Basic Linux Assembly Tutorial
Pre-requirements: running Linux,have installed NASM and ld. Have a text editor you like
Assembly is the most important language any low level developer can learn. Once you understand assembly you can write higher level ends in other languages but there is no plausible alternative for assembly if you are working outside of an established kernel.
Assembly comments are written as “;this is a comment”. On a line anything after “;” is ignored by the assembler.
Here is a basic program that prints “Hello World!” to the console.
Place the following code into helloworld.asm
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | section .data message: db 'Hello World!',10 messageLength: equ $-message section .text global _start _start: mov eax,4 mov ebx,1 mov ecx,message mov edx,messageLength int 80h mov eax,1 mov ebx,0 int 80h |
The “mov” instruction works by taking the parameter on the right and putting it into the variable on the left. “int 80h” is just Linux’s way of telling the CPU you have something important to ask it. Whatever value is in eax tells the CPU what function you want to execute. In this case 1 is the print function. The register ebx tells the kernel to call the write function to the standard output which is the console.
To make the assembly code an executable now, you must first assemble it into an object file which is the format used to translate source code from all languages into a common middle ground.
nasm -f elf helloworld.asm
With an object file created the last step is now to create an executable. The linker takes this object file and checks it to make sure all the references are usable on the system.
ld -s -o helloworld helloworld.o
To run the program
./helloworld
admin | 
July 26, 2009 1:47 pm
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