HSSC-I (11th Class) Computer Science Notes, Chapter - 3, Central Processing Unit (Q/A)

11th Class (HSSC-I) Computer SLO Based Key Point Notes

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Based on National Curriculum Pakistan 2023-2024 and Onward prescribed by Federal Board of Intermediate and Secondary Education, Islamabad, and All Pakistan Boards) 

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Unit 3: Central Processing Unit

Q1. What is a microprocessor?

Ans: 

The microprocessor is the main part of a computer system which is also known as its brain. It is a VLSI circuit that contains thousands of complex circuits that are used to generate control signals to execute instructions. Microprocessor consists of ALU, CU, Registers, Cache, and Buses.

Q2. What is the function of ALU in a computer?

Ans: 

The Arithmetic Logic Unit is the part of the microprocessor where the actual execution of instructions takes place. It consists of two parts Arithmetic Unit and Logic Unit.

Q3. What is the function of ALU?

Ans: 

The Arithmetic Logic Unit is the part of the microprocessor where the actual execution of instructions takes place. It is connected with two registers Data Register and Accumulator. ALU receives input from the Data Register (and from the Accumulator if there are two inputs), and stores results in the Accumulator.

Q4. What is the function of the Control unit in the computer?

Ans: 

The control unit directs and coordinates the activities of all parts of the computer system i.e. I/O, Memory ALU etc. It uses timing and control logic to generate signals.

Q5. Define the bus and name its types.

Ans: 

A bus is a group of parallel wires that transmits binary information amongst various parts of a computer system i.e. I/O, Memory, and Processor. 

There are three types of buses
Data Bus, Control Bus, and Control Bus

Figure 1. Block Diagram of Busses.

Q6. What is the purpose of the control bus?

Ans: 

The control bus carries control signals from the control unit to all other parts. It is a bi-directional bus with having size of 8-16 bits. An 8-bit control bus can support 28 instructions.

Q7. Define the address bus.

Ans: 

Address bus carries address from processor to memory. It is a uni-directional bus having a size of 16-32 bits. A 16-bit address bus can point to 216 Memory locations.

Q8. What is the purpose of the data bus?

Ans: 

Data bus transfers data among various parts of the computer i.e. I/O, Memory, and Processor. It is a bidirectional bus having a size of 32-64 bits. A 32-bit data bus can carry 4 bytes of data at a time.

Q9. Define register.

Ans: 

Registers are the smallest and fastest memory devices found inside the microprocessor which hold the data and address temporarily and pass to other parts of the processor or memory. Some commonly used registers are PC, DAR, D, A, IR, etc.

Q10. Define General Purpose and Special Purpose Registers.

Ans: 

General Purpose Registers: These registers can store data as well as addresses. These are used for arithmetic data movement. E.g. Accumulator, data, counter, and base registers. 

Special Purpose Registers: These registers hold the state of the program and are used by SU to control the operations of the CPU and by OS to control the execution of programs. E.g. PC, IR, MAR, etc.

Q11. What is the purpose of a PC?

Ans: 

Program Counter is a special purpose register that stores the address of instruction next to be executed.

Q12. What is the purpose of DAR (MAR)?

Ans: 

Memory Address Register (or Data Address Register) is a special purpose register that holds the address of data that is to be read from memory.

Q13. What is the purpose of DR?

Ans: 

The data Register is a general-purpose register that provides input to ALU for processing.

Q14. What is the purpose of IR?

Ans: 

An instruction Register is a special-purpose register that holds the instruction just fetched from memory. It then passes this instruction to the instruction decoder for further processing.

Q15. What is the purpose of an Accumulator?

Ans: 

The accumulator is a general-purpose register that holds the output coming from ALU after processing. If required, it also provides input to ALU.

Q16. What is the use of MBR?

Ans: 

Memory Buffer Register temporarily stores the data just read from memory or to be written to memory.

Q17. Differentiate between General Purpose and Special Purpose Registers.

Ans: 

These registers can store data as well as address	These registers hold the state of the program
These are used for arithmetic data movement	These are used by CU to control the operations of the CPU and by OS to control the execution of programs
E.g. Accumulator, data, counter, and base registers	E.g. PC, IR, MAR, etc.

Q18. Define Cache memory.

Ans: 

The cache is a small but high-speed memory that stores the active portion of memory that is frequently required for processing.

Q19. What is meant by instruction code?

Ans: 

A set of bits that indicates some microprocessor operation is known as an instruction or instruction code. An instruction comprises of two parts as follows:

1. Operation Code (OP Code) 
2. Operand

There are three types of microprocessor instructions:

i. Data transfer instructions

ii. Data processing instructions

iii. Program control instructions

Q20. Name the types of instructions.

Ans: 

There are three types of microprocessor instructions

iv. Data transfer instructions

v. Data processing instructions

vi. Program control instructions

Q21. What is data transfer instruction?

Ans: 

Data transfer instructions are used to transfer data from one part to another. Data is transferred generally between two registers or between the register and memory. e.g. MOV, LD, XCHG

22. Define the purpose of following data transfer instructions?

Ans: 

MOV (Move): It moves data from one register to another, from memory to register, or from register to memory. Syntax MOV A,B

LD (Load): It loads the contents of memory to register. Syntax LD A,13

XCHG (Exchange): It exchanges the values of two registers. Syntax XCHG A,B

STO (Store): It stores the values from registers to memory. Syntax STO A

Q23. What is data processing instruction?

Ans: 

Data processing instructions are used to perform arithmetic and logic operations on data. E.g. ADD, SUB, MUL, DIV, AND, OR, NOT, etc.

Q24. Define the purpose of following data processing instructions?

Ans: 

ADD: It adds two values. Syntax ADD A,B

SUB: It subtracts one value from the other. Syntax SUB A,B

MUL: It multiplies two values. Syntax MUL A,B

DIV: It divides one value by another. Syntax DIV A,B

AND / OR / NOT: It performs logical AND OR or NOT operations

Q25. What are program control instructions?

Ans: 

Program control instructions control the order of execution of instructions within a program. E.g. JMP, LOOP etc.

Q26. Define the purpose of following data transfer instructions?

Ans: 

JMP: It transfers the control from a normal sequence to some other instruction.

LOOP: It repeats instructions for a given number of times.

Q27. What is the operation code?

Ans: 

Operation code is the part of a computer instruction that specifies the operation to be performed by the microprocessor.

Q28. Define instruction formats.

Ans: 

The way in which a computer instruction is written is known as instruction format. It specifies the number of bits used for op-code or operands. A computer may have a variety of instruction formats. A simple instruction consists of 16 bits. There are three types of instruction formats:

i. Zero Address instruction

ii. One Address instruction

iii. Two Address instruction

Q29. How many types of instruction formats are there?

Ans: 

There are three types of instruction formats

i. Zero Address instruction

ii. One Address instruction

iii. Two Address instruction

Q30. What is the advantage of using address mode in an instruction?

Ans: 

The main advantage of using address mode in an instruction is that a large number of memory locations can be accessed.

Q31. Define Zero-address instruction.

Ans: 

Zero address instruction requires only an op-code and does not need an operand or operand address. E.g. STOP 

Q32. Define One-address instruction.

Ans: 

One address instruction requires an op-code and only one operand or operand address. E.g. JMP

Q33. Define Two-address instruction.

Ans: 

Two-address instruction requires an op-code and two operands or operand addresses. E.g. ADD

Q34. Define the instruction cycle.

Ans: 

The instruction cycle is the basic operation cycle of a computer to execute various instructions. It defines the steps in which a processor receives an instruction from memory and executes it. It has three steps Fetch, Decode, and Execute.

Q35. What do mean by CISC?

Ans: 

CISC stands for Complex Instruction Set Computer. It is the traditional architecture of the CPU that supports a large variety of instructions. These instructions may have various lengths, formats, and address modes and require complex circuitry to decode. CISC is complex because instructions are used at the hardware level. Examples of CISC processors are the Intel 486 and Pentium series.

Q36. Define RISC.

Ans: 

RISC stands for Reduced Instruction Set Computer. It is the modern architecture of the CPU that supports the same size of instructions and does not use indirect address mode. Its instructions are very simple and execute very fast. RISC CPUs use simple circuitry to decode instructions so are cheaper and easy to manufacture. Examples of RISC processors are IBM PowerPC, Sun SPARC, mobile phones and tablets, etc.

Q37. Differentiate between CISC and RISC architectures.

Ans: 

CISC	RISC
CISC instructions utilize more cycles	RISC instructions utilize fewer cycles
CISC has very complex instructions	RISC has simple instructions
Execution of CISC instructions is slower	Execution of RISC instructions is faster
CISC has more instructions	RISC has fewer instructions
CISC uses both direct and indirect address modes	RISC uses only direct address mode
CISC is generally used in computers	RISC is generally used in mobile phones, tablets, etc.

Q38. Compare Intel P-IV and AMD Athlon processors.

Ans: 

Intel Pentium-IV processor	AMD Athlon processor
High clock speed i.e. 1.7 to 3.0 GHz	Lower clock speed i.e. 1.4 to 2.33 GHz
Bus width is 32 to 64 Bits	Bus width is also 32 to 64 Bits
Cache size is 256 KB to 1 MB	Cache size is 256 KB to 512 KB
Uses both CISC and RISC architectures	Uses only RISC architectures
Has 20 steps execution process	Has 1 steps execution process
Uses generally have 478 pin sockets	Uses generally 462 pin sockets

Q39. Draw a labeled diagram showing the arrangement of buses and registers within the microprocessor.
OR
Label the following diagram

Ans: 

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