Systems Architecture

Question 1

What is the stored program concept?

Answer 1

The idea of a memory unit storing data or programs.
Therefore, more versatile as a singular machine could store different programs for different tasks.
This concept allowed for the creation of general purpose machines.

Question 2

What does the Von Neumann architecture consist of?

Answer 2

A processor.
A memory unit that communicates with processor.
Connections (buses) for I/O devices.
Secondary storage saving/backing up data.

Question 3

What does the processor do?

Answer 3

Executes program instructions to process data.
Handles main memory and I/O operations.

Question 4

How does the processor access program instructions and data? Hint (TFD)

Answer 4

The instructions and data are transferred from secondary to main memory.
They can then be fetched, decoded and executed/processed.
Data can also be provided by I/O devices via I/O controllers.

Question 5

What is cache memory?

Answer 5

Fast access M that stores most frequently used data/instructions.
It is located on the processor.

Question 6

What is main memory?

Answer 6

Stores data/instructions while they are being processed. (RAM/ROM)

Question 7

What are I/O controllers?

Answer 7

Devices that help with communications between peripheral devices and the control unit.

Question 8

What are buses?

Answer 8

Connections that transfer data and signals between components in a computer.

Question 9

What is the address bus?

Answer 9

Either reads or writes data to a memory location.
Is undirectional, one way connection from P to M and I/O C.
Allows the processor to establish a connection with an addressable ‘unit’, whether its a memory location or I/O controller.

Question 10

What is the data bus?

Answer 10

Data is transferred to and from the P, M and I/O C.
It is bi-directional, it allows two-way connection between internal components of the system.

Question 11

What is the control bus?

Answer 11

Sends control signals to manage and orchestrate operations in a computer system.
Including exchanging status signals between components of the computer system, and transmitting clock signals required for the coordination of operations.
It is also bi-directional.

Question 12

What determines how much data can be transferred along the data bus or address bus?

Answer 12

The width of the bus (no. parallel lines).
If width is n bits then 2ⁿ bits can be transferred at a time.

Question 13

What can the control bus be used for? Hint (RASS)

Answer 13

Request communication between units.
Acknowledge a request.
Specify the data type being transferred via the other buses.
Sync the communication between components using the clock pulses.

Question 14

What are some examples of control signals? Hint (RWRGBIC)

Answer 14

M read: Puts data from a M location onto the data bus.
M write: Stores data from the data bus to a M location.
Bus request: Signifies a component needs access to a bus.
Bus grant: Signifies component is informed it can use a bus.
Bus busy: Signifies bus is not available (in use)
Interrupt request: Signifies an error has occurred an requires attention of CPU.
Clock signals: C bus supplies components with clock pulses.

Question 15

What is the ALU?

Answer 15

It’s responsible for all arithmetic calculations and logic operations.

Question 16

What is the control unit?

Answer 16

Organises the sequence in which program instructions are executed and decodes them.
It controls communications within the computer with control signals.

Question 17

What tasks does the control unit carry out? Hint (RW-D-O-A)

Answer 17

Uses C signals to enable data to be read and written to main M.
Decodes instructions to know what needs to be done (ALU, storing etc.)
Organises micro-operations in order for the instruction to carry out.
Uses C signals to determine which ALU operation will carry out.

Question 18

What does the clock do?

Answer 18

Synchronises the operation of the processors components.
Each operation carried out takes a number of clock “cycles”.

Question 19

How does the clock do its job?

Answer 19

Generating regular pulses that emit a signal that oscillates between a low (0) and high (1) state.

Question 20

What is a rising and falling edge (clock)?

Answer 20

Rising edge - changing from a low (0) to a high (1) state.
Falling edge - changing from a high (1) to a low (0) state.

Question 21

How is one clock cycle found?

Answer 21

The time taken between two sequential rising edges is called the clock period and it corresponds to one clock cycle.

Question 22

How is the clock speed calculated?

Answer 22

Clock speed (Hz) = 1 / clock period (s)

Question 23

What does a program being ‘ready’ mean (CPU)?

Answer 23

The program instructions have been translated into machine code.
The program instructions have been loaded (from secondary storage) into the main M (RAM).

Question 24

Describes what happens in the fetch part of the fetch-execute cycle.

Answer 24

Contents of the PC are copied to the MAR. MAR copies address to address bus.

C unit instructs a M read operation so that M location is transferred to the P. The instruction is then transferred and saved to the MDR via the data bus.

Contents of MDR are copied to CIR.

Question 25

What happens in the decode part of the fetch-execute cycle?

Answer 25

C unit splits instruction in CIR into operand and opcode to determine the type of instruction to be carried out.

Question 26

What does the “memory read” control function do?

Answer 26

Puts data from a M location onto the data bus.

Question 27

What does the “memory write” control function do?

Answer 27

Stores data from the data bus to a M location.

Question 28

What does the “bus request” control function do?

Answer 28

Signifies a component needs access to a bus.

Question 29

What does the “bus grant” control function do?

Answer 29

Signifies component is informed it can use a bus.

Question 30

What does the “bus busy” control function do?

Answer 30

Signifies bus is not available (in use)

Question 31

What does the “interrupt request” control function do?

Answer 31

Signifies an error has occurred that requires attention of CPU.

Question 32

How does a compiler work?

Answer 32

It takes the source code as a whole and translates it into machine code.
This machine code can then be run at any time.

Question 33

How does an interpreter work?

Answer 33

It translates one line of code at a time and then executes it.
It does not produce an executable file.

Question 34

What are advantages of using an interpreter?

Answer 34

Errors are found quickly as the interpreter halts at an erroneous line.
Minimal storage: lines are not stored after translation.

Question 35

What are disadvantages of using an interpreter?

Answer 35

Interpreted programs run slower.
The program has to be interpreted every time to be run.
They do not produce an executable file that can be distributed.

Question 36

What are advantages of a compiler?

Answer 36

Compiled programs run quickly as they are already translated.
Provides an executable file.

Question 37

What are disadvantages of a compiler?

Answer 37

They don’t find errors.
There must be available memory to hold the data.
The program must all be recompiled if a change is made.

Question 38

How does having more cache memory improve performance?

Answer 38

More instructions can be stored in cache memory.
This increases the probability of a cache hit.
Fewer fetches from main memory.

Question 39

What are advantages of Harvard architecture?

Answer 39

Reduces bottleneck of data buses as instruction and data can be accessed simultaneously.
Instruction memory can be made exclusively ROM to prevent the program being hacked.

Question 40

What is the role of interrupts?

Answer 40

To suspend the currently executing program.
So that a device that requires the attention of the processor can be serviced.

Question 41

Why must the contents of registers be saved before servicing an interrupt?

Answer 41

So that the currently running task can be returned to.
As the interrupt will clear register values.