NASM Coding: Basics - Examples

2. ret = read (myfile, arr, 64);

   This is a C syscall for reading up to

   64

   characters from a file (or device, like the keyboard) whose file ID number is in

   myfile

   into string

   arr

   . The

   read

   syscall returns the number of successfully read characters, or

   -1

   in case of an error.

   As it was with

   write

   , the

   read

   syscall also takes only $3$ arguments and uses 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

   read

   .

   mov       rax, 0                  ; syscall code 0 is for 'read'
   mov       rdi, 0                  ; file ID 0 is for stdin (= keyboard)
   mov       rsi, arr                ; String variable 'arr' to read into
   mov       rdx, 64                 ; max number of bytes / characters to read
   syscall                           ; invoke operating system to do the reading