x86 Assembly Language is a low-level programming language for the x86 family of microprocessors. It provides a way to write programs that are closely tied to the hardware, offering fine-grained control over the CPU and memory. This module will cover the basics of x86 assembly, including its syntax, instructions, and how to write simple programs.

• Registers: Small storage locations within the CPU used for arithmetic, data storage, and control.
  • General Purpose Registers: EAX, EBX, ECX, EDX, ESI, EDI, EBP, ESP
  • Segment Registers: CS, DS, SS, ES, FS, GS
  • Instruction Pointer: EIP
  • Flags Register: EFLAGS
• Memory Segmentation: Divides memory into segments for code, data, and stack.
• Instruction Set: The set of instructions that the CPU can execute.

• Instructions: Consist of an operation code (opcode) and operands.
  • Example: MOV EAX, 1 (Move the value 1 into the EAX register)
• Labels: Used to mark locations in code.
  • Example: start:
• Comments: Begin with a semicolon (;) and are ignored by the assembler.
  • Example: ; This is a comment

• Hello World Program: A basic program to print "Hello, World!" to the console.

• Data Section: Defines the data used in the program.
  • hello db 'Hello, World!', 0: Defines a string with a null terminator.
• Text Section: Contains the code to be executed.
  • global _start: Defines the entry point of the program.
  • _start:: Label marking the start of the program.
  • mov eax, 4: Moves the syscall number for sys_write into the EAX register.
  • mov ebx, 1: Moves the file descriptor for stdout into the EBX register.
  • mov ecx, hello: Moves the address of the string into the ECX register.
  • mov edx, 13: Moves the length of the string into the EDX register.
  • int 0x80: Interrupt to call the kernel.
  • mov eax, 1: Moves the syscall number for sys_exit into the EAX register.
  • xor ebx, ebx: Sets the return status to 0.
  • int 0x80: Interrupt to call the kernel.

Modify the Hello World program to print "Hello, x86 Assembly!" instead.

Solution:

Write an x86 assembly program to add two numbers and print the result.

Solution:

• Forgetting to Null-Terminate Strings: Always ensure strings are null-terminated when working with syscalls.
• Incorrect Register Usage: Ensure the correct registers are used for specific operations.
• Ignoring the Length of Strings: Always provide the correct length of strings when using syscalls.

In this section, we covered the basics of x86 assembly language, including its architecture, syntax, and how to write simple programs. We also provided practical exercises to reinforce the concepts learned. In the next module, we will explore ARM Assembly Language, another popular assembly language for a different architecture.