Computer Systems

2009-2010

Week 6: The Computer System

Alma Whitfield

2

What we shall do today The computer as a processor (Chptr 2, 3)

Input devices Processing Instructions Outputting

Computer components (Chptr 4) The BUS system Memory Secondary Storage

Computer system simulator (Chptr 14) Threebit

3

What is a computer ? Device that is told stuff INPUT Device that does stuff PROCESS Device that produces stuff OUTPUT Stuff ?

words pictures sounds numbers mouse movements joystick movements

4

Computer as processor Supermarket checkout

Stuff coming in

Stuff coming out

Stuff getting

processed

5

INPUT = WOOL

HUMAN BEING WITH KNITTING NEEDLES

OUTPUT = SOCKS

Human beings as processors Knitting

Stuff coming in

Stuff getting

processed

Stuff coming out

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INPUT=WOOL

HUMAN BEING WITHKNITTING NEEDLES

OUTPUT=SOCKS KNITTING PATTERN

Processors need instructions knit 2, purl 1

Instructions

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Another example Baking a cake

INPUT=INGREDIENTS

HUMAN BEING WITH KITCHEN

OUTPUT=CAKE RECIPE

Instructions

8

Aircraft controlled by computer

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Airbus A320 First “fly-by-wire” commercial

airliner introduced 1988 Virtually all commercial airlines now

use fly-by-wire

10

Computers as processors Aircraft computer

Stuff getting

processed

Stuff coming in

Stuff coming out

11

Computers need instructions

Human with

kitchen

Recipe

Compare with

Hardware

Software

Programs

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Early computers: Harvard Mark I

1944 55 feet long 8 feet high 5-tons Used by the US Navy for gunnery calculations In operation until 1959

PeopleComputer

13

Computer layout: old style

Everything plugged into processor Processor became known as

Central Processing Unit (CPU)

STORAGE DEVICE TO HOLD INSTRUCTIONS

PROCESSORINPUT

DEVICEOUTPUT DEVICE

INPUT OUTPUT

e.g. Keyboard e.g. Monitor

14

KEYBOARD CPU STORAGE DEVICE

MONITOR

INPUTOUTPUT

HARD DISC

Computer layout: new style

Bus(Bundle of wires)

Stuff coming in

Stuff coming out

Stuff getting processed

15

A desktop computer system: with fancy words

Input device

Output device

Processor inside system

box

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Input devices mouse

• scanner

• keyboard

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Output devices printer

• speakers

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Storage devices This is where we store instructions

STORAGE DEVICE TO HOLD INSTRUCTIONS

PROCESSORINPUT

DEVICEOUTPUT DEVICE

INPUT OUTPUT

e.g. Keyboard e.g. Monitor

19

Storage device problems CPU can perform millions of

instructions per second Storage device needs to keep up

the supply of instructions Modern computers need to store

thousands of programs

20

Solution Use two kinds of storage device:

smaller, faster, expensive part

larger, slower, cheaper part

Main memory - very fast (purely electronic), expensive.

RAM

Secondary storage – slower, (electro-mechanical), cheaper.

e.g. hard disc, CD

21

Solution Keep the programs on the

secondary storage Copy a program into main memory

when we want the CPU to run it

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Typical devices attached to the bus CD Drive Hard Disc Drive Floppy Disc Drive CPU RAM (Main memory) Monitor Keyboard Mouse

Can you classify them?

(a) input device

(b) output device

(c) storage device

(d) processor

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Inside the system boxPower supply

Fan on top of CPU

Main memory

DVD and CD Drives

Hard disc

Slots for cardse.g.

Network card

Graphics card

Floppy disc drive

Motherboard(contains the

bus)

24

CD Drive Compact Disc

CD is a plastic disc coated with special optical material

also known as CD-ROM (Read Only Memory)

Data is read from it using a laser light inside a CD Drive

Replaced now by DVD which has more storage capacity

25

Hard disc drive

taking the lid off hard aluminium discs

coated with magnetizable material

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Floppy disc drive Floppy disc is a cheap version of

the hard disc Made of plastic Low capacity Read by a floppy disc drive Very slow Rapidly becoming obsolete

27

CPU

Central processing unit carries out the instructions e.g. Intel Pentium e.g. AMD Pentium clone

Note gold plated legs

28

CPU

CPU installed with fan removed

29

CPU

CPU Socket on the

motherboard

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RAM (Main memory)

Where programs are stored while they are running

Main memory card in slot on motherboard

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RAM (Main memory) A main memory card

Note the gold plated connectors that will slot into

the computer’s bus on the motherboard

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What have we learned so far this week Computer systems consist of devices

attached to a bus Devices include:

processor storage devices input and output devices

Computers execute sets of instructions stored in programs

Programs are copied from Secondary Storage to RAM before execution of instructions

33

KEYBOARD CPU STORAGE DEVICE

MONITOR

INPUTOUTPUT

HARD DISC

Computer layout

Bus(Bundle of wires)

Stuff coming in

Stuff coming out

Stuff getting processed

34

Computer system simulator

35

KEYBOARD CPU STORAGE DEVICE

MONITOR

INPUTOUTPUT

HARD DISC

36

3-bit Simulator Demo A program is a collection of

instructions Instructions are stored in main

memory (RAM) Instructions are fetched into CPU

(Central Processing Unit) Instructions are executed by CPU

possibly resulting in new data being placed in main memory or moved elsewhere

37

3-bit Instruction fetching and executing

done repeatedly This is known as the fetch-execute

cycle

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A computer instruction Instructions are represented at patterns of

0s and 1s 0s and 1s are binary digits binary digit abbreviated to bit

In 3-bit each instruction consists of 8 bits In a standard PC, instructions consists of 32 bits In a higher end machine, instructions can

consist of 64 bits In all cases an instruction has two parts:

operation code (usually abbreviated to op-code)

operand

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A computer instruction op-code

what the instruction will do e.g. add, multiply, move like a verb in English grammar

operand what the instruction will do it to e.g a number like a noun in English grammar

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A computer instruction in 3-bit

0 1 0 0 1 1 0 1

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A computer instruction in 3-bit

0 1 0 0 1 1 0 1

8 bits

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A computer instruction in 3-bit

0 1 0 0 1 1 0 1

8 bits

op-code

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A computer instruction in 3-bit

op-code is 3 bits long – hence the name 3-bit for the simulator

operand is 5 bits long

0 1 0 0 1 1 0 1

8 bits

op-code

operand

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8 available 3 bit op codesMnemonic

Brief meaning

000 STP halt

001 LDD load accumulator direct

010 LDI load accumulator immediate

011 STD store accumulator direct

100 ADD add to accumulator

101 SUB subtract from accumulator

110 JMP jump to next instruction

111 JEZ jump to next instruction if contents of accumulator is zero

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8 available 3 bit op codes Instructions carry out very simple

operations such as adding, storing Simplicity is not just a feature of 3-

bit – true for all kinds of computer system

A program such as a word processor may have millions of them combined to do useful things

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Lets look at the more obvious ones first! ADD – Will add the contents of the

memory address specified to whatever is in the ACCUMULATOR

SUB - Will subtract the contents of the memory address specified from the ACCUMULATOR

STP – Will stop the execution of the instructions

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The CPU Every CPU has at least 3 registers

Instruction register

(IR)

Accumulator

(AC)

Program counter

(PC)

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The CPU Every CPU has at least 3 registers

Instruction register

(IR)

Stores the current instruction when it is fetched from the main memory

Stored here so that the CPU can identify it and carry out the appropriate actions

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The CPU Every CPU has at least 3 registers

Accumulator

(AC)Stores data values fetched from main memory that may be:

• added to

• subtracted from

• moved elsewhere in main memory

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The CPU Every CPU has at least 3 registers

Program counter

(PC)

Contains of the address of the next instruction to be fetched from main memory when current instruction has completed its execution

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3-bit CPU Controls IR - clear the Instruction Register AC - set the Accumulator to 0 PC - set the Program Counter to 0 Run - start the fetch execute cycle Step - fetch and execute the next

instruction (whose address is in the PC)

Stop – stop fetching and executing instructions after current instruction has completed

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Main Memory Program instructions are stored in

main memory during program execution

Main memory was loaded from hard disk when the program was invoked

Instructions are fetched one at a time from main memory into the CPU to be executed

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3-bit Main Memory Our simulator has 32 Memory Locations

operands have 5 bits. These can be used to represent 32 different patterns of 0 and 1. Each different pattern can be used as an address to uniquely identify a memory location

Each location can store 8 bits i.e. one instruction 8 bits is also known as 1 byte

There is also enough memory in each location to store the ASCII code for a character

ASCII is an 8 bit code used to represent alphabetical characters, digits and other items

e.g. 65 is ASCII code for ‘A’ Real Memory has billions of locations.

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3-bit Main Memory Notice that some

of the memory locations have red lines coming from them.

This is to signify that that section of the memory has been reserved for use with the Video Card

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Back to the Instructions ADD – Will add the contents of the

memory address specified to the accumulator

SUB - Will subtract the contents of the memory address specified from the accumulator

It is very important to realise now that when you specify ADD x. The x refers to a memory location’s address. Not a value!

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For Example Question –

What will the AC become once the program has finished executing?

Address 16 holds value 10.

Address 10 holds value 2

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For example Demo

ADD 16 causes contents of location 16 (which is 14) to be placed in the Accumulator

ADD 10 causes contents of location 10 (which is 5) to be added to contents of the Accumulator giving 19

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Lets take a look at the other instructions LDD x

Copies contents of location x into the AC – loading the accumulator

STD x Copies the contents of the AC to

location x – storing the accumulator

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LDD Given the

program in memory, what will the accumulator’s contents be after the program has executed?

Answer 25

60

STD Given the

following program.

What will happen?

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LDI LDI is similar to LDD with one key

difference.

LDI 27 will load 27 into theaccumulator

LDD 27 will load contents of address

27 into the accumulator