Using scanf and printf in assembly

Notes

• We have used printf in c++ already
  • It takes a pointer to a format string
  • And a value or pointer to every argument that matched the format string.
• The call to scanf works much the same way.
  • A pointer to a format string
  • Pointers to all arguments.
  • We need to be careful with strings, as there is no way to limit the input
  • For this reason we will use a different routine for string input.
• In general, this is a band aid solution.
• In any case,
  • rdi contains the address of the format string
  • rsi should contain the first element, or pointer to the first input location.
  • rdx is the third parameter or second element
  • rcx is the fourth parameter or third element
  • r8 is the fifth parameter or fourth element
  • r9 is the sixth parameter or fifth element
  • There can be further parameters but
    1. this is silly
    2. They are more work, but we will learn how later
    3. Break your input/output up over several calls.
• In any case, rax will contain the number of items successfully input or the numbers of bytes output
• Take a look at input.asm.
• For strings, we will use read
  • reference.
  • ssize_t read(int fd, void buf[count], size_t count);
  • The return value is the number of bytes read
  • The fd is a file descriptor
    • 0 is the standard input by default
    • We can deal with others later.
    • But this is defined in my file CONSTANTS.h
  • void buf[count] is a pointer to the allocated memory to hold the input.
  • count is the maximum input size, in bytes, to read
• read is system call 0, see this page
• To read a string
  • place SYS_READ in rax
  • place STDIN in rdi
  • place the address of the buffer in rsi
  • place the size in rdx
  • Call syscall
• The numbers of bytes read is in rax
• See the example in input.asm
• This has a problem, but we will address it later.