Introduction to Assembly Language

Study Notes

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.

Description

This quiz explores the fundamentals of Assembly Language, focusing on the 8086 microprocessor and TASM (Turbo Assembler). You'll learn about the structure of an assembly program, including different segments like data, code, and stack. Perfect for understanding low-level programming and computer architecture.