The machine code of LEGv8 instruction SUB X15,X16,X17 in hexadecimal is

The machine code of LEGv8 instruction SUB X15,X16,X17 in hexadecimal is?

The Correct Answer and Explanation is :

The LEGv8 instruction SUB X15, X16, X17 performs a subtraction of the values stored in registers X16 and X17 and stores the result in register X15. In this instruction, SUB stands for subtract, X15 is the destination register, X16 is the first operand, and X17 is the second operand.

Steps to Determine the Machine Code in Hexadecimal

1. Opcode and Format:
   The SUB instruction in LEGv8 follows the Arithmetic format and is classified under the R-type instructions. The general format for R-type instructions in LEGv8 is:

   | opcode (6 bits) | Rn (5 bits) | Rm (5 bits) | shamt (6 bits) | funct3 (3 bits) | Rd (5 bits) | funct7 (7 bits) |

In this case, SUB is an operation that subtracts, so we need to use the funct3 and funct7 values associated with subtraction in the LEGv8 instruction set.

2. Opcode for SUB:
   The opcode for arithmetic instructions (including subtraction) is 0100011.
3. Register and Immediate Values:

• Rd = X15 = register number 15 → binary 01111
• Rn = X16 = register number 16 → binary 10000
• Rm = X17 = register number 17 → binary 10001

2. Function Codes:

• funct3 for subtraction (SUB) is 000.
• funct7 for subtraction is 0100000.

2. Construct the Machine Code:
   Putting all the fields together:

   | opcode (6 bits) | Rn (5 bits) | Rm (5 bits) | shamt (6 bits) | funct3 (3 bits) | Rd (5 bits) | funct7 (7 bits) |
   | 0100011         | 10000       | 10001       | 000000         | 000             | 01111       | 0100000         |

This gives us the binary machine code for the instruction.

6. Final Hexadecimal Representation:
   The resulting binary instruction 0100011 10000 10001 000000 000 01111 0100000 is:

   0x8A0113F3

Final Answer:

The machine code of the SUB X15, X16, X17 instruction in hexadecimal is 0x8A0113F3.

Explanation:

This breakdown of the LEGv8 instruction illustrates how the binary fields for the opcode, registers, function codes, and other components are structured according to the R-type format for the SUB instruction. This method ensures that the operation is correctly executed by the processor when the program is run.