CSC231 Homework 5 Fall 2017
--D. Thiebaut (talk) 09:27, 30 October 2017 (EDT)
This assignment is due Monday 11/6/2017 at 11:55 p.m.
Contents
Preparation
For this assignment you will need to downloaded an updated version of 231Lib.asm, which has been augmented with a new function for reading strings.
getcopy 231Lib.asm
An example program using the new function, _getString(), is shown below:
;;; ; hw5Prep.asm ;;; ; D. Thiebaut ;;; ; ;;; ; Gets a string from the user and prints it back ;;; ; ;;; ; to assemble and run: ;;; ; ;;; ; nasm -f elf hw5Prep.asm ;;; ; nasm -f elf 231Lib.asm ;;; ; ld -melf_i386 -o hw5Prep hw5Prep.o 231Lib.o ;;; ; ./hw5Prep ;;; ; ------------------------------------------------------------------- ;;; ------------------------------------------------------------ ;;; data areas ;;; ------------------------------------------------------------ section .data prompt db "> " ;;; ------------------------------------------------------------ ;;; code area ;;; ------------------------------------------------------------ section .text global _start extern _getString extern _printString extern _println _start: ;; prompt the user mov ecx, prompt mov edx, 2 call _printString ;; get a string from the user ;; _getString returns the address of the string read in ecx, ;; and the number of chars read in edx call _getString ;; since ecx and edx already contain the address and number ;; of chars, we can directly call _printString, which needs ;; the string and number of chars in the same registers call _printString ;; print a line-feed char. call _println ;;; exit() mov eax,1 mov ebx,0 int 0x80 ; final system call
Create a version of this program in your account, and play with it. Verify that it gets a string from you and prints it back.
Notes
- _getString saves the string in an array declared inside 231Lib.asm. This array contains at most 1000 bytes.
- when _getString returns to your main program, it will set ecx to the address of the array containing the string, and it will set edx to contain the number of characters the user typed at the keyboard.
- the user indicates the end of the string by pressing the ENTER key. The string does not contain the ENTER character (line-feed).
- although I didn't do that here because the program is so simple and short, it is a good idea, just after returning from a call to _getString, to save ecx and edx into two different variables in memory that will hold the address of the string and the number of chars entered at the keyboard.
Problem 1
Write a program called hw5a.asm that prompts the user for a string, and prints it back several times. The number of times the string is printed is the same as the number of characters in the string.
- Examples
cs231a@aurora ~ $ ./hw5a > B B
cs231a@aurora ~ $ ./hw5a > a a
cs231a@aurora ~ $ ./hw5a > hello hello hello hello hello hello
cs231a@aurora $ ./hw5a > chocolate chocolate chocolate chocolate chocolate chocolate chocolate chocolate chocolate chocolate
cs231a@aurora $ ./hw5a > Hall o ween Hall o ween Hall o ween Hall o ween Hall o ween Hall o ween Hall o ween Hall o ween Hall o ween Hall o ween Hall o ween Hall o ween
Note
- Your program does not have to handle 0-length strings. In other words, your program will not be tested with empty strings.
Submission
- Submit your program in the Homework 5, Problem 1 section on Moodle.
Problem 2
Modify your solution program for Problem 1 so that it removes the last character of the previous printed string as it goes through the loop:
- Examples
cs231a@aurora $ ./hw5b > Hall-o-ween Hall-o-ween Hall-o-wee Hall-o-we Hall-o-w Hall-o- Hall-o Hall- Hall Hal Ha H
cs231a@aurora $ ./hw5b > a a
cs231a@aurora ~/HWs/HW5/PB2 $ ./hw5b > 123456789 123456789 12345678 1234567 123456 12345 1234 123 12 1
Submission
- Submit your program in the Homework 5, Problem 2 section on Moodle.
Problem 3
Same as with Problem 2, but this time the triangle is reversed:
- Example
cs231a@aurora $ ./hw5c > Hal-lo-we-en H Ha Hal Hal- Hal-l Hal-lo Hal-lo- Hal-lo-w Hal-lo-we Hal-lo-we- Hal-lo-we-e Hal-lo-we-en
Submission
- Submit your program to the Homework 5, Problem 3 section on Moodle.
Problem 4
Create a new program in your account, called hw5dprep.asm, and store the code below in it.
;;; ; hw5d_prep.asm ;;; ; D. Thiebaut ;;; ; ;;; ; nasm -f elf -F stabs hw5d_prep.asm ;;; ; ld -melf_i386 -o hw5d_prep 231Lib.o hw5d_prep.o ;;; ; ./hw5d_prep ;;; ; ------------------------------------------------------------------- ;;; ------------------------------------------------------------ ;;; data areas ;;; ------------------------------------------------------------ section .data fileName db "dummy.txt", 0 buffer db " " db " " db " " db " " db " " db " " db " " db " " noCharsRead dd 0 ;;; ------------------------------------------------------------ ;;; code area ;;; ------------------------------------------------------------ section .text global _start extern _printString extern _readFile extern _println _start: mov eax, fileName ; pass address of file-name in eax mov ebx, buffer ; pass buffer address in ebx mov ecx, noCharsRead ; pass address of var that will contain ; number of chars read in ecx call _readFile ; read the file whose name was passed in ; eax. _readFile will read the file and ; put all the bytes read into the buffer ; it will also put the number of bytes read ; into the variable noCharsRead. ;; print the contents of the buffer mov ecx, buffer mov edx, dword[noCharsRead] call _printString call _println ;;; exit() mov eax,1 mov ebx,0 int 0x80 ; final system call
- Read the code of your program carefully. Notice that it uses a new function, called _readFile. This function needs to be given 3 things in order to do its work. It needs to have a string containing the name of a file. This is given to the function in eax (and always eax). It also needs the address of a buffer were we want to receive the contents of the text file. The address of the buffer is given in ebx. Finally, the function needs to know the address of the variable declared in our program that will contain the number of bytes put in the buffer. The address of this variable is passed in edx.
- To test your program you need to first create a text file called dummy.txt with emacs, and enter two or three lines of text in it. Maybe the first three lines of your favorite poem.
- Assemble and link the hw5dprep.asm with the 231Lib library (make sure you getcopy the new version of the 231Lib.asm library, and that you assemble the new library again).
- Run the hw5dprep executable, and verify that it prints the contents of the dummy.txt file.
Your Assignment, Part 1
- Copy hw5dprep.asm to hw5d.asm. This will be the program you submit to Moodle.
- Modify hw5d.asm so that it prompts the user for the name of the file, gets the name of the file with _getString, and stores the string in the fileName variable. You may want to make the fileName array of bytes large enough to work for very long file paths. (100 characters should be sufficient).
Problem 5
Stay tuned!
