LEGv8 Machine Language & Instruction Formats (Section 2.5)

1. Humans vs. Computers: Instructions

• Humans write assembly language instructions (e.g., ADD X9, X20, X21).
• Computers store and execute machine language instructions — patterns of high/low electronic signals, represented as numbers.
• Each part of an instruction is a number; placing numbers side by side forms the machine instruction.
• The 32 LEGv8 registers are numbered X0–X31.

2. Assembly → Machine Instruction Example

Assembly instruction:

ADD X9, X20, X21

Machine instruction fields (decimal):

• Opcode (ADD): 1112
• Second source operand: X21 (21)
• Unused field: 0
• First source operand: X20 (20)
• Destination register: X9 (9)

Machine instruction fields (binary):

• 10001011000 (opcode, 11 bits)
• 10101 (X21, 5 bits)