Introduction to Assembly Language

Introduction to Assembly Language

• Assembly Language is a low-level programming language tied to a specific computer architecture.
• The 8086 microprocessor is among the earliest in the x86 family.
• TASM (Turbo Assembler) is a tool for writing and compiling assembly programs for x86 architecture.
• Learning Assembly Language aids in understanding how computers execute instructions on a hardware level.
• It is critical for writing optimized code in performance-sensitive applications and for reverse engineering.

Structure of an Assembly Program

• Assembly programs consist of various segments:
  • Data Segment: Declares variables.
  • Code Segment: Contains executable instructions.
  • Stack Segment: Manages function call stack.
• Example Structure:

  .model small
  .stack 100h
  .data
  message db 'Hello, World!', '$'
  .code
  main proc
    mov ax, @data
    mov ds, ax
    mov ah, 09h
    lea dx, message
    int 21h
    mov ah, 4Ch
    int 21h
  main endp
  end main
  

• Save the file with a .asm extension.

Registers of 8086

• 8086 microprocessor has 16-bit registers used for various operations.
• General Purpose Registers: AX, BX, CX, DX (can be split into two 8-bit registers)
• Segment Registers: CS (Code Segment), DS (Data Segment), SS (Stack Segment), ES (Extra Segment)
• Pointer Registers: SP (Stack Pointer), BP (Base Pointer)
• Index Registers: SI (Source Index), DI (Destination Index)
• Breakdown of General Purpose Registers:
  • AX -> AH | AL
  • BX -> BH | BL
  • CX -> CH | CL
  • DX -> DH | DL

Addressing Modes

• Addressing modes determine how the operand's effective address is computed.
• Immediate Addressing: Operand is part of the instruction (e.g., MOV AX, 1234h).
• Direct Addressing: Address is specified directly (e.g., MOV AX, [1234h]).
• Register Indirect Addressing: Register holds the operand's address (e.g., MOV AX, [BX]).
• Indexed Addressing: Combines a base and index register (e.g., MOV AX, [BX + SI]).

Assembly Instructions

• Data Transfer Instructions:
  • MOV: Transfers data from source to destination (e.g., MOV AX, BX).
• Arithmetic Instructions:
  • ADD: Adds two values (e.g., ADD AX, BX).
  • SUB: Subtracts a value (e.g., SUB AX, 2).
  • MUL: Multiplies unsigned values (e.g., MUL BX).
• Control Flow Instructions:
  • JMP: Unconditional jump (e.g., JMP label).
  • LOOP: Repeats a block a specified number of times (e.g., LOOP label).

Input/Output in Assembly

• Assembly programs use interrupts for hardware interaction:
  • INT 21h for DOS services:
    • AH = 09h: Output string to console.
    • AH = 02h: Output character to console.
    • AH = 01h: Input a character from console.

Loops and Branching

• Assembly supports both conditional and unconditional branching:
  • CMP: Compares two values (e.g., CMP AX, BX).
  • Conditional jumps:
    • JNE: Jump if not equal.
    • JE: Jump if equal.
    • JL: Jump if less than.
    • JLE: Jump if less than or equal.
    • JNZ: Jump if not zero.
    • JZ: Jump if zero.
    • JG: Jump if greater than.
    • JGE: Jump if greater than or equal.
• Loop Example:
  • mov cx, 5 sets loop counter.
  • Loop body executed within a start_loop label, with the loop terminating when CX equals zero.