CSC231 Homework 3 2014

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

---

Preparation

Go through the 3 different loop sections below to get ready for the homework problems for this week.

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

Looping. Version 3

• Now try this program

;;; ; loop3.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
extern _println        
extern _printString


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

                section .data
Array           dd      1, 10, 20, 50, 100, 150, 200, 500, 1000
ARRAYLEN        equ     ($-Array)/4     ; the number of dwords in Array
temp            dd      0               ; a place to save ecx when needed
        
;;;  ------------------------------------------------------------
;;;  code area
;;;  ------------------------------------------------------------

                section .text
                global  _start

_start:


                mov     ecx, ARRAYLEN
                mov     ebx, Array
for:      

;;; display the integer at [ebx]
                mov     eax, dword[ebx]
                add     ebx, 4         ; make ebx point to next int in array
                call    _printDec
                call    _println
                
                loop    for

;;;  exit

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

• Read it carefully. Can you predict what it will output?
• Assemble, link, and run it.
• Verify that your intuition was correct. If it was not, make sure you understand why.
• You are now ready for the assignment.

Problem #1: warming up

• Write an assembly program called hw3_1.asm that uses a loop to print all the numbers from 1 to 20, in increasing order, one per line.

• Submit it to Moodle.

Problem #2: adding a string

• Write an assembly program called Hw3_2.asm that uses a loop to prints ecx in the following way:

1. ecx = 20
2. ecx = 19
3. ecx = 18
4. ecx = 17
...         
20. ecx = 1

(I replaced a large number of lines using ellipses in the list above.)

Problem #3: adding an input

• Your program should be called Hw3_3.asm, and is an extension of Hw3_2.asm.
• It works the same way Hw3_2 operates, but instead of looping a fixed number of times, i.e. 20 times, the program uses a number input by the user to control the loop.
• Example:

./Hw3_3
> 4
1. ecx = 4
2. ecx = 3
3. ecx = 2
4. ecx = 1

or, another example:

./Hw3_3
> 10
1. ecx = 10
2. ecx = 9
3. ecx = 8
4. ecx = 7
5. ecx = 6
6. ecx = 5
7. ecx = 4
8. ecx = 3
9. ecx = 2
10. ecx = 1

In the last example, the prompt generated by the program is > followed by a space, and the user input is 10. The program then loops 10 times.

• Submit your program on Moodle.

Problem #4

• Write a program called Hw3_4.asm that uses a loop to compute the first 40 Fibonacci numbers.
• Your program must use a loop.
• Your solution may use an array, but does not have to. You are free to pick the implementation. But you must you a loop to compute each Fibonacci as the sum of the previous two terms. Of course you start with the first two terms equal to 1; no need to compute these.
• The output of your program should be the following (no extra blank lines before or after):

Fibonacci(0) =1
Fibonacci(1) =1
Fibonacci(2) =2
Fibonacci(3) =3
Fibonacci(4) =5
Fibonacci(5) =8
Fibonacci(6) =13
Fibonacci(7) =21
Fibonacci(8) =34
Fibonacci(9) =55
Fibonacci(10) =89
Fibonacci(11) =144
Fibonacci(12) =233
Fibonacci(13) =377
Fibonacci(14) =610
Fibonacci(15) =987
Fibonacci(16) =1597
Fibonacci(17) =2584
Fibonacci(18) =4181
Fibonacci(19) =6765
Fibonacci(20) =10946
Fibonacci(21) =17711
Fibonacci(22) =28657
Fibonacci(23) =46368
Fibonacci(24) =75025
Fibonacci(25) =121393
Fibonacci(26) =196418
Fibonacci(27) =317811
Fibonacci(28) =514229
Fibonacci(29) =832040
Fibonacci(30) =1346269
Fibonacci(31) =2178309
Fibonacci(32) =3524578
Fibonacci(33) =5702887
Fibonacci(34) =9227465
Fibonacci(35) =14930352
Fibonacci(36) =24157817
Fibonacci(37) =39088169
Fibonacci(38) =63245986
Fibonacci(39) =102334155

• Submit your program on Moodle.