NASM Coding: Basics - Examples

1. ret = write (myfile, arr, 13);

   This is a C syscall for writing the first

   13

   characters of string

   arr

   into a file (or device, like the screen) whose file ID number is in

   myfile

   . The

   write

   syscall returns the number of successfully written characters, or

   -1

   in case of an error.

   Since

   write

   takes only $3$ arguments, we'll use only the registers

   rdi

   ,

   rsi

   , and

   rdx

   to put arguments in, and

   rax

   for the syscall number. After the syscall returns,

   rax

   will include the return value from

   write

   .

   mov       rax, 1                  ; syscall code 1 is for 'write'
   mov       rdi, 1                  ; file ID 1 is for stdout (= screen)
   mov       rsi, arr                ; String variable 'arr' to output
   mov       rdx, 13                 ; number of bytes / characters to output
   syscall                           ; invoke operating system to do the writing