Set 1

Question 1

What are the advantages of RPN?

Answer 1

• Eliminates the need for brackets in sub-expressions
• There is no need for order of precedence of operators (DON’T MENTION BIDMAS)
• The expressions are in a form suitable for evaluation using a stack, as the expression can be evaluated from left to right

Question 2

What is main memory?

Answer 2

Memory that is directly addressable by the processor

Question 3

What is the system bus?

Answer 3

• A set of parallel wires connecting independent components of a computer system
• It is used to pass signals between the components.
• These signals can represent data, address, or control information.

Question 4

What is the stored program concept?

Answer 4

Two parts:
(a) A program must be resident in main memory to be executed.
(b) Machine code instructions are fetched and executed serially (one at a time) by the processor.

Question 5

What are the two types of stored program concept?

Answer 5

1. Von Neumann (where data and instructions are stored in main memory)
2. Harvard Architecture (where instructions and data are stored separately, using separate buses)

Question 6

Why is Harvard Architecture sometimes used in preference to Von Neumann?

Answer 6

Von Neumann:
- Competition for resources as all share same bus

Harvard:
- Instructions and writing/reading data can take place in parallel so can be faster
- Avoid bottleneck of single data/address bus
- Instruction 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 7

Where is Harvard Architecture used?

Answer 7

Embedded systems and digital signal processing systems

Question 8

Describe the role of the control unit

Answer 8

• To control operation of the fetch-execute cycle
• It controls fetching (or loading or storing) operations
• It determines the type of an instruction
• To execute (some) instructions
• To synchronise the operation of the processor
• To send control signals and commands to other components
• To control the transfer of data between registers
• To handle interrupts

Question 9

Describe the fetch part of the fetch decode execute cycle

Answer 9

Fetch:
- Contents of Program Counter transferred to Memory Address Register
- Address bus used to transfer this address to main memory
- Read signal sent along control bus
- Transfer of main memory content uses the data bus
- Contents of addressed memory location loaded into the Memory Buffer Register
- Contents of memory buffer register and copied to the current instruction register
- Increment Program Counter (part of Fetch stage, but not directly involved in fetching an instruction)

Question 10

Describe the decode part of the fetch decode execute cycle

Answer 10

Decode:
- Instruction to decode held by the Current Instruction Register
- The control unit decodes the instruction
- Instruction split into opcode and operand(s)

Question 11

Describe the execute part of the fetch decode execute cycle

Answer 11

Execute:
- If necessary, data is fetched or stored
- The opcode identifies the type of operation to be performed by the processor
- The operation (identified by the opcode) is performed by the control unit.
- The ALU is used for calculation (or comparisons)
- The result (may be) stored in a register (or main memory)
- The status register is updated
- If a branch is required then the program counter is updated
- The control bus will transfer signals to other components to initiate or sequence actions

Question 12

How would you subtract a binary number A from a binary number B?

Answer 12

Convert A into -A using two’s complement. Then use binary addition with -A and B.

Question 13

Why use hexadecimal instead of binary?

Answer 13

• Easier for humans to read/write/remember
• More compact representation
• Fewer digits required for the same value
• Less screen space

(NOT BECAUSE IT TAKES LESS MEMORY SPACE. IT DOESN’T TAKE LESS STORAGE SPACE!)

Question 14

What is the data bus used for? What is its direction?

Answer 14

The data bus is used to transfer data between components.

The data bus is bidirectional

Question 15

What is the address bus used for? What is its direction?

Answer 15

The address bus is used to transfer memory addresses and I/O (input output) locations.

The address bus is unidirectional from the processor

Question 16

What is the control bus used for? What is its direction?

Answer 16

The control bus is used to transport control signals between components.

The control bus is bidirectional

Question 17

Give three examples of control signals

Answer 17

• Clock signals
• Memory read
• Memory write

Question 18

What are I/O controllers?

Answer 18

The circuits that connect the processor to an I/O device. (Providing the correct currents/voltages etc.). Processor cannot be directly connected to the I/O device

Question 19

What is the difference between volatile and non-volatile memory

Answer 19

• Volatile memory maintains its data only while the device is powered.
• Non-volatile memory retains stored information even after power is removed

Question 20

Give an example of non-volatile main memory

Answer 20

ROM (Read Only Memory)

Question 21

What is the instruction set?

Answer 21

• The set of binary codes for the machine operations that a processor has been designed to perform.
• It is specific to the processor

Question 22

What is the purpose of main memory?

Answer 22

Main Memory holds the instructions and data currently being used by the processes.

Question 23

What is the ALU?

Answer 23

The Arithmetic Logic Unit is the part of the CPU that is responsible for executing arithmetic operations (e.g. addition, subtraction, binary shifts) and logic comparisons (e.g. Or / And / Not)

Question 24

What is a register?

Answer 24

Extremely fast, very small, memory in the processor for temporary storage of binary values

Question 25

What does the program counter do?

Answer 25

Stores the address of the next instruction to be executed

Question 26

What does the MAR do?

Answer 26

• Temporarily holds the address of the memory location that data will be read/written to
• The MAR is the processor’s direct connection to the address bus for accessing main memory
• Holds the address of the instruction in the fetch stage as it is fetched

Question 27

What does the MBR do?

Answer 27

• Temporarily stores data read from or written to main memory
• The MBR is the processor’s direct connection to the data bus for accessing main memory
• Holds the instruction in the fetch stage before it is copied into the CIR

Question 28

What is the role of the CIR in the FDE cycle?

Answer 28

• Stores the recently fetched instruction to be decoded by the control unit before it is executed

Question 29

What information is stored in the status register?

Answer 29

• The status register contains many bits (binary flags) that are set (1) or cleared (0) depending on the result of an instruction
• Each bit represents a boolean value, for example if an overflow error has occurred

Question 30

What is the purpose of general purpose registers?

Answer 30

• Have no specific value stored
• Usually used as temporary storage while performing arithmetic operations
• Often store the operands of processor instructions

Question 31

4 reasons secondary storage is needed

Answer 31

• Main memory is volatile, so secondary storage needed to store files that are needed multiple times
• Secondary storage can be used to store larger files, as it usually has a much higher capacity than main memory
• Secondary storage can be used for virtual memory
• Main memory is expensive

Question 32

4 advantages of Harvard architecture

Answer 32

• Instruction and data can be accessed simultaneously
• Instruction and data memory can have different word lengths
• Different technologies can be used to implement instruction and data memory
• For systems with a predetermined use, instruction memory can be implemented as ROM which protects the programs from hacking

Question 33

What is the significance of the leftmost bit in two’s complement?

Answer 33

• The most significant bit (leftmost) is the sign bit, which represents a negative number when set to 1
• The MSB also contributes to the value of the number

Question 34

What is 1111…111 in two’s complement binary?

Answer 34

-1

Question 35

Range of an 8-bit number represented as:
1. unsigned binary number
2. two’s complement number

Answer 35

1. Unsigned: 0 to 255
2. Two’s complement: -128 to 127

Question 36

An 8-bit two’s complement binary number can be between ___ and ___

Answer 36

-128 and 127

Question 37

How to turn a negative number into its positive version in two’s complement (and vice versa)?

Answer 37

1. From the right, every bit up to and including the first 1 stays the same.
2. Everything else gets flipped.

(This is the same as flipping every bit then adding 1)

Question 38

Where is Von Neumann architecture used?

Answer 38

General purpose computing systems