Understanding the Basic Syntax of Assembly Language

Assembly language is a low-level programming language that provides direct control over the computer’s hardware. In this blog post, we will explore the basic syntax of assembly language using a simple program that prints “Hello, world!” to the console.

section .bss     ; uninitialized data variables

section .text    ; code

global _main     ; must be declared for linker (ld)

_main:           ; tells linker entry point
    mov edx, len ; message length
    mov ecx, msg ; message to write
    mov ebx, 1   ; file descriptor (stdout)
    mov eax, 4   ; system call number (sys_write)
    int 0x80     ; call kernel

    mov eax, 1   ; system call number (sys_exit)
    xor ebx, ebx ; exit status 0
    int 0x80     ; call kernel

section .data               ; initialized data
msg db 'Hello, world!', 0xa ; string to be printed
len equ $-msg               ; length of the string

Let’s break down the different sections and instructions used in this program:

Sections

Assembly programs are typically organized into different sections. In this program, we have three sections: .bss, .text, and .data.

• The .bss section is used for declaring uninitialized data variables.
• The .text section contains the executable code.
• The .data section is used for declaring initialized data variables.

Entry Point

The _main: or _start: label denotes the entry point of the program. It is the first instruction executed when the program is run.

Instructions

• mov instructions are used to move data between registers and memory. For example, mov edx, len moves the length of the message to the edx register.
• int 0x80 is a software interrupt instruction that triggers a system call to the kernel. The value 0x80 indicates the system call interface for the Linux operating system.
• eax register holds the system call number. In this program, eax is set to 4 for sys_write (write to console) and 1 for sys_exit (program exit).
• len equ $ - msg calculates the length of the string by subtracting the memory address of `msg` from the current memory address (`$` symbol).

String and Printing

The msg variable holds the string “Hello, world!” terminated with a newline character (0xa). This string is printed to the console using the sys_write system call.

Program Termination

After printing the message, the program exits using the sys_exit system call.

Build and run the asm programs

Nasm is one of the most popular assembly language compilers, it is widely used for various platforms and architectures. In the following steps, we will install NASM on macOS using Homebrew, a popular package manager for macOS. And on Debian-based Linux systems, using the default package manager, apt.

Install nasm on MacOS

1. Open the Terminal application on your macOS system. You can find it in the “Utilities” folder within the “Applications” directory.

2. Install Homebrew by running the following command in the Terminal:

   /bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"
   

3. Once Homebrew is installed, you can use it to install NASM. Run the following command in the Terminal:

   brew install nasm
   

Install nasm on Debian-based systems

To install NASM on a Debian-based system, you can follow these steps:

1. Open a terminal.

2. Update the package lists by running the following command:

   sudo apt update
   

3. Install NASM using the package manager apt with the following command:

   sudo apt install nasm
   

Wait for the installation to complete. NASM and its dependencies will be downloaded and installed on your system. You can verify the installation by running the following command to check the version:

   nasm --version

You should see the version information for NASM displayed in the Terminal, confirming that NASM is successfully installed on your system.

To build and run an assembly program using NASM, follow these steps:

1. Save our assembly source file with the extension .asm, for example, program.asm.

2. Run the following command to compile the source code:

   nasm -f elf64 program.asm -o program.o
   

   This command assembles the assembly source code into an object file (program.o). Adjust the -f option according to your target architecture (e.g., elf32 for 32-bit).

3. Link the object file using a suitable linker, such as ld or GCC (GNU Compiler Collection). Run the following command:

   gcc program.o -o program
   

   This command links the object file and generates an executable file called program. You can replace gcc with the appropriate linker if needed.

4. Run the program by executing the following command:

   ./program
   

   This command executes the assembled and linked program, printing “Hello world!” to the console.

Conclusion

Understanding the basic syntax of assembly language allows you to gain a deeper insight into how programs interact with the underlying hardware. Although assembly can be complex, it offers unparalleled control and optimization possibilities.

Keep exploring and experimenting with assembly to expand your programming horizons.