Higher Computing:
COMPUTER SYSTEMS
Part 2: Computer Structure – 6 hours
Marr College
Higher Computing
Slide 1
INT 2
Five box diagram
The five box diagram represents the basic components of a computer system.
Computer Structure
It represents input devices, processor, main memory, output devices and
backing storage.
Processor
Input
Output
Main Memory
Backing
Store
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Higher Computing
Slide 2
The purpose of the processor
INT 2
The processor – also known as the central processing unit (CPU) - is the ‘brain’
or ‘engine’ of the computer system.
Computer Structure
Its purpose is to interpret instructions and process data contained in computer
programs.
Marr College
Higher Computing
Slide 3
INT 2
Parts of a processor
Computer Structure
The CPU consists of three main parts:
Registers
Control
Unit
The arithmetic and logic unit
performs arithmetic calculations
e.g. / * + - and logic operations
e.g. AND, OR etc.
ALU
RAM
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The control unit has a timer to
that sends signals to fetch,
decode and execute program
instructions.
The registers are temporary
storage areas that hold data being
processed; instructions being
executed; and addresses to be
accessed.
Higher Computing
Slide 4
The purpose of the ALU
The arithmetic and logic unit is a digital circuit that performs arithmetic and
logical operations.
Computer Structure
The ALU can perform many operations including:
Integer arithmetic e.g. addition and subtraction
Bitwise logic operations e.g. AND, OR, NOT, XOR
Control
Unit
* The inputs to the ALU are
the data to be operated on
(called operands)
Operation (opcode)
Input (operand)
ALU
Output (result)
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* and a code from the control
unit indicating which
operation to perform.
* Its output is the result of the
computation.
Higher Computing
Slide 5
The purpose of the Control Unit
Computer Structure
The control unit is the circuitry that controls the flow of data through the
processor, and coordinates the activities of the other units within it.
It can be described as the "brain within the brain", as it controls what happens
inside the processor, which in turn controls the rest of the PC.
It performs the tasks of fetching, decoding, managing execution and then
storing results and has a timer to synchronise these events.
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Higher Computing
Slide 6
The Address Bus
• Carries address information from the CPU to main memory and any other
attached devices
Computer Structure
• It is uni-directional i.e. one-way only
• The width (i.e. the number of wires) determines the number of memory locations
the CPU can address
A 32 bit address bus has 32 parallel wires each switched on (1) or off (0)
that can address locations starting from:
0000 0000 0000 0000 0000 0000 0000 0000
(decimal 0)
up to and including address:
1111 1111 1111 1111 1111 1111 1111 1111
(decimal 232 –1)
making a total of 232 addresses
Every time a wire is added to the width of the address bus, the address range
doubles
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Higher Computing
Slide 7
The Data Bus
Computer Structure
• Carries data to and from the CPU,
main memory and any other devices
attached
• It is bi-directional i.e. two-way
• The number of wires determines the
quantity of data that the bus can
carry so increasing the number of
wires in the data bus increases the
quantity of data it can carry
• A typical 32-bit data bus can carry
32-bits of data or instructions at a
time
In Higher Computing we make
the assumption that:
The size (number of wires) on
the data bus determines the size
of the memory locations in
RAM e.g.
A 32 bit data bus means each
memory location in RAM stores
32 bits (i.e. 4 bytes equivalent)
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Higher Computing
Slide 8
The Control Bus - control lines
Signals are sent out and received on the control bus.
Computer Structure
The control bus is not really a ‘bus’ as it does not
transfer data or addresses.
It is made up of discrete wires each with a specific
function:
• Read and Write signals are initiated to fetch – execute instructions in
memory
• Clock line carries a series of clock pulses at a constant rate to keep the
CPU in step (clock rate measured megahertz or gigahertz)
• Reset halts the execution of the stored program. Internal registers are
cleared and the machine reboots.
• Interrupts are signals usually from I/O devices that halt program
execution temporarily. The CPU may ignore them e.g. printer out of
paper
• Non-maskable Interrupts cannot be ignored e.g. power failure
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Higher Computing
Slide 9
The buses: Address, Data and Control
Computer Structure
Address bus
Other
registers
Memory
Address
Register
Data bus
Memory Data
Register
Arithmetic
and Logic
Unit (ALU)
Main
Memory
Control Bus (Read / Write)
Control Unit
Clock pulses
Electronic
clock
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Higher Computing
Slide 10
The fetch-execute cycle
Computer Structure
The fetch-execute cycle of the processor refers to the sequence that is
completed for each instruction in a program.
•
Fetch Sequence
•
Move the value in the program counter to the memory address
register
Send the value in the memory address register to memory via the
address bus
Return the value stored in memory via the data bus
Store the value in the memory data register
Copy the instruction from the memory address register to the
instruction register
Increment the program counter. The instruction in the instruction
register is then Decoded
•
•
•
•
•
•
Execute Sequence
•
Instruction is executed
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Higher Computing
Slide 11
The purpose of the Registers
The registers are small, fast storage areas that temporarily hold data.
Instructions or addresses.
Computer Structure
The registers in the CPU have three main functions. They are to hold data
being processed, instructions being executed, and addresses being accessed.
• memory address register (MAR) –
holds the address of a location in memory
• memory data register (MDR) – holds
data just read from or written to memory
• program counter (PC) – holds the
address of the next instruction to be
fetched
• Instruction register (IR) – holds the
current instruction being executed
• general purpose registers – can be used
by programmers
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Higher Computing
Slide 12
Computer Memory - Cache
Cache Memory
Computer Structure
Is a small area of ‘super fast’ access memory, between the processor and
main memory, which stores frequently used instructions and data.
1. Cache fetches data from next to
current addresses in main memory
Main
Memory
(RAM)
Cache
4. If not, the CPU has to fetch next
instruction from main memory - a
much slower process
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2. CPU checks to see whether
the next instruction it requires
is in cache
CPU
3. If it is, then the instruction is
fetched from the cache – a very
fast position
Higher Computing
Slide 13
INT 2
Main memory – RAM
Main memory is commonly referred to as RAM (random access memory).
Computer Structure
RAM is used to hold program instructions and data before and after processing
by the CPU.
RAM is volatile i.e. loses it contents
when switched off.
Reading from RAM is slower than
accessing registers or cache.
Use of cache avoids slower fetches from
RAM.
RAM chip
Increasing memory size (capacity in Mb/Gb) improves system performance as
more programs and data can be held.
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Higher Computing
Slide 14
INT 2
Main memory - ROM
Another type of memory is Read Only Memory (ROM).
Computer Structure
ROM is used to store the bootstrap loader program that locates the
operating system on the hard disc when the computer ‘boots up’.
Features - ROM
• ROM data is permanently etched on
chip
• Read-only so data cannot be changed
• Data not lost when computer switched
off
ROM chip
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Higher Computing
Slide 15
INT 2
Backing storage
Computer Structure
Backing storage is where the computer permanently saves computer
programs and data.
This includes devices such as the hard disc and media such as DVD.
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Higher Computing
Slide 16
Distinguishing between different parts of memory
Computer Structure
Registers – fastest
access time as internal
to CPU
Cache – slower than
registers but fast as no
READ needed
RAM – slower than
registers and cache
Backing store –
slowest speed of access
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Higher Computing
Slide 17
Memory
Task
Computer Structure
Investigate the following elements of computer memory:
•
•
•
•
Registers
cache
main memory
backing storage
Distinguish between the above elements of memory according to:
•
•
Function
speed of access
Produce a brief word processed report or powerpoint presentation.
Include an image/diagram of each component.
Marr College
Higher Computing
Slide 18
Addressability
Computer Structure
Main memory consists of a number of
storage locations, each of which is
identified by a unique address.
The ability of the CPU to identify each
location is known as its
addressability.
Each location stores a word i.e. the
number of bits that can be processed
by the CPU in a single operation. Word
length may be typically 16, 24, 32 or
as many as 64 bits.
A large word length improves system
performance, though may be less efficient
on occasions when the full word length is
not used
Marr College
Higher Computing
Slide 19
Calculating memory capacity of a computer
Computer Structure
Memory capacity can be calculated if we know:
•
the number of lines on the address bus
•
and the number of bits stored in each memory location
Note: in Higher we assume the number of bits in a memory location to be the
same as the number of bits the data bus can carry.
Formula
Amount of storage locations = 2
Memory capacity = 2
width of the address bus
the width of the address bus
* width of data bus
Example
A computer has a 24 bit address bus and a 16 bit data bus. Calculate the
maximum amount of memory this computer can use.
224 * 2 bytes
= 33,554,432 bytes
33,554,432 / 1024
= 32,768 kilobytes
32768 / 1024
= 32 megabytes
Marr College
Higher Computing
Slide 20
Calculating memory capacity of a computer
Questions
Computer Structure
Calculate the total memory requirements of the following computer systems:
a) Processor has a 16-line address bus and each location stores 16 bits.
b) Processor has a 16-line address bus and each location stores 32 bits.
c) Processor has a 32-line address bus and a 24-line data bus.
d) Processor has a 36-bit address bus and a word length of 32 bits.
Remember: Location size = data bus size = word length / size
Marr College
Higher Computing
Slide 21
INT 2
Desktop computer
Processing Power - measured
by clock speed e.g. 3 Ghz.
Computer Structure
Memory size - typically 3 Gb.
Backing storage – large hard
disc typically 750 Gb – 1 Tb. Also,
CD-RW and DVD-RW.
Input devices – keyboard,
microphone, mouse, webcam etc.
Output devices – monitors,
printers, speakers, modem etc
Typical uses – applications
(word, excel), email, internet,
gaming
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Higher Computing
Slide 22
INT 2
Laptop computer
Processing Power - measured by
clock speed e.g. 1.75 Ghz – 2.4
GHz
Computer Structure
Memory size - typically 512 Mb –
1 Gb.
Backing storage – large hard disc
typically 40 - 100 Gb. Also, CDRW and DVD-RW.
Input devices – keyboard,
touchpad
Output devices – LCD integrated
monitor, printers, speakers,
modem etc
Typical uses – applications (word,
excel), email, internet, gaming.
Portable to work on the move.
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Higher Computing
Slide 23
INT 2
Palmtop computer
Processing Power - measured by clock speed e.g. 200 – 500 Mhz
Computer Structure
Memory size - typically 32 – 64 Mb. Can extend with memory card.
Backing storage – Same as memory (battery powered)
Input devices – stylus and touch screen
Output devices – integrated LCD screen
Typical uses – personal organiser and connectivity to PC.
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Higher Computing
Slide 24
INT 2
Mainframe computer
A large and powerful computer that deals with very high volumes of data
processing.
Computer Structure
Features
Processing power: several processors
RAM: 32 GB or more
Backing storage: 100s of GB, tape
drives
I/O: keyboard, printers, monitors
Uses
Used by large organisations to provide remote access to central computer via
terminals e.g. a bank’s ‘cashpoint’ machines are directly connected to a
mainframe at head office.
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Higher Computing
Slide 25
INT 2
Embedded computer
Computer Structure
Embedded computers are special-purpose systems where the computer is
embedded within the machine it controls e.g. a computer system in a car.
They perform a specific task
Considered a
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Higher Computing
Slide 26
System Performance:
INT 2
Clock Speed
Clock Speed
Computer Structure
Is the number of clock pulses a CPU generates per second. Measured in
gigahertz i.e. 1GHz = 1 billion pulses per second.
These pulses synchronise the steps
of the fetch-execute cycle e.g. a
clock pulse starts a ‘fetch’, or
triggers placing data in the MDR.
The faster the clock speed – the
more operations can be executed
per second.
Clock speed indicates processing power but some instructions need more
clock pulses on one processor than on another.
Marr College
Higher Computing
Slide 27
System Performance:
MIPS rate
Millions of Instructions per Second (MIPS)
Computer Structure
MIPS roughly measures the number of machine code instructions that a CPU
can execute per second.
Pros
Rough measure of processor performance e.g.
Intel
Intel
Intel
Intel
8080:
486 DX2:
Pentium III:
Core 2 X6800:
500 kIPS at 2 MHz
54 MIPS at 66 MHz
1354 MIPS at 500 MHz
27,079 MIPS at 2.93 GHz
Cons
No standard way of measuring MIPS as does not take into account complexity
of an nstruction e.g. some instructions require more time than others.
Refers only to CPU speed, not other factors such as I/O
Therefore a machine with a high MIP rate may not run an application faster
than a CPU with a lower rate due to other limiting factors.
Marr College
Higher Computing
Slide 28
System Performance:
MIPS rate
How to calculate a CPU’s MIPS rating
There is a simple formula to calculate MIPS.
Computer Structure
Formula
MIPS = n / (t x 1,000,000)
Where n is the number of instructions executed and t
Example
No standard way of measuring MIPS as does not take into account complexity
of an nstruction e.g. some instructions require more time than others.
Refers only to CPU speed, not other factors such as I/O
Therefore a machine with a high MIP rate may not run an application faster
than a CPU with a lower rate due to other limiting factors.
Marr College
Higher Computing
Slide 29
System Performance:
MIPS rate
Millions of Instructions Per Second (MIP)
Computer Structure
A clock speed of 200 MHz does not mean that 200 million instructions are
executed per second. Because of this, clock speed isn’t an accurate measure
of performance.
It may therefore be the case that two processors have the same clock speed
but different MIP rates.
In order to get a better picture of performance we measure the CPUs ability in
how many million instructions it can process per second, also referred to as the
machine cycle time.
Marr College
Higher Computing
Slide 30
System Performance:
MIPS rate
Working out MIPS rate
Computer Structure
It can take at least five clock pulses to execute an instruction.
Example – a CPU with a clock speed of 200 MHz
=>
200,000,000 clock pulses / 5 pulses per instruction
=>
40,000,000 instructions per sec.
Thus CPU = 40 MIPS
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Higher Computing
Slide 31
System Performance:
MIPS rate
Computer Structure
Problems with MIPS rate
•
Using MIPS rate as a comparison factor also has problems.
•
MIPS rate depends on what sort of instructions are being carried out
•
There is no standard set and so some manufacturers could use
instructions which require less clock pulses to complete thus giving the
impression of a faster MIPS rate than it actually is.
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Higher Computing
Slide 32
System Performance:
FLOPS
Floating Point Operations per Second (FLOPS)
Computer Structure
Measures how many floating point operations are processor can perform per
second.
FLOPS are more accurate than
MIPS.
They measure a number of clearly definable arithmetic tasks carried out per
second.
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Higher Computing
Slide 33
System Performance:
Application-based Test (Benchmarks)
Computer Structure
Benchmarks
This involves running various
application programs on different
computer systems and observing which
system runs these programs the
fastest.
Most reliable measure of
processor performance because it
provides the user with actual
evidence of how well different
processors perform complex
operations at high speed.
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Higher Computing
Slide 34
Factors that affect system performance
Computer Structure
Data bus width – Increasing the data bus width increases processor
throughput i.e. an 8-bit bus can transfer 1 byte of data; a 16-bit can transfer 2
bytes etc.
Cache memory - Using / increasing cache (SRAM) means less ‘fetches’ from
slower main memory (DRAM).
Peripheral speed – Slow transfer rates slow the system down so better to
choose peripherals with faster transfer rates.
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Higher Computing
Slide 35
Current trends in computer hardware
Faster clock speeds – improved CPU throughput as millions more instructions
per second.
Computer Structure
More RAM – more sophisticated software can be run, and improved
multitasking i.e. more programs loaded simultaneously.
More backing storage capacity means more data can be stored
permanently.
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Higher Computing
Slide 36