Question

What are the key design issues for a microprocessor instruction set?        

Why do you consider the above design issues to be important?

Answer 1

Key design issues of microprocessor instruction set:

1. Registers
2. Instruction formats
3. Operations repertoire
4. Operands
5. Literal values
6. Addressing modes
7. Decision making

1. Registers:

• The size and number of the registers required by the microprocessor are one of the design issues.
• If the number of the registers increases, then the implementation of the microprocessor becomes more complicated.
• If few registers are used, then the main memory space will run out.
• The implementation of the special registers like Program counter involves more design issues.

2. Instruction format:

The instruction format involves operands, operators, literal values and address location etc. The major issues are:

1. The length of the instruction.
2. What type of the instruction format?
3. Memory space required for the instruction.
4. The length and number of the addresses.

3. Operations repertoire:

The design issues are to find the type of the operation is used for the microprocessor, the number of the operations required and how complex of the operations are required for the designing the microprocessor.

4. Operands:

• How many operands are required for the each instruction?
• What are the data types of the operands?

5. Literal values:

• Some of the instructions require literal values for the execution of the instruction.
• The key issues are:

• To find the amount of space required for the literal values in the memory.
• What type of the registers is used to store the literal values?

6. Addressing modes:

The design issues are:

• What type of the addressing mode is used to design the microprocessor?
• How to use the addressing mode?
• To determine the number of registers and the amount of the memory required for the each addressing mode.

7. Decision making:

• Some of the instructions require a decision to execute the instruction like IF instruction.
• The key issue is to keep track of the address of the Program counter.
• The Program counter holds the address of the next instruction to be implemented.