Computer structurepowerpoint

CS Topic 2 - Computer Structure v2 1
Here is a diagrammatic representation of a computer
system. Note the direction of data flow between the
different parts of the system.
Input Output
Backing
Storage
Main Memory
RAM & ROM
Processor

There are three sets of electrical lines which connect all the
parts.
The parts which go together to make up the computer system
need to be able to communicate with one another.
These sets of lines are called buses.

The Processor "sees" all the other parts of the computer,
(ROM & RAM, Backing storage, Input and Output devices)
as one continuous block of locations each with its own
unique address.
Addressability
Each location has a different binary number which identifies it.
This is its address.
Giving every part of the computer a unique address allows
the processor to communicate easily with any other part of
the computer system and is known as addressability.

The three buses which carry signals around the computer
system each have their own particular function.
1. Address Bus
This is used by the Processor to indicate which location has
to be accessed.
It is a one-way bus from the processor as the processor
dictates all movement of signals.
The number of lines on the address bus determines the
maximum amount of memory locations which can be accessed.

A 2 line address bus would allow addresses.
2 lines
00
01
10
11
because 22
= 4.
because 23
= 8.
X lines on the address bus allows addresses.
A 24 line address bus allows
224
= 16Mb of addresses.
A 32 line address bus allows
232
= 4Gb of addresses.
4
A 3 line address bus would allow addresses.8
3 lines
000
001
010
011
100
101
110
111
2x
Address bus width and addressable memory

the size of each memory location.
Calculating maximum memory size
To calculate maximum memory size we need to consider two
elements:
the number of memory locations.(addresses)
The following are assumed in the Higher course.
The size of each memory location
= data bus width
= memory word size.

Consider the following example:
Calculate the maximum memory size of a computer with a
24 bit address bus and a memory word size of 16 bits.
A 24 bit address bus allows 224
addresses = 16 MB of addresses.
Each address contains 16 bits = 2 bytes
Maximum memory size =16 MB x 2 bytes = 32 MB

2. Data bus
This is the bus which is used to transfer the actual
data to and from the locations.
It is a two-way bus as data may be going to the processor
(Read) or coming from the processor(Write).

Control bus
With both the data and the address buses, each of the lines
operate together, in parallel, at the same time, as a unit.
e.g. It would not make sense to look at one of the lines on
the address bus.
You have to look at them all to see the full address.
The Control bus is a collective name for a number of discrete
lines each of which has a different function and operates at
different times.
They are best viewed as a number of individual lines.

3.Clock line
This line sends a regular series of pulses at a speed
measured in
We’ll start with three of the lines on the control bus:
1. Read line
The processor activates this line to show that it wants to be
sent (read) data from another part of the computer system.
2. Write line
The processor activates this line to show that it wants to
send (write) data to another part of the computer system.
Hertz(Hz).
Every event in the computer is timed to take place at
particular points within the on/off cycle of each pulse.
It controls the speed and timing of all operations in the computer.

Other lines on the control bus include:
4. Reset line
This line resets (clears the contents of all) the registers
inside the processor. This prepares it for carrying out a
new task.
5. Interrupt line
Devices send a signal in on this line to interrupt the
processor when they need attention.
e.g. the printer may be out of paper.

The two most basic operations which the processor carries
out are and
The processor works on data which is stored in main memory.
It therefore spends a lot of time transferring data to and from
the main memory.
memory read memory write.
The processor uses the three buses to communicate
with the main memory.

Memory Read
1. Processor copies the required address onto the address bus.
2. Processor activates the read line on the control bus.
3. Memory controller copies the data from the required address
onto the data bus and the data is transferred from the main
memory to the processor.
Memory Write
1. Processor copies the required address onto the address bus.
3. Processor activates the write line on the control bus.
4. Memory controller copies the data from the data bus and
puts it into the required memory location.
2. Processor copies the data onto the data bus.

The following components are generally found inside a
Processor.
Register
Arithmetic &
Logic unit. (ALU)
Control
Unit. (CU)
Register
Register
Register
The three buses(although not shown here) continue inside
the processor to allow the components to communicate.

1. Registers
A register is a storage location located inside
the processor. A modern processor has many
registers.
Registers are used to hold :
● data which is being processed
● instructions which are being executed
● addresses which are about to be accessed.
Register
Register
Register
Register

2. Control Unit (CU) Control
Unit. (CU)
The Control Unit (CU) is responsible for
● sending out the pulses on the control bus clock line to
synchronise the timing of events and direct the fetching
and executing of instructions.
● decoding the instructions when they arrive from the RAM
and executing them. (i.e. carrying them out)

3. Arithmetic & Logic Unit (ALU)
Arithmetic &
Logic unit. (ALU)
The Arithmetic & Logic Unit is responsible for:
● carrying out the calculations on data being processed by
a program.
● carrying out the logical decision making on data being
processed by a program. This uses operators such as
AND, OR, NOT.

The sets of instructions are called and the
processor spends its time fetching and executing
instructions one at a time.
Any problem can be solved by defining a sequence of
instructions which are input and stored in
A unit, called the , then fetches each instruction
in turn and executes it.
RAM.
Processor
programs
This is known as the fetch-execute cycle.

The steps in the fetch part of the cycle are always the same.
1. The processor sets up the with the
address of the next instruction to be fetched.
2. The processor sends a on the control bus.
3. The instruction is then copied onto the from
the correct memory location to a register in the CPU.
4. The CU(Control unit) the instruction and begins
to it.
address bus
read signal
The steps in the execute part of the cycle depend on the
instruction to be executed.
data bus
decodes
execute

A computer has to store data and instructions. It uses
uses various types of memory to do this.
Registers
Cache memory
Main memory
Backing storage
Speed of access
fastest
slowest

Registers1.Registers
The registers are storage locations located
inside the processor.
Registers are used to hold :
● data which is being processed
● instructions which are being executed
● addresses which are about to be accessed.
They allow the fastest access times as they
are physically on the same chip as the
processor.
Cache memory
Main memory
Backing storage

Registers
Cache memory
Main memory
Backing storage
2.Cache memory
Cache memory is the second fastest type
of memory.
It is small amount of a fast type of Ram called
static RAM(SRAM) which is used to store
frequently accessed data and instructions.
The processor looks here for data before going out to the
main memory. If the data is here then it can be very
quickly accessed.
Modern computers will have around 1 or 2 Mb
of cache memory.

Registers
Cache memory
Main memory
Backing storage
3. Main memory
Main memory is the third fastest type of
memory.
It is made of a large amount of a type of Ram
called dynamic RAM(DRAM) which is used to
store programs and data. It also contains a
small of amount of ROM for operating system
use.
If the data or instructions cannot be found in the
cache then the processor will access it from the
main memory.
Modern computers will have between 1 GB
and 4GB of main memory.

Registers
Cache memory
Main memory
Backing storage
4. Backing Storage
Backing storage is the slowest type of
memory. It is needed because it keeps its
contents even when the power is switched
off unlike the types of memory listed so far.
There are many types of backing storage including Hard
disk, DVD, CD,Floppy disk, magnetic tape, and solid state
devices such as flash cards/memory sticks.
Backing stores also tend to have a larger capacity than
the faster types of memory. i.e. they can store more.
The speed of access varies according to the backing store
used.
(except for ROM)

How can you compare the performance of two computers?
There are many ways to do this and some are better than others.
We will be looking at four methods used for comparison:
1. Clock speed
3. FLOPS
4. Application based tests
2. MIPS

1. Clock speed
3. FLOPS
4. Application based
tests
2. MIPS
The clock speed is often given as a
measure of computer performance but it
really only measures processor speed.
It seems reasonable to assume that a Pentium IV 3.6 Ghz
is faster than a Pentium IV 2.8 GHz. These processors
are both made by Intel, have the same instruction set and
basically work in the same way.
When comparing different makes and types of processor,
however, the one with the faster clock speed may not be
“better”. The slower processor may work more efficiently.
Comparing clock speeds alone is a poor measure of performance.

1. Clock speed
3. FLOPS
tests
2. MIPS
Why can’t we just keep increasing the
clock speed?
•The processor can overheat and this will cause damage.
• The other components may not be able to keep up with
the processor and so “bottlenecks occur”.

1. Clock speed
3. FLOPS
tests
2. MIPS
MIPS measures how many instructions the processor
executes per second.
(millions of instructions per second)
This seems like a good idea but different processors have
different instruction sets and so they are difficult to compare.
A manufacturer may choose a program with simple, fast
instructions in it to make their processor look good.
It doesn’t take into account the size and complexity of
the instructions chosen for the tests.

1. Clock speed
3. FLOPS
tests
2. MIPS
(floating point operations per second)
This measure involves getting the processor to carry out
floating point operations.(calculations on real numbers) and
measures how many it can do each second.
Floating point operations are carried out in the same way
by most processors so this is a better method than clock
speed or MIPS.
FLOPS only measures calculating speed and does not
consider the type of applications that a user
may want to run on the computer.
(programs)

1. Clock speed
3. FLOPS
tests
2. MIPS
The tasks include loading large files, editing graphics, carrying
out calculations etc. These are called benchmark tests and
they show how well each processor can carry out specific
tasks. (See handout)
The best measure of performance involves running a set of tests
using typical examples of commonly used applications.
Computers are given a score for each test depending on how
well they perform and so overall scores can be compared.
This is a very realistic way of measuring performance.

The type of processor used plays a large part in determining
system performance. However, other parts of the computer
also have an effect on how well the computer performs.
We will be looking at three factors which have an effect:
1. Data bus width
3. Data transfer rate to/from
peripherals.
2. Cache memory

1. Data bus width
3. Data transfer rate
to/from peripherals
2. Cache memory
Data and instructions are transferred using the data bus.
A 64 bit data bus can carry twice as much data at a
time as a 32 bit data bus.
A wider data bus means that more data can be
transferred at a time and this therefore improves system
performance.
This assumes that the other components e.g. registers
can cope with the larger amounts of data.
Also less fetches from RAM are required which also
improves system performance.

1. Data bus width
to/from peripherals
2. Cache memory
As you have already seen, cache memory is a small fast
type of RAM which sits between the Processor and the
main memory(RAM).
Level 1 Cache is the fastest type of cache as it is
physically on the same chip as the processor.
Level 2 Cache is on a separate chip and so it is slower to
access than Level 1. Both types are much faster than
main memory RAM.
Increasing the amount of cache memory improves the
system performance.

1. Data bus width
to/from peripherals
2. Cache memory
A peripheral is any device that is not part of the essential
computer. (the processor, memory, and bus paths).
Some peripherals are mounted in the same case with the
main part of the computer, e.g.
Other peripherals are outside the computer case, e.g.
the hard disk drive, DVD drive, and network interface card (NIC)
the printer, mouse, keyboard,scanner, attached by a wired or
wireless connection

1. Data bus width
to/from peripherals
2. Cache memory
Peripheral devices transfer at different speeds and so these
can have an effect on system performance too.
The processor communicates with peripheral devices. e.g.
loading a file from a hard disk, scanning an image etc.
Using peripherals with fast data transfer rates will improve
system performance.
A hard disk drive spinning at 10000 rpm
generally performs better than a 7200 rpm drive.
http://www.pcguide.com/ref/hdd/index.htm
(revolutions per minute)
Peripheral
device interfaces also transfer at different speeds.

Computer technology is constantly improving with new ideas
and techniques to improve performance.
The following three trends have been seen for many years:
1. Increasing clock speeds
3. Increasing backing storage
capacity.
2. Increasing main memory

capacity.
Clock speeds have increased rapidly over the years and are
now measured in .
A new desktop or laptop computer will have a processor with
a clock speed of around
AMD processors tend to have slower clock speeds than
Intel processors but AMD argue that their processors are
more efficient.
Have a look at the websites for current figures and
products. www.intel.co.uk www.amd.com
Over the last year both
and
have announced that they will concentrate on other
ways to improve system performance.
Gigahertz
2.8 to 3.4 GHz.
Intel
AMD

capacity.
The technology for making Main memory (RAM) continues
to improve and capacity has increased while costs continue
to fall.
A new desktop or laptop computer will have between
and of RAM fitted as standard.
Increasing the amount of main memory(RAM) is a very
effective way of improving system performance.
If a computer does not have enough RAM then it has to
make use of the Hard disk drive for temporary storage too.
1GB
4GB
This is called and is much slower to access
than RAM.
virtual memory

capacity.
The technology for making Backing storage also continues to
improve and capacity has increased while costs continue to fall.
A new desktop computer will have a Hard disk drive of
anywhere between and fitted as standard.250 GB 1 TB
Laptop computers tend to have smaller Hard disk drives with
capacities from to150 GB 500 GB .
DVDs and solid state storage devices such as flash cards,
memory sticks and memory cards also continue to increase
in capacity while decreasing in price.

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