HG

Chapter 4 Notes - Data Transfer and Arithmetic Instructions

Overview of Chapter Four

  • Focus on data transfer between memory and CPU, and data addressing methods.
  • Discuss arithmetic instructions and their effects on flags.
  • Explain addressing modes and how to create loops.

Data Transfer Instructions

  • Data Transfer Instructions are the fundamental commands for moving data.
  • Operand types:
    • Immediate values (encoded directly in the instruction)
    • Registers
    • Memory locations
  • Move Instruction: Fundamental to data transfer.
  • Rules for moving data:
    • Reports that both operands must be of the same size.
    • Moves cannot be performed between two memory locations.
    • Representation of numbers and how they transfer from memory to register.

Operand Types Explained

  • Immediate: A fixed value encoded directly into the instruction (e.g., 8 can represent:
  • 8 bits (byte)
  • 16 bits (word)
  • 32 bits (DWORD)).
  • Immediate values are signed.
  • Registers: Named storage locations in CPU (like EAX, AL, etc.).
  • Register names are encoded as numbers for the CPU.
  • Memory: Reserved locations to store data (e.g., var1 with an assigned value).

Data Access and Instruction Encoding

  • Immediate values are part of instruction encoding, stored in the instruction code rather than in data segments.
  • Example of immediate encoding and its sizes differs:
  • EAX register and its encoding: Certain bits represent sizes for destination registers.
  • Dereferencing: Refers to accessing a variable's value based on its memory address.

Move Instruction Usage

  • Move Syntax:
  • Clear understanding of source and destination (e.g., from register to memory).
  • Register (
  • Immediate (
  • Immediate to Immediate or Memory to Memory is NOT Allowed.

Errors and Data Transfer

  • Common Error Examples:
  • Size mismatch (e.g., AL (1 byte) to EAX (4 bytes) leads to errors).
  • Instructions cannot be directed to memory or instruction pointers:
  • Certain registers (segment, instruction pointer) cannot be used as destinations.

Data Transfer Strategies

  • Zero-out Method: Before moving data, zero out existing registers to avoid leftover data corruption.
  • Using MOVZX and MOVSX to automatically handle sign extension during transfers.
  • MOVZX fills higher bits with zeros (for unsigned values).
  • MOVSX fills with sign bits based on the most significant bit.

Exchange Instruction

  • Exchange Purpose:
  • Allows swapping of values between two registers without using additional space to store values temporarily.

Summary of Key Points

  • Data transfer instructions include operand types, move instruction rules, immediate handling, dereferencing, and transfer strategies (zeroing, extending).
  • Recognize common errors in size mismatch and register limitations.
  • Understand and apply exchange effectively to minimize code complexity.