CSC231 Homework 5 Fall 2017

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

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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).