7 - Fundamentals of Computer Organisation and Architecture

Question 1

Explain the role of the Arithmetic Logic Unit.

Answer 1

The ALU is part of the processor that processes and manipulates data. It performs arithmetic and logical operations on data supplied in registers, storing the result in a register.

Question 2

Explain the role of the Control Unit.

Answer 2

The CU controls operation of the fetch-decode-execute cycle and sends control signals to other components of the cycle.

Question 3

Explain the role of general-purpose registers.

Answer 3

General-purpose registers have no specific purpose and can be used by the programmer to store data, as needed.

Question 4

Explain the role of the clock.

Answer 4

The system clock is a unit inside the processor that provides regular clock pulses that are used to sequence the operation of the processor and the movement of data around the other components of the computer.

Question 5

Explain the role of the Program Counter.

Answer 5

The PC stores the memory address of the next instruction to be fetched and executed and is incremented as part of the fetch-decode-execute cycle.

Question 6

Explain the role of the Current Instruction Register.

Answer 6

The CIR stores the instruction that is currently being executed by the processor. The “decode” step is also performed on the CIR.

Question 7

Explain the role of the Memory Address Register.

Answer 7

The MAR stores the memory location where data in the MBR is about to be written to or read from. This is also the processor’s direct connection to the address bus for accessing main memory.

Question 8

Explain the role of the Memory Buffer Register.

Answer 8

The MBR holds the data that has just been read from or is about to be written to main memory. This is also the processor’s direct connection to the data bus for accessing main memory.

Question 9

Explain the role of the Status Register.

Answer 9

The SR stores single bit condition codes each of which indicates the outcome of arithmetic and logical operations carried out in the ALU, for example an arithmetic operation may produce a positive, negative, zero result, a carry or overflow and the corresponding condition codes are set (made 1).

Question 10

What are registers?

Answer 10

Registers are memory locations within the processor that support fast access as well as rapid manipulation of their contents.

Question 11

What are the major components in a processor?

Answer 11

• Arithmetic Logic Unit
• Control Unit
• Clock
• General purpose registers
• Dedicated registers, including:
• Program Counter
• Current Instruction Register
• Memory Address Register
• Memory Buffer Register
• Status Register

Question 12

Explain how the Fetch-Execute cycle is used to execute machine code programs.

Answer 12

A machine code program is made up of machine code instructions which are fetched from main memory, one at a time, and executed in the processor.

Question 13

Describe what happens in the Fetch stage of the Fetch-Execute cycle.

Answer 13

1) The address of the next instruction to be executed (held in the PC) is copied to the MAR.
2) The address bus is used to transfer this address to main memory.
3) The instruction held at that address in main memory is transferred via the data bus to the MBR.
4) The contents of the PC are incremented to hold the address of the next instruction to be executed.
5) The contents of the MBR are copied to the CIR. This frees up the MBR for the execute phase.

Question 14

Describe what happens in the Decode stage of the Fetch-Execute cycle.

Answer 14

• The instruction held in the CIR is decoded by the CU.
• The instruction is split into opcode and operand.

Question 15

Describe what happens in the Execute stage of the Fetch-Execute cycle.

Answer 15

• The opcode identifies what type of instruction it is.
• Instructions are executed by the relevant part of the processor.
• Further memory fetches are carried out if required.
• For maths or logic instructions, the ALU executes the instruction.
• The result of computation is stored in the accumulator.
• The status register is updated.

Question 16

Describe the role of interrupts.

Answer 16

An interrupt is a signal that is sent to the processor to request immediate attention. When the processor receives an interrupt, it suspends what it is doing and runs the process associated with the interrupt.

Question 17

Describe the role of Interrupt Service Routines.

Answer 17

An ISR is a small program designed to respond to each interrupt. These can be provided by either the operating system or I/O device drivers.

Question 18

Describe the effects of interrupts on the Fetch-Execute cycle.

Answer 18

1) The processor receives the interrupt.
2) The processor completes the Fetch-Execute cycle of the instruction that it was running when it received the interrupt.
3) The current contents of the processor registers (including the PC) are saved to memory.
4) The origin of the interrupt is identified so that the appropriate ISR is called.
5) All other lower priority interrupts are put on hold to allow the ISR to finish running.
6) The PC is updated with the address of the first instruction of the ISR.
7) The ISR completes its execution.
8) The processor registers are reloaded with the values that were saved to memory.
9) The lower priority interrupts that were put on hold are re-established.
10) The PC is set to point to the address of the next instruction that needs to be executed in the program that the processor was running when it received the interrupt.

Question 19

What is a volatile environment?

Answer 19

A volatile environment is a snapshot of the information needed to execute a process from its current instruction. This contains the special purpose and general purpose registers (including the PC).

Question 20

Explain what happens when an interrupt occurs?

Answer 20

The volatile environment is saved on the stack. The source of the interrupt is identified so the appropriate ISR is called. After the ISR has finished its execution, the volatile environment is restored.

Question 21

Why is it important to save the volatile environment while the interrupt is being serviced?

Answer 21

Running the ISR causes the contents of processor registers to change. Unless the volatile environment is saved before these changes occur, restoring the contents of the affected registers will be impossible as will returning the processor to the exact state it was in just before executing the ISR.

Question 22

Name 6 factors that affect processor performance.

Answer 22

• Multiple cores
• Cache memory
• Clock speed
• Word Length
• Address bus width
• Data bus width

Question 23

Explain the effect on processor performance of multiple cores.

Answer 23

By having multiple cores in a computer system, parallel processing can take place. This is where a task is split up into different parts which are executed at the same time in different cores. Therefore by increasing the number of cores, the speed of execution is increased (as long as those instructions can be executed in parallel).

Question 24

Explain the effect on processor performance of cache memory.

Answer 24

The cache is a small amount of fast memory placed near the processor which stores copies of data from frequently used memory locations, data to be written to main memory and pre-fetched instructions. The cache can be accessed much faster than main memory. By increasing the size of the cache, more data and instructions can be stored there, resulting in programs being able to run faster.

Question 25

Explain the effect on processor performance of clock speed.

Answer 25

A processor executes a particular instruction in a set number of clock ticks or clock cycles. Therefore increasing the clock speed causes instructions to be executed at a faster rate.

Question 26

Explain the effect on processor performance of word length.

Answer 26

Increasing the word length means that the processor can handle more data per instruction. This is useful as processing an instruction may require multiple fetches of data from main memory. Therefore, by being able to pass larger amounts of data to the processor with every pass, the system is likely to carry out instructions faster. Increasing the word length means that the processor can access a larger amount of memory through direct accessing and could also allow the processor to have a larger instruction set.

Question 27

Explain the effect on processor performance of the address bus width.

Answer 27

The size of the address bus represents the amount of unique memory locations that are addressable. Increasing the width of the address bus will increase the number of addressable locations and therefore allows more memory to be installed on the computer.

Question 28

Explain the effect on processor performance of the data bus width.

Answer 28

Increasing the width of the data bus means that more bits and therefore more data can be passed down it with each pulse of the clock. Therefore increasing the width of the data bus increases the rate of data transfer.

Question 29

What are the different definitions of processor performance.

Answer 29

• Speed/Throughput - Number of instructions executed per second.
• Scalability - could this processor cope with, say, more memory?
• Response Time - How long does it take for a single request to be fully completed?
• Power consumption.

Question 30

What is the three box model?

Answer 30

The three box model is the simplest way to represent the internal components of a computer system.

system bus
←—————————————→
↕↕↕
processormaininput/
memoryoutput

Question 31

What are the basic internal components of a computer?

Answer 31

The internal subsystems are:
• Processor or Central Processing Unit (CPU)
• Main memory (RAM)
• I/O controllers - input only, output only, both input and output
• Buses

Question 32

What is the role of the processor, and how does it relate to the other internal components?

Answer 32

The processor executes machine instructions that have been fetched from main memory locations. The processor selects a memory location by placing the address of the location on the address bus. The data processed by machine instructions is fetched along the data bus from main memory and the results of processing are returned in the same way. The control bus is used by the processor to assert actions and to allow devices (such as the keyboard controller) to grab the attention of the processor via the interrupt mechanism when a key is pressed.

Question 33

What is the role of main memory?

Answer 33

Main memory is memory that can be accessed directly by the processor. The main memory consists of a contiguous block of read/write, randomly accessible storage locations where instructions or data are stored as binary sequences.

Question 34

What is the role of the address bus?

Answer 34

The address bus is unidirectional and is used to select a specific memory location containing a word of data or an instruction. It does this by carrying the address of the desired location on its bus. The address bus is also used to address I/O ports during input/output operations.

Question 35

What is the role of the data bus?

Answer 35

The data bus provides a bidirectional path for moving data and instructions between system components.

Question 36

What is the role of the control bus?

Answer 36

The control bus is a bidirectional bus which is used to transport control signals that manage and orchestrate the operations that take place in a computer system between components. This includes exchanging status signals between the components and transmitting clock signals required for the coordination of instructions.

Question 37

What is the role of the I/O controllers?

Answer 37

An I/O controller is a board of electronics that enables the processor to control and communicate with a peripheral device through a I/O port. It translates signals from the device into the format required by the processor which allows the flexibility to add new devices without having to redesign the processor as each device operates using different electronic signals. It is also used to buffer data being sent between the processor and the device, so that the processor does not have to wait for the individual device to respond.

Question 38

What is the system bus?

Answer 38

The system bus is a set of parallel wires connecting independent components of a computer system and is used to pass signals between components. The signals can represent data, addresses or control information.

Question 39

What is the difference between von Neumann and Harvard architectures?

Answer 39

In von Neumann architecture, instructions and data share the same memory which the processor communicates with over a shared bus called the system bus. In Harvard architecture, instructions and data are allocated separate memories. The processor is connected to both memories by separate buses so that each memory can be accessed simultaneously.

Question 40

What are the advantages of Harvard architecture.

Answer 40

• Instructions and data are handled quicker as they do not have to share the same bus.
• Avoid bottleneck of single data/address bus.
• Instructions and data memory can have different word lengths.
• Avoids possibility of data being executed as code, which is one method that can be exploited by hackers.

Question 41

Where is von Neumann architecture used?

Answer 41

Von Neumann architecture is used extensively in general purpose computing systems.

Question 42

Where is Harvard architecture used?

Answer 42

Harvard architecture is extensively used in embedded systems such as digital signal processing (DSP) systems.

Question 43

What is meant by the stored program concept?

Answer 43

A program must be resident in main memory in order to be executed. Instructions are fetched and executed one at a time by the processor. Programs can be moved in and out of memory.

Question 44

Describe and explain the process with which data is originally saved onto a read only optical disk and how this is read by a consumer.

Answer 44

Pits and flats are stamped into aluminium layer. The laser in the disk drive reads the pits and flats. The pits represent 0s and flats represents 1s. The binary values are then read.

Question 45

What does the term peripheral mean?

Answer 45

An external device not directly under the control of the processor.

Question 46

What three parts do I/O controllers consist of?

Answer 46

The controller consists of three parts:
• The I/O port - to allow exchange of signals between the processor and the peripheral.
• Electronics that interface the controller to the system bus.
• Electronics appropriate for sending signals to the device connected to the computer.

Question 47

Why is it not sensible to connect peripherals directly to the processor?

Answer 47

• Each peripheral operates in a different way.
• Not sensible to design a processor to control every possible peripheral.
• A new type of peripheral would require the processor to be redesigned.
• Peripherals may operate at a different voltage from the processor.
• Peripherals will usually operate at a slower rate than the processor.

Question 48

Describe the principles of operation of a laser printer.

Answer 48

The print drum is coated in static charge. The printer generates a bitmap image from the data. Laser beams are directed on the print drum via a rotating octagonal mirror. The laser is modulated and neutralises electric charge on the drum where image should be dark. Toner is given charge and the charged drum picks up toner. There is one drum mechanism for each colour. Toner is then transferred from drum to paper. The toner is then fused to the paper by heated rollers.

Question 49

Describe the principles of operation of a hard drive.

Answer 49

The hard disk is a magnetic medium. Binary digits are represented by magnetising spots on the disk. The hard disk is made up of platters and is divided into tracks (which are concentric circles) and sectors. The drive head moves radially, while the disk continually spins at a high speed. Data is read/written as correct sector passes under the read/write head. There is one head per platter and the head is parked when not in use.

Question 50

Ignoring any differences in price and assuming that both drives have the same capacity, state the reasons why you might choose a solid-state drive rather than a traditional hard disk drive?

Answer 50

• Faster access speed.
• Silent operation.
• Less heat generated.
• Less power required.
• More robust due to no moving parts.

Question 51

Why are CD-ROM disks suitable for music storage?

Answer 51

• Easily portable.
• Enough storage to hold music files.
• Can be read by many devices.
• Can’t be overwritten.
• Small form factor.
• Can be stored in a small space.
• Non-volatile.
• Durable.

Question 52

What does an opcode represent?

Answer 52

It indicates the basic machine operation.

Question 53

What does an operand represent?

Answer 53

The value that the instruction operates on.

Question 54

What is the difference between direct and immediate addressing?

Answer 54

Direct addressing means that the operand is the memory address or register number of the datum whereas immediate addressing means the operand is the datum.

Question 55

During the decode and execute stages of the fetch-decode-execute cycle the instruction that is being processed is stored in the CIR. Explain why the instruction could not be processed directly from the MBR.

Answer 55

To execute the instruction other data may need to be fetched from main memory. Further memory fetches would overwrite the contents of the MBR.

Question 56

How does the addressing mode relate to the operand of an instruction?

Answer 56

It indicates how the value in the operand should be interpreted.

Question 57

When the processor writes data to the main memory it will make use of the address, control and data buses. Explain how each of these buses will be used during this write process.

Answer 57

The address of the memory to be written to is placed on the address bus.
The data to be written is placed on the data bus.
The signal to write is placed on the control bus.
The control bus carries a clock signal to synchronise the memory and processor.
When the write signal is received by the memory on the control bus, the data from the data bus is stored into the location identified by the address bus.

Question 58

A RFID system is made up of a transponder built into an RFID tag and an interrogator or reader. Explain the principle of operation of this RFID system.

Answer 58

The RFID reader transmits a signal to the antenna of the RFID tag. The RFID tag is energised by the signal from the reader. The transponder in the tag sends the data signal which is received by the reader.

Question 59

Explain the purpose of the check digit in a bar code.

Answer 59

To check the bar code is valid.

Question 60

Describe the principles of operation of a bar code reader, excluding the use of the check digit.

Answer 60

For a photodiode system a laser is shone at the bar code. A moving mirror moves the light beam across the bar code and the light is reflected back. White bands reflect more light than black bands. A light sensor measures the amount of reflected light which is converted into an electrical signal and analysed to determine the value encoded in the bar code.

Question 61

What is an instruction set?

Answer 61

The set of bit patterns for which machine operations have been defined. An instruction set is processor specific.

Question 62

Describe the principles of operation of a digital camera.

Answer 62

Lens focuses light onto an array of sensors on the sensor chip. Each sensor produces an electrical signal. The signal represents a pixel. The image is captured when the shutter is pressed. An ADC converts measurement of light intensity into binary data.
Firmware performs data processing to “tidy up” image. Colour filter is used to generate data separately for red, green, blue colour components. Image is recorded as an array of pixels.

Question 63

Describe the principles of operation of a solid state drive (SSD)

Answer 63

Most commonly uses NAND flash memory. Data is stored using floating gate transistors. This is a type of transistor that can trap and store charge. Data must be read in pages and written/erased in blocks. Can use single-level cells that store one bit or multi-level cells that store two bits.

Question 64

Describe how data is written to and read from a CD-R disk.

Answer 64

To write data a high powered laser makes sections less reflective.

To read a low powered laser is used to read data back from the disk

The difference between pits and lands indicates the 1s and 0s. Data is stored as a continuous spiral track.

Question 65

What are the development in the design of hard disk drives that has enabled an increase in storage capacity?

Answer 65

• More platters
• Greater density of each platter
• More tracks on a platter
• Change to perpendicular magnetic domains
• Ability to write smaller magnetic domains
• Use of different alloy materials for the platter

Question 66

What are the hardware components of a solid-state drive?

Answer 66

• NAND memory
• Controller
• SATA interface

Question 67

What are the differences between primary and secondary storage?

Answer 67

• Secondary storage is non-volatile
• Secondary storage is not directly accessible to the processor
• Capacity of primary store is limited by width of the address bus whereas there is no limit on capacity of secondary store
• Data in primary store can be accessed more quickly than data in secondary store

Question 68

Describe the principles of operation of an optical disk drive that is used to read data from an optical disk, ushc as CD-ROM or DVD-ROM

Answer 68

With optical disks, data is stored on a spiral track. The disk spins at variable angular velocity. A low power laser is shone at the disk. Light is focussed on spot on track. Some light is reflected back from the disk. The amount of light reflected back is measured. Continuation of land/pit on the disk reflects light whereas transition between land and pit scatters light. Transition between land and pit indicates a 1 and continuation of land/pit represents 0.