Assembly Language Basics

Overview

• Assembly Language: Low-level language close to machine code.

• Focus: Understanding how assembly language works, specifically x86 assembly, rather than mastering a specific type.

Memory Architecture

• Little-Endian: Values are written with the least significant byte first.

  • Example: 32-bit double word 0x1BA583D4 is written in memory as D4, 83, A5, 1B.

Instruction Format

• Operands: x86 instructions can have 0-3 operands. Typically follows the format mnemonic destination, source.

  • Mnemonic: Reserved name used for operations (e.g., MOV for copying values).

  • Opcode: The actual operation code interpreted by hardware.

  • Example: MOV EAX, 0xCODE5AFE - MOV is the mnemonic, EAX is the destination, and 0xCODE5AFE is the source operand.

Registers

• Data Registers: Used for arithmetic, logical, and other operations.

  • 32-bit Registers: EAX, EBX, ECX, EDX

    • 16-bit Versions: AX, BX, CX, DX

      • 8-bit Versions: AH, AL, BH, BL, CH, CL, DH, DL

  • Primary Accumulator: EAX (used in I/O and arithmetic operations).

  • Base Register: EBX

  • Count Register: ECX (used for loop counts).

  • Data Register: EDX (also used for I/O operations).

• Pointer Registers:

  • 32-bit: EIP, ESP, EBP

    • 16-bit: IP, SP, BP

  • EIP: Instruction pointer (offset address of the next instruction).

  • ESP: Stack pointer (points to the top of the stack).

  • EBP: Base pointer (references parameter variables in the stack).

• Index Registers:

  • ESI: Source index (used for string operations).

  • EDI: Destination index (used for string operations).

• Control Registers (Flags):

  • ZF (Zero Flag): Set to 1 if the result of an operation is zero; 0 otherwise.

Segments

• Code Segment (CS): Contains executable code.

• Data Segment (DS): Contains data.

• Stack Segment (SS): Contains stack data.

• Extra Segments (ES, FS, GS): For additional data storage.

64-bit vs. 32-bit Registers

• 64-bit Registers: Use R prefix (e.g., RAX, RBX, RCX, RDX).

  • New Registers: R8 to R15.

  • Stack Size: Larger.

  • Flags: RFLAGS register.

Stack

• Purpose: Used for temporary data storage, passing arguments to functions, and saving register values.

• Instructions:

  • PUSH: Stores a value on the stack.

  • POP: Removes a value from the stack.