Calling Functions in Assembly

CS 301: Assembly Language Programming Lecture, Dr. Lawlor

As a reminder, on our x86-64 linux machines:
These are the same registers used by your foo function, as well as any other functions you call.

Defining Functions in Assembly

The instruction "call" is used to call another function.   A called function can then return using "ret".  Here's an example:

mov edi,7 ; pass a function parameter
call otherFunction  ; run the function below, until it does "ret"
add eax,1 ; modify returned value

otherFunction: ; a function "declaration" in assembly
	mov eax,edi ; return our function's only argument

(Try this in NetRun now!)

Notice how a function declaration in assembly looks exactly like a goto label.  The only real difference is you can get back from a function by calling "ret" to return to whoever called you.

Getting There
Getting Back
call somewhere
ret  *
jmp somewhere
jmp backToYou

* How does it know where to return to?  Call stores the return address to jump back to on the stack, which we'll talk about at the next lecture.

External Functions in Assembly

By default, the assembler assumes functions you call are defined later in the same file, like we did above.  To call an external function, such as NetRun's "print_int", or a standard C library function like "exit", you need to tell the assembler the function is "extern".  "extern" isn't actually an instruction--it doesn't show up in the disassembly--it's just a message to the assembler, often called a pseudoinstruction.  In C++ or C, header files tend to contain declarations of extern functions, and so play the same role as extern in the assembler.

Here's how we'd call the UNIX function "exit", which ends the program:

extern exit ; tell assembler function is defined elsewhere
call exit ; call the function

(Try this in NetRun now!)

Here's how we'd call the standard C library function "getchar", which reads one ASCII character from cin, and returns it in eax.

extern getchar
call getchar

(Try this in NetRun now!)

NetRun includes a couple of useful functions designed to be easy to call from assembly.  For example, "read_input" parses an integer from cin, and returns it in eax:
extern read_input
call read_input
; eax has the read-in integer now
add eax,10000

(Try this in NetRun now!)

To show integers, "print_long" takes one long integer in edi, and prints it on the screen:
mov edi, 17 ; function argument goes into print_long in edi
extern print_long
call print_long

(Try this in NetRun now!)

One caution: if you call any function, that function has a perfect right to use eax, ecx, edx, esi, edi, and the other registers you can use.  This makes it a little tricky to store data across a function call.  For example, print_long will use eax, so this won't actually return 5:
mov eax, 5 ; I hope to return this
mov edi, 3 ; function argument for print_long
extern print_long
call print_long
; CAUTION: print_long trashed eax!

(Try this in NetRun now!)

The solution is to save our registers before calling functions--we'll see how to do this next lecture!