#include <stdio.h> //for the printf

int main() // This is the program entry. It can't run otherwise.
{
     printf("Hello world!\n"); // Prints Hello World! on the screen. The \n makes a newline.
     return 9; // Stops the program with code 9
}
    

.calculate: // A label 
        mov r8, rcx     // mov rcx to r8
        add r8, 8       // add 8 to r8 (r8 now is equal to x + 8 )
        mov r9, rdx   // rdx -> r9
        add r9, 8      // add 8 to r9
        mov rcx, rax // etc
        mov rax, 0
        mov rdx, 0
    

Truth be told, the move from 32-bit and 64-bit is just learning the differences and caveats between the two; they're compatible 99% of the time. But I tried my best to look for a decent 64-bit tutorial since I knew you explicitly did not want 32-bit.

format PE console

entry start

include 'win32a.inc'

Hello db 'Hello World!',0

start:
        mov eax, Hello
        call [printf]
        call [ExitProcess]


; Import section
section '.idata' import data readable
 
library kernel32, 'kernel32.dll', \
        msvcrt,'msvcrt.dll'

import kernel32, \
       ExitProcess,'ExitProcess'
 
import  msvcrt, \
        printf, 'printf'      


    

P §░◄Φv÷♣╨☻■⌂►t Φ☼
    

start:
    push Hello
    call [printf]
    push 0
    call [ExitProcess]    

start:
    push Hello
    call [printf]
    add esp,4
    push 0
    call [ExitProcess]    

The 64-bit calling convention (for some reason referred as "ABI") is a pathetic trash design to begin with, especially the one by Microsoft.
-- snip--

/end rant sorry

Code:

start:
    push Hello
    call [printf]
    add esp,4
    push 0
    call [ExitProcess]
    

start: 
    push Hello 
    call [printf]  
    push 0 
    call [ExitProcess]
    

Thank you for correcting that error and explaining.

So I don't move the hello string into a register. I just push it in the stack?

system error wrote:

Code:

start:
    push Hello
    call [printf]
    add esp,4
    push 0
    call [ExitProcess]
    

This code shows the same results as:

Code:

start: 
    push Hello 
    call [printf]  
    push 0 
    call [ExitProcess]
    

What am I not understanding?

entry start

include 'win32a.inc'

Hello db 'Hello World! The number is %d.',0

subtract:
        sub ax,2
        push eax
        push Hello
        call [printf]
        add esp,4
        push 0
        call [ExitProcess]


start:
    mov eax, 4
    add eax,6
    push eax
    push Hello 
    call [printf]
    add esp,4
    call subtract




section '.idata' import data readable
 
library kernel32, 'kernel32.dll', \
        msvcrt,'msvcrt.dll'

import kernel32, \
       ExitProcess,'ExitProcess'
 
import  msvcrt, \
        printf, 'printf'    

    

format PE console

entry start

include 'win32a.inc'

Hello db 'Hello World! The number is %d.',0

subtract:
        sub eax,1
        push eax
        push Hello
        call [printf]
        add esp,4
        push 0
        call [ExitProcess]


start:
    mov eax, 4
    add eax,6
    push eax
   ; push Hello
   ; call [printf]
   ; add esp,4
    call subtract




section '.idata' import data readable
 
library kernel32, 'kernel32.dll', \
        msvcrt,'msvcrt.dll'

import kernel32, \
       ExitProcess,'ExitProcess'
 
import  msvcrt, \
        printf, 'printf'