Instruction Set Architecture and Control Unit Design: Two Marks Important Questions and Answers

Digital Principles and Computer Organization:

Chapter 8: Instruction Set Architecture and Control Unit Design

Two Marks Questions with Answers

1. What is an Instruction Set Architecture (ISA)?

 Answer:  ISA is the interface between hardware and software. It defines the set of instructions a CPU can execute, their formats, data types, addressing modes, and behaviour.

2. Why is ISA important for compatibility and portability?

 Answer: Programs written for one ISA can run on any processor that implements the same ISA. This ensures software portability across different hardware platforms.

3. Explain the relation between ISA, hardware, and software.

 Answer:

• Software uses the ISA to write and execute programs.

• ISA acts as a bridge by defining instructions, registers and memory access methods.

• Hardware implements the ISA in CPU components, memory and I/O devices to execute instructions.

4. What are the key components of an ISA?

 Answer:  Key components include :

1. Instruction Set (arithmetic, logical, data transfer, control flow)

2. Instruction Format (opcode, operands, immediate values)

3. Data Types (integer, floating point, characters)

4. Registers (number, type, size)

5. Addressing modes and Memory model

6. I/O mechanisms, interrupts, exceptions

7. Privilege levels and parallelism support

5. What does an instruction format specify ?

 Answer: It specifies the opcode, operand fields, immediate values and control bits. It determines instruction length and decoding complexity.

6. What is CISC architecture ?

 Answer: CISC (Complex Instruction Set Computer) architecture uses a large set of complex instructions where each instruction can perform multiple low–level operations, such as memory access, arithmetic and logic, in a single instruction.

7. What is RISC architecture ?

 Answer: RISC (Reduced Instruction Set Computer) architecture uses a small and simple set of instructions, each designed to execute very quickly, usually within one clock cycle.

8. Give one example each of CISC and RISC processors.

 Answer:

CISC processor : Intel × 86 family

RISC processor : ARM, MIPS, or PowerPC

9. State one main difference between CISC and RISC.

 Answer: CISC has many complex instructions that may take multiple cycles to execute, while RISC has fewer and simpler instructions that typically execute in one clock cycle.

10. What are addressing modes ?

 Answer: The different ways that a processor can access data are referred to as addressing schemes or addressing modes.

11. List different addressing modes.

 Answer:

1. Register Mode

2. Absolute Mode or Direct Mode

3. Immediate Mode

4. Indirect Mode

5. Index Mode

6. Relative Mode

7. Autoincrement Mode

8. Autodecrement Mode

12. Differentiate direct and indirect addressing mode.

 Answer: In direct addressing mode the address of the location of the operand is given explicitly as a part of the instruction. On the other hand, in indirect addressing mode, the effective address of the operand is the contents of a register or the main memory location whose address is given explicitly in the instruction.

13. State the basic addressing modes supported by RISC processor.

 Answer: The RISC processor has three basic addressing modes :

• Register addressing

• Immediate operand and

• Relative to PC addressing for branch instructions.

14. What is relative addressing mode? When is it used?

Here, the referenced register is program counter (PC) and hence this addressing mode is also known as PC–relative addressing.

The effective address is determined by adding the contents of PC to the address field. EA = PC + Address part of instruction.

The address part is a signed number so that it is possible to have branch target location either before or after the branch instruction. This addressing mode is commonly used to specify the target address in branch instructions.

• Example : JNZ BACK: This instruction causes program execution to go to the branch target location identified by the name BACK, if the branch condition is satisfied.

15. What are addressing modes?

Part of the programming flexibility for each processor is the number and different kind of ways the programmer can refer to data stored in the memory or I/O device. The different ways that a processor can access data are referred to as addressing schemes or addressing modes.

16. What do you mean by effective address?

An address computed by the processor when executing a memory access or branch instruction or when fetching the next sequential instruction is known as Effective Address (EA). An effective address can be made up from as many as three elements : The base, index and displacement.

17. List any four addressing modes ?

1. Immediate addressing

2. Register addressing

3. Base or displacement addressing

4. PC–relative addressing

5. Pseudodirect addressing

18. What is immediate addressing mode?

The operand is given explicitly in the instruction.

• Example : MOV #20, A : This instruction copies operand 20 in the register A. The sign # in front of the value of an operand is used to indicated that this value is an immediate operand.

19. What is indirect addressing mode ?

In this addressing mode, the instruction contains the address of memory which refers the address of the operand.

20. What is displacement addressing mode ?

This addressing mode combines the capabilities of direct addressing and register indirect addressing. In this addressing mode, instruction has two address fields : Value and referenced register. The effective address is computed by adding contents of  referenced register to value.

EA = value + (R)

Three common variation of displacement addressing are :

■ Relative addressing

■ Base register addressing

■ Index addressing.

21. What is base register addressing mode ?

The effective address of the operand is the contents of a register or the main memory location whose address is given explicitly in the instruction.

Example : MOV (R0), A : This instruction copies the contents of memory addressed by the contents of register R0  into the register A.

22. What is index addressing mode ?

In this addressing mode, the address field references the main memory and the referenced register contains a positive displacement from that address. EA = Memory address + (R).

The indexing is a technique that allows programmer to point or refer the data (operand) stored in sequential memory locations one by one. It is an efficient mechanism for performing iterative operations.

• Example : MOV [R1 + RI], R : In this instruction main memory address is given by register R1 and the referenced register RI gives the positive displacement. The contents of the memory address generated by the addition of main memory address and displacement is copied to register R.

23.What is autoincrement addressing mode?

The effective address of the operand is the contents of a register specified in the instruction. After accessing the operand, the contents of this register are incremented to address the next location.

• Example : MOV R0, (R2)+ : The above instruction copies the contents of register R0 into the memory location whose address is specified by the contents of register R2. After copy operation, the contents of register R2 are automatically incremented at by 1.

24 What is autodecrement addressing mode ?

The contents of a register specified in the instruction are decremented and then they are used as an effective address to access a memory location.

• Example : MOV – (R0), R1 : This instruction, initially decrements the contents of register R0 and then the decremented contents of register R0 are used to address the location. Finally, the contents from the addressed memory location are memory copied into the register R1.

25. What is stack addressing mode ?

A stack is linear array of reserved memory locations. It is associated with a pointer called Stack Pointer (SP).

In stack addressing mode, stack pointer always contains the address of Top Of Stack (TOS) where the operand is to be stored or located. Thus, the address of the operand (source or destination) is the contents of stack pointer.

This addressing mode is the special case of register indirect addressing where referenced register is a stack pointer.

Usually, stack grows in the direction of descending addresses, (descending stack), starting from a high address and progressing to lower one. In this stack, SP is decremented before any items are appended (pushed) on stack and SP is incremented after any items popped from the stack.

• Example : PUSH R : This instruction decrements SP and copies the contents of register R on to the top of stack pointed by stack pointer.

26. Brief about relative addressing mode with an example.

Here, the referenced register is program counter (PC) and hence this addressing mode is also known as PC–relative addressing.

The effective address is determined by adding the contents of PC to the address field. EA = PC + Address part of instruction.

The address part is a signed number so that it is possible to have branch target location either before or after the branch instruction. This addressing mode is commonly used to specify the target address in branch instructions.

• Example : JNZ BACK: This instruction causes program execution to go to the branch target location identified by the name BACK, if the branch condition is satisfied.

27. What is meant by an addressing mode? Mention most important of them.

Part of the programming flexibility for each processor is the number and different kind of ways the programmer can refer to data stored in the memory or I/O device. The different ways that a processor can access data are referred to as addressing schemes or addressing modes.

An address computed by the processor when executing a memory access or branch instruction or when fetching the next sequential instruction is known as Effective Address (EA). An effective address can be made up from as many as three elements : The base, index and displacement.

28. Distinguish between autoincrement and autodecrement addressing mode.

Autoincrement addressing mode : The effective address of the operand is the contents of a register specified in the instruction. After accessing the operand, the contents of this register are incremented to address the next location.

• Example : MOV R0, (R2)+ : The above instruction copies the contents of register R0 into the memory location whose address is specified by the contents of register R2. After copy operation, the contents of register R2 are automatically incremented at by 1.

Autodecrement addressing mode : The contents of a register specified in the instruction are decremented and then they are used as an effective address to access a memory location.

• Example : MOV – (R0), R1 : This instruction, initially decrements the contents of register R0 and then the decremented contents of register R0 are used to address the location. Finally, the contents from the addressed memory location are memory copied into the register R1.

29.  State the need for indirect addressing mode. Give an example.

 Answer: In indirect addressing mode, the instruction specifies the address where the address of the operand is located. In case of register indirect addressing, the address of the operand is specified by the contents of the register. These operating modes are needed when we need to access arrays. Specifically, in register indirect addressing, registers are used as pointers and they are very useful for dealing with arrays or tables of simple data values.

Consider Fig. 8.7.1, where a data structure consists of a sequence of consecutive byte values. The pointer register contains (1000)_H, corresponding to the first item in the structure. To access item 2 in the structure we only need to increment the value in the pointer register.

30. State the advantages of hardwired control unit.

 Answer:

• Hardwired control unit is fast because control signals are generated by combinational circuits.

• The delay in generation of control signals depends upon the number of gates.

• It has greater chip area efficiency since its uses less area on–chip.

31. State the disadvantages of hardwired control unit.

 Answer:

• More the control signals required by CPU; more complex will be the design of control unit.

• Modifications in control signal are very difficult. That means it requires rearranging of wires in the hardware circuit.

• It is difficult to correct mistake in original design or adding new feature in existing design of control unit.

32. What is microprogramming ?

 Answer: Microprogramming is a method of control unit design in which the control signal selection and sequencing information is stored in a ROM or RAM called a control memory CM.

33. What is control memory ?

 Answer: A memory that is part of a control unit is referred to as a control memory. It stores the sequences of micro–operations to be performed to execute microinstructions.

34. Give full form of CAR.

 Answer: CAR stands for Control Address Register.

35. What is microprogram sequencing ?

 Answer: Determining the address of the next microinstruction to be executed using microprogram sequencer is called microprogram sequencing.

37. What is address sequencing ?

 Answer: Each computer instruction has its own microprogram routine in control memory to generate the micro–operations that execute the instruction. To execute a particular computer instruction, accessing a corresponding microinstruction routine, sequencing the microinstructions within the routine and if necessary branching from one routine to another is known as address sequencing.

37. What is microinstruction ?

 Answer: Each word in the control memory is a microinstruction which specifies the control signals to be activated to perform one or more micro–operations.

38. What is microcode ?

 Answer: The translation of symbolic microprogram to binary produces a binary microprogram called microcode.

39. What is microprogram ?

 Answer: A sequence of one or more micro–operations designed to perform specific operation, such as addition, multiplication is called a microprogram.

40. What is pipeline register?

 Answer: The control data register holds the present microinstruction while the next address is computed and read from memory. This data register is sometimes called a pipeline register. It allows the execution of micro–operations specified by the control word and the generation of the next microinstruction simultaneously.