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Part 2: Sending Function Arguments and Recieving Values Back

In Printing Command Line Arguments, we saw how to define and call functions in x86-64 Linux Assembly.

Now I wanted to know how to provide arguments to a function and return values. In other words, when this is done in a higher level language, how is it translated at the Assembly code level?

The answer is pretty straightforward. In x86-64 Assembly, there is a convention defining which registers are defined to be used for the first, second, third and so forth.

All further arguments are also pushed on to the stack.

Return values are expected to be stored on in register $rax.

Here is an example of code that utilized the conventional registers to use return values and pass arguments to print out a programs command line arguments:

section .data

section .text
    global _start
_start:
  call .getNumberOfArgs   ; expects return value in $rax
  mov rdi, rax
  call .printNumberOfArgs ; expects value to be in 1st argument, i.e. $rdi
  call .exit

.getNumberOfArgs:
  pop rbx         ; this is the address of the calling fxn. Remove it from the stack for a moment
  pop rax         ; get argc from stack
  push rbx        ; return address of calling fxn to stack
  ret

; expects value to be in 1st argument, i.e. $rdi
.printNumberOfArgs:
  pop rbx         ; this is the address of the calling fxn. Remove it from the stack for a moment
  add rdi, 48     ; convert number of args to ascii (only works if < 10)
  push rdi        ; push the ascii converted argc to stack
  mov rsi, rsp    ; store value of rsp, i.e. pointer to ascii argc to param2 of sys_write fxn
  mov rdx, 8      ; param3 of sys_write fxn is the number of bits to print
  push rbx        ; return the address of the calling fxn to top of stack.
  call .print
  ; clean up the number that was pushed onto the stack. Retaining the return address currently on top of stack
  pop rbx
  pop rcx
  push rbx
  ret
  
.print:           ; print expects the calling location to be at the top of the stack
  mov rax, 1
  mov rdi, 1
  syscall
  ret             ; return to location pointed to at top of stack

.exit:
  mov rax, 60
  mov rdi, 0
  syscall

There are some additional well-defined calling conventions are helpful to follow. See here.

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