1. Higher Computing
Computer Structure

2. What you need to know about computer systems:
ALU and control unit
Registers, cache and main memory
Function and speed of access to memory
Data and address buses
Control lines
System performance
Current trends in computer hardware

3. The Processor The processor is the main part of the computer
It consists of the:
Control unit
Arithmetic and logic unit
Registers

4. The computer structure
Control bus not shown
data bus
RAM
ROM
Printer, plotter, scanner
VDU and keyboard
Disk, CD/DVD
Digital I/O
A/D
D/A
Microprocessor
Process control
Clock pulses
Electronic
clock
Address bus

5. The processor structure
Main
memory
Other
registers
Memory
address
register
Address bus
Memory
data
register
data bus
A L U
Control unit
Control bus (read and write)
Clock pulses
Internal processor bus
Electronic
clock

6. Buses
There are three sets of wires that connect the processor to the memory and input/output devices. These are called buses, the three buses are:
The data bus
The address bus
The control bus

7. The data bus
The data bus carries data to and from the processor, main memory and other devices attached to the data bus
The data bus carries data in both directions depending on whether we are reading or writing data
Data is told which way to go by the control bus

8. The address bus
The processor uses the address bus to tell the memory which address is to be used
It carries the address information from the processor to the main memory and any other devices attached
The address bus is unidirectional (one way)

9. More on the address bus
The number of wires in the address bus (the width of the bus) determines the number of storage location which the processor can address
A typical microcomputer with 32 wires in the address bus can address up to 4,294,967,296 (from 0 to 232-1) memory locations

10. Addressability Data is stored in storage locations
Each piece data which can be stored in its own storage location in main memory is called a word
A word is the number of bits that the processor can process in a single operation (clock pulse)
Each storage location has its own unique address
The method used to identify each unique address is called addressability

11. The control bus
The control bus is made up of a number of separate wires each with its own function
Read
Write
Clock
Interrupt
Non-maskable interrupt
reset }
The fetch-execute cycle

12. The fetch-execute cycle
The fetch-execute cycle consists of two parts
The fetch part is where the instruction is copied into the control unit of the processor and decoded (read)
The execute part occurs next and is the instruction being carried out (write)

13. The fetch-execute cycle
The processor sets up the address bus with the required address
The processor activates the read line
The instruction is transferred from the memory to the processor by using the data bus (fetch or read)
The instruction is decoded
The instruction is executed (write)

14. The fetch-execute cycle
The processor sets up the address bus with the required address
Other
registers
Memory
address
register
Address bus
Memory
data
register
A L U
Control unit
Clock pulses
Main memory
Internal processor bus
Electronic
clock

15. The fetch-execute cycle
Other
registers
Memory
address
register
Address bus
Memory
data
register
A L U
Control unit
The processor activates the read line
Control bus (read)
Clock pulses
Main memory
Internal processor bus
Electronic
clock

16. The fetch-execute cycle
Other
registers
Memory
address
register
Address bus
The instruction is transferred from the memory to the processor by using the data bus (fetch). It is stored in the MDR
Memory
data
register
data bus
A L U
Control unit
Control bus (read)
Clock pulses
Main memory
Internal processor bus
Electronic
clock

17. The fetch-execute cycle
Memory
address
register
data
Control unit
A L U
Other
registers
Electronic
clock
Clock pulses
Internal processor bus
The instruction which is in the MDR is transferred via the internal processor bus to the control unit and decoded

18. The fetch-execute cycle
Main
memory
Memory
address
register
data
Control unit
A L U
Other
registers
Electronic
clock
Clock pulses
Internal processor bus
Control bus (write)
The instruction is then executed

19. The Control Bus - Clock
The clock line carries a series of pulses at a constant rate
The pulses are used to keep the processor and its related components in step with one another
The clock rate is the frequency at which the clock generates pulses and it is measured in hertz (Megahertz or Gigahertz)

20. The Control Bus - Interrupts
An interrupt is a signal to the processor from an input or output peripheral device
An interrupt cause a break in the execution of the processor’s current program
An example of an interrupt would be a message from the printer stating it was out of paper
A non-maskable interrupt is an interrupt that cannot be ignored

21. The Control Bus - Reset
The reset line on the processor is used to return the whole computer system to a state, as if it has just been switched on
Reset is usually used when the computer has “frozen”

22. Computer memory - Registers
The registers are a group of storage locations inside the processor
Registers hold:
Data being processed
Instructions being executed
Addresses to be accessed
Accessed instantly

23. Registers Memory address register Address bus Other registers Memory
data
register
data bus
A L U
Control unit

24. Computer memory - Cache
Cache a small amount of memory
Used as a temporary store for often used instructions
Two types
Level 1 cache built on the chip, very fast data transfer
Level 2 cache situated close to the chip, not just as fast

25. Computer memory
1. Cache fetches the next instruction from main memory in advance of the processor needing it
Main
memory
2. Processor checks to see if the next instruction is in cache
Cache
Memory
(SRAM)
Microprocessor
3. If the instruction is in the cache , it is fetched from the cache, a very fast process
4. If the instruction is not in the cache, it has to be fetched from main memory, a much slower process

26. Computer memory
2. Processor checks to see if the next instruction is in cache
1. Cache fetches the next instruction from main memory in advance of the processor needing it
Main
memory
Microprocessor
Cache
Memory
(SRAM)
3. If the instruction is in the cache , it is fetched from the cache, a very fast process
4. If the instruction is not in the cache, it has to be fetched from main memory, a much slower process

27. Computer memory – Main memory
Random Access Memory
Dynamic Random Access Memory (DRAM)
Needs to have its contents constantly refreshed (about a thousand times per second)
Static Random Access Memory
Retains its contents as long as there is power

28. Computer memory – Video RAM
Video RAM holds the data to be displayed on the screen
VRAM is sometimes contained on a separate graphics card

29. Computer memory – ROM Read Only Memory: holds data permanently Types:
Programmable ROM: empty on manufacture programmed by the user
Erasable Programmable ROM: can be programmed then erased then reprogrammed
Flash ROM: can be reprogrammed while its still inside the computer, used on devices such as digital cameras

30. Comparison of types of memory 1
Four types of usable memory in
order of closeness to the CPU
LOW
HIGH
HIGH
Registers
Cost per byte of storage
Cache
Storage capacity
Speed of access
RAM
Hard disk
HIGH
LOW
LOW

31. Comparison of types of memory 2
Element of computer memory
What do they hold
Speed of access (relative)
Nano-seconds
Registers
Cache- SRAM
Main memory
DRAM
ROM
Hard disk Virtual memory
Element of computer memory
What do they hold
Speed of access (relative)
Nano-seconds
Registers
Data, instructions and addresses
Immediate 1
Cache- SRAM
Frequently accessed instructions
Very fast
Level 1= 5
Level 2= 15
Main memory
DRAM
Programs and data when computer is on
Fast 50
ROM
Data even if computer is off
50 – 100
Hard disk Virtual memory
Programs and data permanently stored
Slow
1,000,000
Data, instructions and addresses
Immediate 1
Frequently accessed instructions
Very fast
Level 1= 5
Level 2= 15
Programs and data when computer is on
Fast 50
Data even if computer is off
Fast
50 – 100
Programs and data permanently stored
Slow
1,000,000

32. How long is a nanosecond?

33. Computer performance
Computers are tested to see fast they process data
Generally speaking the faster the processor (measured in Gigahertz) then the faster the processing
This is not always the case the computer can only process data as fast as its slowest part
What could slow the processing of data down?

34. Measures of performance
The measures of computer performance
Clock speed
Millions of Instructions per Second - MIPS
Floating Point Operations Per Second - FLOPS
Application based tests

35. MOP – clock speed
Everything the processor does is kept precisely in time with the clock
Simply put
If the processors clock rate is 1 Gigahertz it will be able to fetch 1000 million instructions per second
2 Gigahertz it will be able to fetch 2000 million instructions per second

36. MOP - MIPS Millions of Instructions Per Second
A measure of performance based on the average number of machine code instructions executed per second

37. MOP - FLOPS Floating Point Operations Per Second
A measure of the arithmetical calculating speed of a computer system

38. MOP – Application based tests
ABTs are more applicable for the everyday user
A benchmark is established and different computers are compared
How long each computer takes to complete a particular task e.g. scale and rotate a graphic, do a 1000 calculations in a spreadsheet, reformat a 1000 page word-processed document

39. MOP - evaluation
Hardware such as printers can evaluated to see how many pages per minute they can print
Benchmark tests are designed to reveal how the whole system performs and is not based merely on the processors clock speed

40. Factors affecting performance
A systems performance may be enhanced by:
Increasing:
Cache memory
Data bus width
The rate of data transfer
The clock speed
Video RAM
Adding more:
main memory
Processors

41. Factors affecting performance (dbw)
Data bus width
Increasing the DBW can increase the overall performance of the system

42. Factors affecting performance (cache)
The use of cache memory
As it is much faster for the processor to access data and instructions held cache if the size of the cache can be increased then the overall system performance will improve

43. Factors affecting performance
The rate of data transfer to and from peripherals can have a major effect on system performance
The transfer rate is controlled by the type of interface connecting the peripheral to the processor

44. Factors affecting performance (cs)
Increasing the clock speed of a processor can have a significant impact on the computers overall performance
Increasing the clock speed of the processor will in itself not increase the overall performance of the system if the data bus cannot deliver enough data to the processor so that it can work at its maximum performance

45. Factors affecting performance (mm)
Adding more main memory
Additional memory will allow more data to be held in immediate access store rather than on backing storage.
It is much faster to access data from RAM than from hard disk

46. Factors affecting performance (VRAM)
Video RAM is specialized RAM which is used on video cards.
Video RAM is dual-ported, which means it can be accessed by two different devices simultaneously. This enables data to be read from video RAM (i.e. sent to the computer monitor) at the same time data is written to video RAM.
Graphics performance may be improved by increasing the Video RAM

47. Factors affecting performance (proc)
Adding more processors to a computer system is an effective method of increasing performance
Some programs can take advantage of the presence of a second processor and this can have a significant effect on improving performance

48. A factor not affecting performance
Increasing the address bus width will increase the total number of memory locations which the processor can address but:
This will have no effect on the performance

49. Factors affecting performance
Tactic
Effect on computer system performance
Increase clock speed
Increase data bus width
Increase cache memory
Increase width of address bus
Increase RAM
Increase VRAM
Increase rate of data transfer to and from peripherals
Tactic
Effect on computer system performance
Increase clock speed
increase
Increase data bus width
Increase cache memory
Increase width of address bus
none
Increase RAM
slight increase
Increase VRAM
increase graphics performance
Increase rate of data transfer to and from peripherals
increase
increase
increase
none
slight increase
increase graphics performance
increase

50. Current trends in computer hardware
Increasing clock speeds
Although this increases performance technical problems are being experienced such as heat dissipation and increased power consumption
Using more than one processor in a single computer can alleviate this or using two processors on a single chip –”dual core”

51. Current trends in computer hardware
Increasing memory
Both operating systems and applications are continually demanding more RAM
The cost of RAM has decreased
Free RAM refers to the amount of RAM available to applications after the operating system has been loaded

52. Current trends in computer hardware
Increasing backing storage capacity
The cost of backing storage has decreased
The demand for backing storage of greater capacity has increased
The size of a typical hard disk on a home computer has increased
The variety of backing storage available has increased

53. Types of interface - USB
Universal Serial Bus
Plug and play capabilities, hot swapping (no need to reboot the computer)
Provides power to low consumption peripherals
In many cases no device drivers needed
Lower cost than firewire
Over 1 billion devices in use

54. Types of interface – Firewire (IEEE 1394 interface )
A serial bus interface
For high speed communications
Real time data transfer
Frequently used in a personal computer with digital audio and digital video peripheral devices
Developed by Apple (firewire) and Sony (i.Link)
More expensive than USB

55. Transfer rates of interfaces
Type of interface
Maximum transfer rate Megabits per second
USB 1
1.5
USB 2
480
USB 3 (2008)
4800 – 5000 (5Gbits)
Firewire 400
400
Firewire 800
800
Firewire 1600 (2008)
1600
Firewire 6400 (2009+)
6400
Type of interface
Maximum transfer rate Megabits per second
USB 1
USB 2
USB 3 (2008)
Firewire 400
Firewire 800
Firewire 1600 (2008)
Firewire 6400 (the future)
1.5
480
4800 – 5000 (5Gbits)
400
800
1600
6400