CSC231 Homework 3 2014

--D. Thiebaut (talk) 15:17, 29 September 2014 (EDT)

---

Preparation

Looping. Version 1

• Create the following program in your beowulf2 directory:

;;; ; loop1.asm
;;; ; D. Thiebaut
;;; ;
;;; ; a simple program using a loop in which the ecx
;;; ; register is printed.
;;; ;
;;; ; to assemble and run:
;;; ;
;;; ;     nasm -f elf loop1.asm
;;; ;     nasm -f elf 231Lib.asm
;;; ;     ld -melf_i3896 -o loop1 loop1.o 231Lib.o
;;; ;     ./loop1
;;; ; -------------------------------------------------------------------

extern _printDec                       ; the function that prints eax in decimal
extern _println                          ; the function that brings the cursor to the next line.

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

                section .text
                global  _start

_start:
                mov     ecx, 10
for:            mov     eax, ecx
                call    _printDec
                call    _println
                loop    for

;;;  exit()

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

• Run it.
• Verify that it displays the different values of ecx, the loop counter.

Looping. Version 2

• This time we write the same program as the one above, but add some code to make it display "ecx = 10", "ecx = 9", etc.
• So the body of the loop will look something like this:

                mov     ecx, msg
                mov     edx, MSGLEN
                call    _printString

You should recognize the way _printString works: you pass the address of the string in ecx, the length of the string in edx, and you call the function.

• The problem with this code is that it modifies ecx, which is our loop counter. So we'd better save ecx before we execute this code and restore it right after. We use a variable called temp for this purpose:

;;; in the data segment:
temp           dd        0                    ; safe place to save ecx

;;; back in the code segment:

                mov     dword[temp], ecx
                mov     ecx, msg
                mov     edx, MSGLEN
                call    _printString
                mov     ecx, dword[temp]

• Update your program with the new code, indicate that _printString is an extern function, and assemble, link, and run your new program.
• Verify that its output looks like this:

ecx = 10
ecx = 9
ecx = 8
ecx = 7
ecx = 6
ecx = 5
ecx = 4
ecx = 3
ecx = 2
ecx = 1