global   main 
section .data
     message1:          db  "Hello World!", 0x0A
     message1_length:   equ $ - message1        

     STDIN:      EQU     0
     STDOUT:     EQU     1
     STDERR:     EQU     2

     SYS_READ:   EQU     0
     SYS_WRITE:  EQU     1
     SYS_EXIT:   EQU     60

     SUCCESS:    EQU     0

section   .text
main:  
          ; store a big number in the entire register
          mov       rax, 01111111111111111h 
          ; store a smaller number in the lower 1/2
step1:
          mov       eax, 022222222h
          ; store a smaller number in the lower 1/4
          mov       rax, 01111111111111111h 
          mov       ax, 03333h
          ; store a small number in the lower 1/8
          mov       rax, 01111111111111111h 
          mov       al, 044h
          ; store a small number in the second 1/8
          mov       rax, 01111111111111111h 
          mov       ah,  077h

          ; let's try some negative numbers
          mov       rax, 01111111111111111h 
          mov       al, -1
          mov       rax, 01111111111111111h 
          mov       ax, -2
          mov       rax, 01111111111111111h 
          mov       eax, -3
          mov       rax, 01111111111111111h 
          mov       rax, -4

          ; do a print to show the RAX register is wiped out after a system call
          mov     rax, SYS_WRITE
          mov     rdi, STDOUT
          mov     rsi, message1
          mov     rdx, message1_length
          syscall

          ; exit the program properly
          mov       rax, SYS_EXIT     ; system call for exit
          mov       rdi, SUCCESS      ; exit code 0
          syscall                     ; invoke operating system to exit