CSC231 Homework 5 Fall 2017

--D. Thiebaut (talk) 09:27, 30 October 2017 (EDT)

---

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

1. _getString saves the string in an array declared inside 231Lib.asm. This array contains at most 1000 bytes.
2. 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.
3. the user indicates the end of the string by pressing the ENTER key. The string does not contain the ENTER character (line-feed).
4. 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
 

• 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

• 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

• Submit your program to the Homework 5, Problem 3 section on Moodle.