CSC231 Homework 5 Fall 2017

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

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This assignment is due Monday 11/6/2017 at 11:55 p.m.

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
 

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

• Write a program called hw4d.asm that prompts the user for a string, and displays the checksum of this string, in decimal.

• The checksum of a string is the exclusive or of all the bytes in a string. This is an operation used mostly in communication, when bytes of data are sent over a network. In general, when a packet of bytes is sent, the exclusive or of all the bytes is computed, and sent at the end, as an extra byte. The receiving device, upon receiving the data, will compute the exclusive or of all the data bytes, and compare the checksum it computed to the checksum that was sent. If the two checksums are different, very likely a transmission error occurred and the receiving device will ask for the packet to be transmitted again.

• Here is how you would compute the checksum of a string of 5 bytes: 0x41, 0x42, 0x43, 0x44, 0x45. I will use binary, as it is simpler:

 checksum <-- 0x41  = 0100 0001 
 checksum <-- checksum xor 0x42    = 0100 0001 xor 0100 0010 = 0000 0011
 checksum <-- checksum xor 0x43    = 0000 0011 xor 0100 0011 = 0100 0000
 checksum <-- checksum xor 0x44    = 0100 0000 xor 0100 0100 = 0000 0100
 checksum <-- checksum xor 0x45    = 0000 0100 xor 0100 0101 = 0100 0001

The checksum of the 5 bytes is 0100 0001, or 0x41, or 65. The program will print 65 as the checksum for the string ABCDE.

• Examples

cs231a@aurora ~ $ ./hw5d 
> hello there
44

cs231a@aurora ~ $ ./hw5d 
> ABCDE
65

cs231a@aurora ~ $ ./hw5d 
> AA
0

cs231a@aurora ~ $ ./hw5d 
> Hello!
99

cs231a@aurora ~ $ ./hw5d 
> I like milk chocolate better than dark chocolate.
80

Problem 5

Stay tuned!