sizeof есть!!! :)

;--- "hello world" for 64-bit Linux, using SYSCALL.
;--- assemble: JWasm -elf64 -Fo=Lin64_1.o Lin64_1.asm
;--- link:     gcc Lin64_1.o -o Lin64_1
stdout    equ 1
SYS_WRITE equ 1
SYS_EXIT  equ 60
    .data
string  db 10,"Hello, world!",10
    .code
_start:
    mov edx, sizeof string
    mov rsi, offset string
    mov edi, stdout
    mov eax, SYS_WRITE
    syscall
    mov eax, SYS_EXIT
    syscall
    end _start