“Hello World” with assembly language, nasm.

Recently, I’ve gotten interested in assembly language. The purpose is to understand programming more deeply. At this time, I decided to use nasm because it is easy to install. The environment is macOS.

Preparation

$ brew install nasm
$ touch assemble.asm

The code

global    _main
section   .text
_main:    mov       rax, 0x02000004         
          mov       rdi, 1                  
          mov       rsi, message            
          mov       rdx, 13              
          syscall                           
          mov       rax, 0x02000001         
          xor       rdi, rdi                
          syscall
section   .data
message:  db        "Hello, World", 10

Output

$ nasm -fmacho64 assemble.asm
$ ld assemble.o -lSystem -o assemble
$ ./assemble
Hello, World

Explanation

・global: this file’s code can be used from other object files.

・main: entry point

・section .text: machine language is written in this section.

・mov: copy content A to content B (ex. mov contentB contentA)

・syscall: make kernel execute the written code

・xor: the exclusive OR. It is written in order to initialize rdi.

・rax: a register for system call number. 0x02000004 is “write”. 0x02000001 is “read”

・rdi: a register for file descriptor. 1 is standard output.

・rsi: a register for reading an address for message.

・rdx: a register for byte of an output object.

・section .data: used for declaring initialized data or constants

・db: 8 bytes

・10: means new line in ASCII

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https://www.linkedin.com/in/tomoharu-tsutsumi-56051a126/