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## Assembly Language - Environment Setup This chapter will guide you through setting up the NASM assembly development environment on a Linux system and completing your first assembly program. * * * ## Linux Environment Preparation If you are using a Windows system, it is recommended to install WSL (Windows Subsystem for Linux) or use a virtual machine to obtain a Linux environment. macOS users can install Linux in a virtual machine or use a Docker container. All examples in this tutorial are based on the Ubuntu/Debian system. For other distributions, please adjust the package manager commands accordingly. * * * ## Installing the NASM Assembler On Ubuntu/Debian systems, use the apt package manager to install NASM: $ sudo apt update $ sudo apt install nasm After installation, verify that NASM is installed successfully: $ nasm -version NASM version 2.16.01 If you see the version number output, NASM has been successfully installed. * * * ## Installing the Linker The assembler generates object files (.o files), which cannot be run directly. You need a linker to convert the object files into executable files. We use the linker that comes with GCC: $ sudo apt install gcc Alternatively, you can directly use the GNU linker ld: $ ld --version In this tutorial, we mainly use gcc for linking because it automatically handles some low-level details, making it more suitable for beginners. * * * ## Installing the GDB Debugger GDB (GNU Debugger) is the most commonly used debugger on Linux. It can execute programs step by step, view register states, and inspect memory contents. $ sudo apt install gdb Verify GDB installation: $ gdb --version GNU gdb (Ubuntu 12.1-0ubuntu1) 12.1 * * * ## First Assembly Program: Hello, tutorial Create a new file `hello.asm` and enter the following content: ## Example ; File path: hello.asm ; First assembly program: print Hello, tutorial! ; NASM syntax, Linux 32-bit section.data; Data segment: stores initialized data msg db'Hello, tutorial!',0xA; Define string msg, 0xA is newline character len equ$- msg ; Calculate string length: current address - msg starting address section.text; Code segment: stores executable instructions global _start ; Declare _start as program entry point _start:; Program entry ; System call sys_write (4): output string to stdout mov eax,4; System call number 4 = sys_write mov ebx,1; File descriptor 1 = stdout (standard output) mov ecx, msg ; Address of string to output mov edx, len ; String length int 0x80; Trigger interrupt, execute system call ; System call sys_exit (1): exit program normally mov eax,1; System call number 1 = sys_exit mov ebx,0; Return value 0 = normal exit int 0x80; Trigger interrupt, execute system call * * * ## Compiling and Running The entire compile and run process is shown in the figure:  Use the following commands to compile and run this program: $ nasm -f elf32 hello.asm -o hello.o # Assemble: convert .asm to .o object file $ ld -m elf_i386 hello.o -o hello # Link: convert .o to executable file $ ./hello # Run programHello, tutorial! The functions of the three commands above: | Step | Command | Description | | --- | --- | --- | | 1. Assemble | `nasm -f elf32 hello.asm -o hello.o` | -f elf32 specifies 32-bit ELF output format, -o specifies output file name | | 2. Link | `ld -m elf_i386 hello.o -o hello` | -m elf_i386 specifies 32-bit link mode, generates executable file hello | | 3. Run | `./hello` | Execute program in current directory | * * * ## Using GCC to Link (Recommended Method) If your environment's ld configuration is complex, you can use gcc to link, as it automatically handles C runtime and other details: $ nasm -f elf32 hello.asm -o hello.o $ gcc -m32 hello.o -o hello -nostartfiles $ ./hello Hello, tutorial! The `-nostartfiles` option tells GCC not to add default startup code, because we have defined our own `_start` entry point. * * * ## Code Structure Analysis Let's understand the program section by section: | Section | Code | Description | | --- | --- | --- | | Data segment declaration | `section .data` | Declare data segment, stores variables and constants | | String definition | `msg db 'Hello, tutorial!', 0xA` | db defines byte sequence, 0xA is newline ASCII code | | Length calculation | `len equ $ - msg` | $ represents current address, subtract msg address to get string length | | Code segment declaration | `section .text` | Declare code segment, stores instructions | | Entry declaration | `global _start` | Export _start as global symbol, the linker needs it | | Entry label | `_start:` | Location where program begins execution | > `int 0x80` is the Linux 32-bit system call instruction. It triggers a software interrupt, causing the kernel to execute the system call we specified through eax. This is the bridge between user programs and the operating system kernel. * * * ## Using GDB for Debugging Add debugging information during compilation, then use GDB to execute step by step: $ nasm -f elf32 -g hello.asm -o hello.o # -g adds debugging information $ ld -m elf_i386 hello.o -o hello $ gdb ./hello (gdb) break _start # Set breakpoint at _start(gdb) run # Run program(gdb) stepi # Single step one instruction(gdb) info registers # View all registers(gdb) x/s msg # View msg string content(gdb) quit # Exit GDB | GDB Command | Function | | --- | --- | | `break _start` | Set breakpoint at _start label | | `run` | Run program, stop at breakpoint | | `stepi` | Single step one machine instruction | | `info registers` | Display current values of all registers | | `x/s address` | View content at specified address in string format | * * * ## Common Issues > If you are compiling 32-bit assembly on a 64-bit system, you need to install 32-bit compatibility libraries: `sudo apt install gcc-multilib` > If ld reports error "cannot find entry symbol _start", check whether your code correctly has `global _start` and whether the label name is exactly the same (note the underscore).