CSC231 Homework 5 2014

--D. Thiebaut (talk) 07:30, 29 October 2014 (EDT)

---

Preparation

The two programs below illustrate how you would put a function, f(), in a separate file, and call it from a main() program kept in another file.
The specific details that are important and that make the system work:

• there is only one global _start label, and it is in main. The linker (ld) will complain if there are more than one _start label, as it is where Linux will start running your program.
• The label f in f.asm must be global, so that example.asm can access it.
• The label f in example.asm must be extern, so that nasm will not complain that it is not finding the label f used in the call instruction.
• Both example.asm and f.asm should be assembled separately, and linked together with the 231Lib library.

Example.asm

;;; ; example.asm
;;; ; D. Thiebaut
;;; ;
;;; ; A simple program to demonstrate how to link a main
;;; ; program and an extern function.
;;; ;
;;; ; to assemble and run:
;;; ;
;;; ;     nasm -f elf -F  stabs example.asm
;;; ;     nasm -f elf -F  stabs f.asm
;;; ;     ld -melf_i3896 -o example example.o f.o 231Lib.o
;;; ;     ./example
;;; ; -------------------------------------------------------------------


;;;  ------------------------------------------------------------
;;;  data areas
;;;  ------------------------------------------------------------

                section .data
a               dd      100
b               dd      200
c               dd      300
result          dd      0

;;;  ------------------------------------------------------------
;;;  code area
;;;  ------------------------------------------------------------

                section .text
                global  _start
extern          _printDec
extern          _println
extern          f
        
_start:
;;; compute f( a ) = 2*a
                push    dword[a]
                call    f
                call    _printDec
                call    _println
        
;;; compute f( 3 ) = 2*3 = 6
                push    dword 3 
                call    f
                call    _printDec
                call    _println

;;; compute f( b + f (c) + 1 )
                push    dword[c]
                call    f
                inc     eax
                add     eax, dword[b]
                push    eax
                call    f
                call    _printDec
                call    _println
                
       
;;;  exit()

                mov     eax,1
                mov     ebx,0
                int     0x80    ; final system call

f.asm

;;; ; f.asm
;;; ; D. Thiebaut
;;; ; A simple program with one global function that computes
;;; ; f(a) = 2*a
;;; ;
;;; ; -----------------------------------------------------------


                section .text
                global  f

;;; ;------------------------------------------------------------
;;; ; f(x): a function that receives a parameter through
;;; ; the stack and that computes y = 2*x
;;; ; x is at [ebp+8]
;;; ; returns the result in eax.
;;; ; does not modify any of the other registers (besides eax)
;;; ;------------------------------------------------------------
f:              push    ebp
                mov     ebp, esp
                mov     eax, dword[ebp+8]
                add     eax, eax
                pop     ebp
                ret

Assembly/Link Steps

 nasm -f elf example.asm
 nasm -f elf f.asm
 nasm -f efl 231Lib.asm
 ld -melf_i386 example.o  f.o  231Lib.o  -o example

Output

200
6
1602

Advantages

The advantage of this setup is that if we put several functions in a separate file, they can become a nice collection of utility functions, and serve as a library. This is what we are doing with 231Lib.asm, containing _printDec, _println, and _printString.
The second advantage is that I can test your functions by linking them to my own test program. If your functions are supposed to save all the registers they use but don't, my test program can easily check and report the problem.

Problem #1

Write a program called Hw5_1.asm that contains 1 function called
f1
that will compute f1( x ) = 3 * x - 1, and returns the result in eax.

Use f.asm above as an example. Your program cannot contain a main program with a label _start. You have to create it on your own main program to test your function.

You only submit Hw5_1.asm to Moodle, and keep your test program.

Problem #2

Write a program called Hw5_2.asm that contains 1 function called
f2
that computes f2( x ) = x + (x-1) + (x-2) + ... + 2 + 1, and returns the result in eax. We assume that x will always be strictly positive, and your function does not have to deal with negative numbers.

Submit your program to Moodle, and keep your test program.

Problem #3

Write a program called Hw5_3.asm that contains 2 functions, f3 and f4, defined as follows:

• f3( x ) = 2 x + 10
• f4( x ) = 3 f3( x ) + f3( x-1 )

Submit your program to Moodle, and keep your test program.

Problem #4

Write a program called Hw5_4.asm that contains 3 functions, f5, f6, and f7, defined as follows:

• f5( x ) = 2 x + 10
• f6( x ) = 3 f5( x ) + f5( x-1 )
• f7( x ) = 2 ( f6( x ) - f5( x ) )

Submit your program to Moodle, and keep your test program.