A simple example of linking assembly programs to C programs.

called_hello.asm

section .data
        msg db "Hello, World.asm","\0x0a" ;

section .text
        global hello

hello:
        mov rax, 1
        mov rdi, 1
        mov rsi, msg
        mov rdx, 17 ; 17 characters in msg
        syscall

        ret

calls_hello.c

void hello(void);

int main(void)
{
        hello();
        return 0;
}

run.sh

#!/bin/bash

nasm -f elf64 called_hello.asm -o called_hello.o
gcc -c calls_hello.c -o calls_hello.o

ld called_hello.o calls_hello.o