Computer System: Two Marks Important Questions and Answers

Digital Principles and Computer Organization:

Chapter 3: Computer System

 

Two Marks Questions with Answers

 

1. What is the difference between computer structure and computer function ?

 Answer: Computer structure refers to the physical arrangement and interconnection of components such as the CPU, memory, I/O devices and system buses.

Computer function describes the operations performed by each component–such as processing data, storing data, moving data and controlling activities.

2. What are the four basic functions performed by every computer system ?

 Answer: The four basic functions are :

1. Data processing – Performing arithmetic and logical operations.

2. Data storage – Temporarily or permanently storing data and instructions.

3. Data movement – Transferring data within the system or to the outside world.

4. Control – Directing and coordinating the operations of all components.

3. Explain the main components of a CPU.

 Answer: The CPU consists of three major parts:

• Arithmetic and Logic Unit (ALU) : Performs arithmetic operations (addition, subtraction) and logical operations (AND, OR, NOT).

• Control Unit (CU) : Directs the execution of instructions and controls data flow within the CPU.

• Registers : Small, fast storage locations that temporarily hold data and instructions during processing.

4. What is the role of main memory in a computer system?

 Answer: Main memory stores the data and instructions that are currently being used by the CPU. It provides fast and temporary storage, allowing the CPU to access information quickly. This makes it essential for smooth and efficient program execution.

5. What is a system bus? Name its types.

 Answer: A system bus is a communication pathway that connects the CPU, memory, and I/O devices, allowing them to exchange data. The types of system buses are :

• Data bus : Carries actual data.

• Address bus : Carries address information to identify memory locations.

• Control bus : Carries control signals used to manage operations of the computer.

6. Define computer architecture.

 Answer: Computer architecture is defined as the functional operation of the individual H/W unit in a computer system and the flow of information among the control of those units.

7. Define computer H/W.

 Answer: Computer H/W is the electronic circuit and electro mechanical equipment that constitutes the computer.

8. What are components of a computer system?

 Answer: The principle components of computer system are hardware and software. Hardware components are further classified as input unit, output unit, control unit, memory unit and ALU. Software components includes: system software and application software.

9. Name the functional units of a computer.

 Answer: The functional units of a computer are :

1. Input unit

2. Output unit

3. Control unit

4. Memory unit

5. Arithmetic and logical unit

10. What is meant by Central Processing Unit (CPU) ?

 Answer: The arithmetic and logic unit in conjunction with called Central Processing Unit (CPU).

11. What is the function of input unit ?

 Answer: A computer accepts a digitally coded information through input unit using input devices such as keyboard and mouse.

12. What are the functions of control unit ?

 Answer: The control unit co–ordinates and controls the activities amongst the functional units. The basic function of control unit is to fetch the instructions stored in the main memory, identify the operations and the devices involved in it, and accordingly generate control signals to execute the desired operations.

13. What is the function of arithmetic and logic unit?

 Answer: The arithmetic and logic unit (ALU) is responsible for performing arithmetic operations such as add, subtract, division and multiplication, and logical operations such as ANDing, ORing, Inverting etc.

14. What are the three types of buses in a system bus?

 Answer: A system bus consists of:

1. Data bus – Bidirectional; carries actual data between CPU, memory and I/O.

2. Address bus – Unidirectional; carries the address of memory/I/O to be accessed.

3. Control bus – Carries control signals such as Memory Read, Memory Write, I/O Read, etc., to coordinate operations.

15. What determines the maximum addressable memory of a system?

 Answer: The width of the address bus determines the maximum addressable memory. Example : A 32–bit address bus can address 2³² = 4 GB of memory.

16. Why is the data bus width important?

 Answer: The width of the data bus (8, 16, 32, 64 bits) determines how many bits can be transferred in one operation.

A wider bus allows more data movement at once, improving system performance.

17. What is a single bus structure?

 Answer: In a single bus structure, the CPU, memory and I/O devices are all connected to one common bus that carries address, data and control signals.

Only one pair of devices can communicate at a time.

18. Why are multiplexed address/data lines used in single bus systems?

 Answer: To reduce the number of pins and wiring.

•  First part of cycle → address is placed

• Second part → data is transferred using the same lines

Latches are used to store the address.

19. State two advantages of a single bus structure.

 Answer:

1. Low cost – Fewer lines and simple wiring.

2. Flexibility – Easy to add new peripheral devices.

20. What is the main limitation of a single bus structure?

 Answer: Only one communication can occur at a time, causing a bottleneck.

More devices → more contention → reduced performance.

21. Why do modern systems use multiple bus hierarchies?

 Answer: Because as devices increase, a single bus experiences :

• Bus sharing delays

• Propagation delays

• Bus bottlenecks

Multiple buses divide the load and increase performance.

22. Name the three buses in a traditional bus hierarchy.

 Answer:

1. Local bus – Connects CPU to cache/memory.

2. System bus – Connects CPU, main memory and I/O.

3. Expanded bus – Connects slower peripherals (printers, disk drives).

23. What is the purpose of a high–speed bus in a multiple bus hierarchy?

 Answer: A high–speed bus supports high–bandwidth devices such as video controllers, SCSI/FireWire, and high–speed LANs. It isolates these devices so they do not overload the main system bus, improving overall performance.

24. What are the main classes of computers based on size and performance?

 Answer: The major classes of computers are :

1. Microcomputers (PCs, laptops)

2. Minicomputers (small organizations, labs)

3. Mainframe Computers (banks, railways)

4. Supercomputers (weather forecasting, scientific research)

25. What is a microcomputer? Give two examples.

 Answer: A microcomputer is a small, low–cost computer that uses a microprocessor as its CPU. It is meant for personal and office use.

 Examples : Desktop PC, Laptop.

26. What is a minicomputer?

 Answer: A minicomputer is a mid–sized computer that supports multiple users simultaneously.

It is used in small organizations, research labs and institutes to run shared applications.

27. What is a mainframe computer? State one use.

 Answer: A mainframe is a powerful computer that can handle large volumes of data and supports hundreds of users at the same time.

Use Banking transactions, airline reservations, railway ticketing.

28. What is a supercomputer? Mention one application.

 Answer: A supercomputer is the fastest and most powerful type of computer, capable of processing trillions of calculations per second.

Application : Weather forecasting, space research, nuclear simulations.

29. What is Von Neumann Architecture ?

 Answer: Von Neumann Architecture is a computer design where program instructions and data share the same memory and the same bus system. The CPU fetches instructions and data sequentially from the same memory, which may cause the Von Neumann bottleneck.

30. What is the Von Neumann bottleneck ?

 Answer:  The Von Neumann bottleneck refers to the limitation of data transfer speed caused by using the same bus for instructions and data. Because only one transfer can happen at a time, the CPU may remain idle waiting for data or instructions.

31. What is Harvard Architecture ?

 Answer: Harvard Architecture is a computer design where program instructions and data are stored in separate memories and accessed using separate buses. This allows simultaneous access to data and instructions, improving speed.

32. What is the instruction execution cycle?

 Answer: The Instruction Execution Cycle is the sequence of steps a CPU follows to execute each instruction.

It mainly includes Fetch, Decode, and Execute, where the CPU retrieves the instruction, interprets it, and performs the required operation.

33. What are the main phases of the Instruction Cycle ?

 Answer: The three main phases are :

1. Fetch – The CPU reads the next instruction from memory.

2. Decode – The Control Unit interprets the instruction.

3. Execute – The CPU performs the required operation (arithmetic, logical, memory access, etc.).

34. What happens during the Fetch phase ?

 Answer:

During the fetch phase :

• The Program Counter (PC) sends the address of the next instruction to memory.

• The instruction is read from memory and placed in the Instruction Register (IR).

• The PC is incremented to point to the next instruction.

35. What is the role of the decode What is the role of the decode phase in the instruction cycle ?

 Answer: During decode, the control unit analyzes the instruction stored in the IR, identifies the operation to be performed and determines the required operands and addressing mode.

36. What happens during the Execute phase?

 Answer: In the execute phase, the CPU carries out the operation specified by the instruction. This may include ALU operations (add, subtract), data transfer (load/store), or control operations (branch, jump).

37. What is the role of the Program Counter in the Instruction Cycle ?

 Answer: The Program Counter (PC) holds the address of the next instruction to be executed. After each fetch, it is incremented automatically to point to the next instruction, unless a branch or jump occurs.

38. What is the Instruction Register (IR)?

 Answer: The instruction register stores the current instruction fetched from memory. It holds the instruction during the decode and execute phases.

39. What is an interrupt cycle in instruction execution?

 Answer: The interrupt cycle is an additional step where, before fetching the next instruction, the CPU checks for pending interrupts. If an interrupt exists, the CPU saves its state and transfers control to the interrupt service routine.

40. What is the purpose of the control unit in the instruction cycle?

 Answer: The Control Unit directs each step of the instruction cycle by generating control signals for fetching, decoding and executing instructions.

41. What is performance ?

 Answer: The performance is an important attribute of a computer. It is an important criteria for selection of a computer.

42. What is execution time ?

 Answer: Execution time or response time is the total time required for the computer to complete a task including disk access, memory access, I/O activities, operating system overlead, CPU execution time and so on.

43. What is CPU execution time ?

 Answer: CPU execution time or simply CPU time is the time the CPU spends computing for particular task and does not include time spend waiting for I/O or running other programs.

44. What is user CPU time ?

 Answer: CPU time spent in the program, called user CPU time

45. What is system CPU time ?

 Answer: CPU time spent in the operating system performing tasks on behalf of the program, called system CPU time.

46. How CPU execution time for a program is calculated?

 Answer: CPU execution time for a program = CPU clock cycles for a program × Clock cycle time

• Alternatively, because clock rate and clock cycle time are inverses,

CPU clock cycles for a program = CPU execution time for a program / Clock rate

47. List the different components that affects the performance of a program.

 Answer: The performance of a program also depends on the algorithm, the language, the compiler, the architecture and the actual hardware.

48. Define CPI.

 Answer: The term clock cycles per instruction is the average number of clock cycles each instruction takes to execute, is often abbreviated as CPI.

CPI = CPU clock cycles/Instruction count.

49. Define MIPS.

 Answer: MIPS is a way to measure the processor speed. The processor speed can be measured in terms of million instructions per second (MIPS). It is given as

MIPS rate = 1 / (Average time required for the execution of instruction × 10⁶)

50. Define MIPS rate.

 Answer: The rate at which the instructions are executed at a given time. It is given by

 MIPS rate =  N×R / C×10⁶

51. Define throughput and throughput rate.

 Answer: Throughput is defined as the total amount of work done in a given time. Throughput rate is defined as the rate at which the total amount of work done at a given time.

52. Define SPEC rating.

 Answer: The SPEC rating is given by

SPEC rating = Running time on the reference computer / Running time on the computer under test

53. State and explain the performance equation.

 Answer: Suppose that the average number of basic steps needed to execute one machine instruction is S, where each basic step is completed in one clock cycle. If the clock cycle rate is R cycles per second, the program execution time is given by

T = (N× S) / R

This is often referred to as the basic performance equation.

54. What does the term hertz refer to ?

 Answer:

The electrical unit for this measurement of CPS is hertz (Hz).

55. Define processor clock.

 Answer: The processor clock is a timing device that sets the clock speed for executing instructions.

56. Define clock rate.

 Answer: The clock rate is given by, R = 1/P, where P is the length of one clock cycle.

It is specified in megahertz (MHz) – millions of cycles per second or gigahertz (GHz) – billions of cycles per second.

57. Define response time and throughput.

 Answer:

Response time is the time between the start and the completion of the event. Also referred to as execution time or latency. Throughput is the total amount of work done in a given amount of time.

58. What is SPEC?

 Answer: The SPEC is nonprofit organization. SPEC stands for System Performance Evaluation Corporation.