Instruction format and Generate HEX codes for | Mov Bx, [BX] [SI] | Mov Ax, Dx

Introduction

x86 assembly language, a low-level programming language that operates at the heart of your computer’s architecture. In this blog post, we’ll explore the instruction format and generate HEX codes for two common x86 instructions “Mov Bx, [BX] [SI] & Mov Ax, Dx”.

Instruction Format

Before we delve into the specific instructions, let’s understand the general format of x86 instructions. The x86 instruction format typically consists of:

1. Operation Code (Opcode): This part specifies the operation to be performed.
2. ModR/M: Stands for “Modifier Register/Memory.” It provides additional information about the operands.
3. SIB (Scale Index Base): Used for more complex addressing modes involving scaling, indexing, and basing.
4. Displacement: An offset or displacement value for memory addressing.
5. Immediate Data: Immediate values used directly in the instruction.

Instruction 1: Mov Bx, [BX] [SI]

Let’s break down the instruction “Mov Bx, [BX] [SI]” into its components:

• Opcode: The opcode for the MOV instruction is typically 8B (in hexadecimal).
• ModR/M: In this case, the ModR/M byte is used to specify the addressing mode and the operands. The destination register is BX, and the source operand is located at the address formed by adding the contents of registers BX and SI.
• SIB: Since there is an index (SI) involved, a SIB byte might be present, but in this case, it’s not necessary.
• Displacement: Since there is no displacement, this part is 0.
• Immediate Data: There is no immediate data for this instruction.

Now, let’s assemble these components:

• Opcode: 8B
• ModR/M: Assuming BX is the destination and [BX] [SI] is the source, the ModR/M byte would be 33 in hexadecimal (21 in decimal).
• SIB: Not applicable (0)
• Displacement: 0
• Immediate Data: Not applicable

Putting it all together, the HEX code for “Mov Bx, [BX] [SI]” is 8B 33 00 00.

Instruction 2: Mov Ax, Dx

Now, let’s look at the “Mov Ax, Dx” instruction:

• Opcode: Again, the MOV opcode is 8B.
• ModR/M: Here, the destination register is AX, and the source register is DX.
• SIB: Not applicable.
• Displacement: 0
• Immediate Data: Not applicable.

Assembling the components:

• Opcode: 8B
• ModR/M: Assuming AX is the destination and DX is the source, the ModR/M byte would be C2 in hexadecimal (194 in decimal).
• SIB: Not applicable (0)
• Displacement: 0
• Immediate Data: Not applicable

The HEX code for “Mov Ax, Dx” is 8B C2 00 00.

Conclusion

Understanding x86 assembly language can be challenging, but breaking down instructions into their components helps demystify the process. By grasping the structure and meaning of each part, you’ll be well on your way to mastering low-level programming.