General-purpose fictional computer: MMIX, MMIXAL Hello World Example

string  BYTE   "Hello, world!",#a,0   string to be printed (#a is newline and 0 terminates the string)
  Main  GETA   $255,string            get the address of the string in register 255
        TRAP   0,Fputs,StdOut         put the string pointed to by register 255 to file StdOut
        TRAP   0,Halt,0               end process