An assembly program is usually divided into sections. Each section has its use: .text is for writing instructions, .data is for declaring global variables.

An assembly program can be divided into multiple files. One of them should contain the _start label. It is the entry point, it marks the first instruction to be executed. This label should be declared global.

global  _start

section .data
msg     db      'Hello World!', 0Ah     ; assign msg variable with your message string

section .text

_start:
    mov     edx, 13     ; number of bytes to write - one for each letter plus 0Ah (line feed character)
    mov     ecx, msg    ; move the memory address of our message string into ecx
    mov     ebx, 1      ; write to the STDOUT file
    mov     eax, 4      ; invoke SYS_WRITE (kernel opcode 4)
    int     80h

    mov     ebx, 0      ; return 0 status on exit - 'No Errors'
    mov     eax, 1      ; invoke SYS_EXIT (kernel opcode 1)
    int     80h

Comments start with a semicolon.

section, global, and db are directives.

mov and int are instructions.

Save the code above as helloworld.asm. To compile and execute the assembly program, use the following commands

nasm -f elf32 helloworld.asm
ld -m elf_i386 helloworld.o -o helloworld
./helloworld

The option -f of the NASM command is for specifying the output file format. elf32 is output format for 32-bit linux object file.

The option -m of the ld command is for specifying machine architecture.