Computer Structure

Question 0

The Structure of the Central Processing Unit (CPU)

Answer 0

• In a microcomputer, this consists of a microprocessor, containing:
  
  • control unit - controls the fetching, decoding and executing of instructions
  • arithmetic and logic unit (ALU) - carries out the calculations and the logic operations
  • registers - group of storage locations within the microprocessor that are used temporarily to hold:
    
    • data being processed
    • instructions being executed
    • addresses to be accessed

Question 1

The Two state machine

Answer 1

The processing and storage devices have only two states ‘ON’ and ‘OFF’, represented using the binary system - ‘1’ and ‘0’.

Question 2

What are Buses?

Answer 2

Buses are the sets of wires connecting the CPU to the main memory and peripherals.

Question 3

What kind of Buses are there?

Answer 3

• Address Bus - carries address information from the CPU to main memory or peripherals. It is unidirectional. The width of the address bus determines the maximum amount of memory that the computer can have. This is 2 ^ n where n
  is the width of the address bus.
• Data Bus - carries data between the CPU and the memory or peripherals. During a write operation data is carried from the CPU and in a read operation, to the CPU. It is bi-directional. The width of the data bus determines how much data can be transferred in a single operation.
• Control Bus is made up of a number of separate wires, connecting the control unit to memory and peripherals.

Question 4

What kind of wires are there?

Answer 4

• Read and Write wires
• Clock wire carries clock pulses, which keep the CPU and other units in step. The clock rate is measured in hertz (MHZ or GHz).
• Interrupt wires carry signals to the CPU, which can cause the CPU to break in the carrying out of its program. The CPU may be able to ignore the interrupt (maskable) or not (non-maskable interrupt - NMI).
• Reset line is used to return the CPU to its initial state, e.g. used to recover from ‘crashes’.

Question 5

The Fetch Execute Cycle

Answer 5

• Program instructions are got (fetched) one at a time from main memory, decoded and then carried out (executed).
• The CPU fetches the instructions and data in the program from memory. This is a read operation.
• The program may require data to be put into memory. This is a write operation.

Question 6

Memory Read Operation

Answer 6

1. The required memory address is put in the Memory Address Register.
2. The control unit activates the Read Line.
3. The contents of the memory at the address are put on the data bus and into the Memory Data Register.

Question 7

Memory Write Operation

Answer 7

1. The required memory address is put in the Memory Address Register.
2. The value to be written is put into the Memory Data Register
3. The control unit activates the Write Line.
4. The contents of the Memory Data Register are transferred by the data bus to the required memory address.

Question 8

Memory - Registers, Cache and Main Memory

Answer 8

• Main memory consists of memory locations each with a unique address.
• Each location can hold a word.
• A word is the number of bits of data that can
  be processed in a single operation by the CPU. The word length is the same as the width of the data bus.
• There are two types of main memory — RAM and
  ROM.

Question 9

Random Access Memory

Answer 9

• RAM is volatile memory - data held in RAM is lost when the power is switched off. RAM holds the computer’s programs and data while they are
  being processed.
• DRAM (Dynamic RAM) is normal RAM and needs to have its contents refreshed otherwise the contents are lost.
• SRAM (Static RAM) doesn’t need refreshed as long as power is applied. It is used for holding date etc. when the computer is switched off; power being supplied by a rechargeable battery. SRAM is also fast and used for cache memory.
• Cache memory is memory close to (or in) the processor, which is used to hold often-used instructions to speed up processing.
• VRAM (Video RAM) is used to hold the data, which makes up the screen display. It is related to the number of colours and screen resolution. It may be part of the main memory or, more usually, on a separate graphics card.

Question 10

Read Only Memory

Answer 10

• ROM is permanent memory - data is retained when the power is switched off.
• ROM is used to hold programs and data permanently, the data being fixed during manufacture. In computers with a disc-based operating system, ROM is used to hold the bootstrap loader program, which loads the operating system from disc.
• In some games machines, ROM cartridges are used to hold software and ROM is used in many digital automated systems.

Question 11

Speed of Memory Access

Answer 11

• Processor registers are accessed fastest
• Cache memory in the processor next,
• Then the cache outside the processor
• The main memory and slowest is virtual memory on disc.

Question 12

How is System Performance measured?

Answer 12

• MIPS (millions of instruction per second)
• FLOPS (floating point operations per second)
• Benchmarks (timing the computer carrying out tasks such as recalculating in a spreadsheet or redrawing complex graphics)

Question 13

Factors that affect System Performance

Answer 13

• Bus Width
  
  • Increasing the data bus width will increase the amount of data fetched in each cycle and hence increase system performance.
  • Increasing the width of the address bus increases the maximum amount of memory that the computer could have. More memory allows more programs and data to be loaded at the same time, which can improve productivity. Also with a larger memory, slower virtual memory does not need to be used as often.
• Clock Speed
  
  • The clock generates a series of pulses at a constant rate, which keeps everything that the CPU does in time.
  • The faster the clock speed, the faster the operations will be carried out.
  • Remember that the overall speed is determined by the slowest component.
• Memory
  
  • Adding more memory can speed up the computer.
  • Also some memory has faster access time than other types.
  • Use of cache memory made of fast static RAM can also improve performance.

Question 14

Speed of data transfer to peripheral devices

Answer 14

• Speeds of Interfaces increasing e.g.
  
  • USB 1 -> USB 2
  • Firewire 400 -> Firewire 800
• Cables are being replaced by Wireless communication between CPU and peripheral e.g. WIFI and Bluetooth