memory
– E.g. Sim cards/Flash memory
– Instructions/data can be erased electronically

and replaced.

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System buses

• A bus is a set of parallel wires connecting two or

more independent components of a computer
system in order to pass signals between them.

• The system is split into three separate buses:

– the data bus
– the address bus
– the control bus.

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Big picture

• Where are instructions that are currently in use

stored?

• What else is stored in main memory?
• How are instructions executed by the

processor?

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Activity 1 - Answers

PC

MDR

ALU

Accumulator

0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101

INP
Add 5
INP
STO 8
INP
ADD 8

Address bus

Data bus

​MAR

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Von Neumann architecture

• In the early 1940s, computers stored data

that was worked on in memory, the program
was not stored and they used paper tape,
punch cards or switches.

• The mathematician John Von Neumann and

colleagues had an idea to store the program
instructions and the data all in memory.

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Von Neumann architecture

An example of how the program and the
data are stored in the same memory to allow
data flow between memory and the
processor:

Program

Program

Program

Program

Data

Data

Data

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Processor - Registers

• PC – Program Counter

– Holds the location of the next instruction/data address in

main memory

• MAR – Memory Address Register

– The contents of the PC are copied here and then

transferred along the address bus

PC

MAR

Address bus

Main

Memory

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Processor - Registers

• Accumulator, holds the result of arithmetic or logical calculations from ALU

MDR- This holds the data fetched from main memory that is needed by the processor while executing instructions.

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Learning objectives

• To describe the importance of the Program

Counter in the Fetch-Execute cycle

• To understand how Fetch – Decode - Execute

works in more detail

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What happens in the FDE cycle?

• Fetch part of the cycle:

– Program counter is incremented for each

instruction of the program being executed.

– The contents of the Program Counter are put

into the MAR (Memory Address Register).

– The address is transferred along the address

bus to main memory (this address indicates
which part of memory to fetch the data/
instructions from.

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What happens in the FDE cycle?

• Fetch part of the cycle:

– The data/instruction that has been addressed is

transferred back to the processor along the data
bus.

– This is held in the MDR (Memory Data Register).

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What happens in the FDE cycle?

• Decode/Execute part

– The instruction is split into an Op-Code and

an Operand.

– The instruction is carried out by the ALU

(Arithmetic Logic Unit).

Multiple Choice

Which of these describes a classic von Neumann architecture?

• Data transfers and instruction fetches between memory can be done at the same time

• Uses a separate memory address for instructions and data

• Processor needs one cycle to complete an instruction

• Uses the same physical memory address for instructions and data

Multiple Choice

Which of these describes a classic von Neumann architecture?

• The processor only requires one cycle to complete an instruction

• Data transfers and instruction fetches from RAM cannot be performed at the same time

• Uses separate memory address space for instructions and data

• Data transfers and instruction fetches can be performed at the same time

Multiple Choice

The address of an instruction or data pointed at by the memory address register is sent to which register during a fetch operation?

• Accumulator

• Memory address register

• Program counter

• Memory data register

Multiple Choice

What is the purpose of the accumulator?

• To fetch the next instruction to calculate data

• To store the results of calculations

• To decode an instruction to calculate a total

• To execute a calculation

Multiple Choice

The value in the program counter is passed to which register during a fetch operation?

• Program counter

• Memory data register

• Accumulator

• Memory address regsiter

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Topic 1.1 Systems architecture
Sub topic 1.1.1 - Architecture of the CPU
Lessons 4-5 of 9 - Von Neumann architecture

Multiple Choice

What does the Memory Address Register (MAR) hold?

• Address of the instruction or piece of data to be fetched or stored

• Address of the next instruction to be executed

• The current instruction to be executed

• Data being transferred to and from main memory

Multiple Choice

What does the Memory Data Register (MDR) hold?

• Data being transferred to and from main memory

• Address of the instruction or piece of data to be fetched or stored

• The current instruction to be executed

• Address of the next instruction to be executed

Multiple Choice

Holds the address of the next instruction or the address of data to be fetched.

• Memory data register

• Accumulator

• Program counter

• Memory address register

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Keywords

• Von Neumann

architecture

• MAR (Memory Address

Register)

• MDR (Memory Data

Register)

• Program Counter
• Accumulator

• ALU (Arithmetic Logic Unit)
• CU (Control Unit)
• Cache
• Fetch/execute
• Buses

Multiple Choice

One purpose of the memory data register is to…

• Hold the results of calculations

• Hold data fetched from, or to be written to the memory

• Hold the address of data ready for use by the memory data register

• Hold the address of an instruction passed from the program counter

Multiple Choice

Holds data fetched from or written to the RAM.

• Program counter

• Memory data register

• Memory address register

• Accumulator

Multiple Choice

What does the program counter hold?

• Address of the instruction or piece of data to be fetched or stored

• Address of the next instruction to be executed

• The current instruction to be executed

• Data being transferred to and from main memory

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Lesson 5 – Activity 2

• FDE cycle