CSC231 Homework 3 2014
--D. Thiebaut (talk) 15:17, 29 September 2014 (EDT)
This assignment deals with loop that use the ecx register and the loop instruction, and with indirect, and based indexed addressing modes. This assignment is due Tuesday, Oct. 6th, 2014, at 11:55 p.m. The first 3 loop sections are just for you to review loops, and do not need to be submitted.
Contents
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):
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181 6765 10946 17711 28657 46368 75025 121393 196418 317811 514229 832040 1346269 2178309 3524578 5702887 9227465 14930352 24157817 39088169 63245986 102334155
- Submit your program on Moodle.
Problem #5
- Write a program called Hw3_5.asm that works the exact same way as Problem #4, but displays a string in addition to the fibonacci terms.
- Same restrictions as for Problem #4
- 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.
Problem #6
- Assume that you have a program that uses a loop to compute Fibonacci numbers.
- The Fibonacci terms get stored in an array called Fib as they are computed. The array is an array of dwords.
- In the loop, the program computes
Fib[ i ] = Fib[ i-1 ] + Fib[ i-2 ];
- If we are not worried about possible overflow of the arithmetic, i.e. we are not worried that the computation might become incorrect at some point, and if we assume that the array can be as large as we want (we have a lot of RAM in our computer), how many Fibonacci terms can we expect to compute in 1 second if our computer has a 2.5 GHz Pentium processor?
- Write your answer as a paragraph that you will submit on Moodle, in the Homework 3 Problem 6 section. Make sure you explain your answer fully. Just putting down a number without justification, even is the number is in the acceptable range, will be worth the smallest possible grade over 0. You may want to add a snipet of assembly code to support your explanations.
