DYNAMIC STRING LENGTH CALCULATION

;alive.asm
section .data
      msg1   db    "Hello, World!",10,0       ; string with NL and 0
      msg1Len      equ    $-msg1-1     ; measure the length, minus the 0
      msg2   db    "Alive and Kicking!",10,0  ; string with NL and 0
      msg2Len      equ    $-msg2-1     ; measure the length, minus the 0
      radius dq    357                 ; no string, not displayable?
      pi     dq    3.14                ; no string, not displayable?
section .bss
section .text
      global main
main:
    push           rbp            ; function prologue
    mov            rbp,rsp        ; function prologue
    mov            rax, 1         ; 1 = write
    mov            rdi, 1         ; 1 = to stdout
    mov            rsi, msg1      ; string to display
    mov            rdx, msg1Len   ; length of the string
    syscall                       ; display the string
    mov            rax, 1         ; 1 = write
    mov            rdi, 1         ; 1 = to stdout
    mov            rsi, msg2      ; string to display
    mov            rdx, msg2Len   ; length of the string
    syscall                       ; display the string
    mov            rsp,rbp        ; function epilogue
    pop            rbp            ; function epilogue
    mov            rax, 60        ; 60 = exit
    mov            rdi, 0         ; 0 = success exit code
    syscall                       ; quit