MUL instruction | IMUL instruction in 8086 Assembly Language with examples

Introduction

This tutorial will guide you through the MUL and IMUL instructions in 8086 Assembly Language. These instructions are essential for performing multiplication operations in assembly programming. We will cover their usage, how to calculate flag bits, and provide practical examples to enhance your understanding.

Step 1: Understanding the MUL Instruction

The MUL instruction is used for unsigned multiplication in 8086 Assembly Language. It multiplies the accumulator (AL or AX) by a specified operand.

How to Use the MUL Instruction

• Syntax:

  MUL operand
  

• Operands: Can be:
  • 8-bit (AL)
  • 16-bit (AX)
  • 32-bit (EAX) in 32-bit mode

Example of MUL Instruction

1. Load a value into AX:

   MOV AX, 5
   

2. Multiply AX by another value:

   MUL 3 ; AX now contains 15
   

Calculating Flag Bits

• CF (Carry Flag): Set if the result is too large to fit in the destination.
• OF (Overflow Flag): Set if the result exceeds the range of the destination operand.

Step 2: Understanding the IMUL Instruction

The IMUL instruction is used for signed multiplication. It operates similarly to MUL but takes into account the sign of the operands.

How to Use the IMUL Instruction

• Syntax:

  IMUL operand
  

• Operands: Similar to MUL, but can also take two operands.

Example of IMUL Instruction

1. Load a signed value into AX:

   MOV AX, -5
   

2. Multiply AX by another signed value:

   IMUL -3 ; AX now contains 15 (as a signed value)
   

Calculating Flag Bits for IMUL

• The flags behave similarly to the MUL instruction:
  • CF: Set if the signed result is too large.
  • OF: Set if the signed result is out of range.

Step 3: Converting Multiplication Expressions to Assembly

To convert a multiplication expression into assembly language, follow these steps:

1. Identify the operands and their values.
2. Determine whether the operation is signed or unsigned.
3. Use the appropriate instruction (MUL for unsigned and IMUL for signed).
4. Load the operands into the accumulator registers before performing the multiplication.

Example Conversion

Expression: A * B where A = 6 and B = 4.

1. Load A into AX:

   MOV AX, 6
   

2. Multiply by B:

   MUL 4 ; AX now contains 24
   

Conclusion

In this tutorial, we explored the MUL and IMUL instructions used for multiplication in 8086 Assembly Language. Key takeaways include understanding how to use these instructions, calculate their flag bits, and convert multiplication expressions into assembly code. With this knowledge, you can effectively perform multiplication operations in your assembly programs.

Consider practicing with different values and expressions to solidify your understanding of these instructions.