.equ '\0' это то же, что .equ '0'

.equ NOL, '0'
.equ NUL, "\0"
.equ ABZAC, '\n'

.section .text
.globl _start
_start:
  movq $NOL, %rcx
  movq $NUL, %rdi
  movq $ABZAC, %rax
  movq $60, %rax
  syscall

tmp.s: Assembler messages:
tmp.s:2: Warning: '\0' in quoted symbol name; behavior may change in the future

Disassembly of section .text:

0000000000000000 <_start>:
   0:	48 c7 c1 30 00 00 00 	mov    $0x30,%rcx
   7:	48 c7 c7 00 00 00 00 	mov    $0x0,%rdi
   e:	48 c7 c0 0a 00 00 00 	mov    $0xa,%rax
  15:	48 c7 c0 3c 00 00 00 	mov    $0x3c,%rax
  1c:	0f 05                	syscall

Constants
A constant is a number, written so that its value is known by inspection, without knowing any context. Like this:

.byte  74, 0112, 092, 0x4A, 0X4a, 'J, '\J # All the same value.

Strings
A string is written between double-quotes. It may contain double-quotes or null characters. The way to get special characters into a string is to escape these characters: precede them with a backslash `\' character. For example `\\' represents one backslash: the first \ is an escape which tells as to interpret the second character literally as a backslash (which prevents as from recognizing the second \ as an escape character). The complete list of escapes follows.


\b
Mnemonic for backspace; for ASCII this is octal code 010.
\f
Mnemonic for FormFeed; for ASCII this is octal code 014.
\n
Mnemonic for newline; for ASCII this is octal code 012.