LearnAsm

introduction to 64 bit assembly

NASM - The Netwide Assembler

section .data
    prompt: db "Please enter your name: "
    prompt_len: equ $-prompt

$ evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using JMP $

$$ evaluates to the beginning of the current section; so you can tell how far into the section you are by using ($-$$)