Monash College

MCD4700 - Introduction to Computer Systems,
Networks and Security
Assignment 1 - Trimester 1, 2020

Submission guidelines
This is an individual assignment, group work is not permitted

Deadline​: ​15 April 2020, 7:55pm

Submission format:​ docx for the written tasks, LogiSim circuit files for task 1,
MARIE assembly files for task 2. All files must be uploaded electronically via Moodle.

Late submission:
● By submitting a ​Special Consideration Form​ or visit this link:
https://lms.monashcollege.edu.au/course/view.php?id=1331
● Or, without special consideration, you lose 5 marks of your mark per day that you
submit late (including weekends). Submissions will not be accepted more than 5 days
late.

This means that if you got ​Y​ marks, only Y-n*​5​ will be counted where ​n​ is the number
of days you submit late.


In-class interviews:​ See instructions for Task 2 for details.

Marks:​ This assignment will be marked out of 100 points, and count for 20% of your
total unit marks.

Plagiarism:​ It is an academic requirement that the work you submit be original. If there is
any evidence of copying (including from online sources without proper attribution),
collaboration, pasting from websites or textbooks, ​Zero marks​ may be awarded for the
whole assignment, the unit or you may be suspended or excluded from your course. Monash
Colleges policies on plagiarism, collusion, and cheating are ​available here​ or see this link:
https://www.monashcollege.edu.au/__data/assets/pdf_file/0010/17101/dip-assessment-polic
y.pdf

Further Note: When you are asked to use Internet resources to answer a question, this ​does
not mean copy-pasting text​ from websites. Write answers in your own words such that
your understanding of the answer is evident. Acknowledge any sources by citing them.

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Monash College MCD4700 Assignment 1
1. Boolean Algebra and Logisim Task ​(35 marks)

Input Output
A B C D Z1 Z2
0 0 0 0 0 0
0 0 0 1 1 0
0 0 1 0 0 0
0 0 1 1 1 0
0 1 0 0 0 1
0 1 0 1 1 1
0 1 1 0 0 1
0 1 1 1 0 0
1 0 0 0 0 0
1 0 0 1 1 0
1 0 1 0 0 0
1 0 1 1 1 0
1 1 0 0 0 1
1 1 0 1 0 0
1 1 1 0 0 1
1 1 1 1 1 1
Truth table for Z1 and Z2

Step 1: Boolean Algebra Expressions ​(10 marks)
Write the Boolean equation for your truth table in sum of products form (your choice). First,
think about how to deal with the two outputs. Then, describe each single row in terms of
Boolean algebra. Finally, combine the boolean terms for each row into a larger term. Briefly
explain these steps for the truth table given.
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Monash College MCD4700 Assignment 1
Step 2: Logical Circuit in Logisim ​(10 marks)
Model the resulting Boolean terms from Step 1 in a single Logisim circuit, using the basic
gates AND, OR, NOT. Use the original boolean expression for drawing the circuit (do not
simplify the expression). You can use logic gates with more than two inputs.
To do this task you have to use a template Logisim file “task_1.2_template” that has been
provided with this assignment. Do not forget to rename the file before saving your circuit in it.
And, ‘2’ marks will be deducted if you​ change the labels (or adding any extra label)​.



Step 3: Optimised Circuit​ ​(15 marks)
The goal of this task is to find a minimal circuit (using only universal ​NAND​ gates). Based on
the truth table and Boolean algebra terms from Step 1, optimize the function using Karnaugh
maps.

You need to create two Karnaugh maps, one for each output. Your documentation should
show the maps as well as the groups found in the maps and how they relate to terms in the
optimized Boolean function.

Then use Logisim to create a logic circuit. Don’t use any other gates, other than ​NAND​.

Use the template Logisim file “task_1.3_template” that have been provided with this
assignment. Do not forget to rename it.














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Monash College MCD4700 Assignment 1
2. MARIE ​(65 marks)
In this task you will develop a MARIE application that performs some manipulation of strings.
We will break it down into small steps for you.

Most of the tasks require you to write code, test cases and some small analysis. The code
must contain comments, and you submit it as .mas files together with the rest of your
assignment. The test cases should also be working, self-contained MARIE assembly files
(without requiring much input from the user). The analysis needs to be submitted as part of
the main PDF file you submit for this assignment.

In-Class interviews: ​You will be required to demonstrate your code to your tutor after the
submission deadline. Failure to demonstrate will lead to “​zero marks” ​being awarded to the
entire programming part of this assignment.

Name as a String
This section introduces the concepts you need for the rest of the assignment. A string is a
sequence of characters. It’s the basic data structure for storing text in a computer. There are
several different ways of representing a string in memory and how to deal with strings of
arbitrary length.

For this assignment, we will use the following string representation:

● A string is represented in a contiguous memory location, with each address containing
one character.
● The characters are encoded using the ASCII encoding.
● End of a string is marked by the ASCII character ‘.’ (i.e. dot or full-stop).
● A string can be of any arbitrary length, and will be terminated by a ’.’, and it may
contain any of the following: alphabets (A-Z, a-z), numbers (0-9) and ASCII Space
Character (Hex 020).


Here is an example. A string “John Nguyen.” will be represented in memory (written as
hexadecimal numbers):

04A 06F 068 06E 020 04E 067 075 079 065 06E 02E
J o h n N g u y e n .


Note that, for a string with ​n characters, we need (​n+2 ​) words of MARIE memory in order to
store all the characters belonging to that string (including space character and ‘.’).

In MARIE assembly language programming, we can make use of the ADR command, the
HEX keyword and a label “myString” to put this string into memory:
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Monash College MCD4700 Assignment 1

myStringAddr, ADR myString
myString, HEX 04A /’J’
HEX 06F /’o’
HEX 068 /’h’
HEX 06E /’n’
HEX 020 /’Space’
HEX 04E /’N’
HEX 067 /’g’
HEX 075 /’u’
HEX 079 /’y’
HEX 065 /’e
HEX 06E /’n’
HEX 02E /’.’

2.1 A MARIE program for printing a string ​(10 marks)
Prepare a MARIE program called ​P​rintString that can print an ASCII ‘.’ terminated
string (using the “Output” instruction) containing your “First Name” and “Last Name”.

Start by using a label “​PrintStringAddr​” that holds the start address of a string (like,
myStringAddr​). The code should then load a character from that address, output it, then
increment the address by one, and keep doing that upto the character loaded from the
address is a ‘.’ (which signals the end of the string). The string may look similar to the output
below.

YourFirstName LastName


To receive full marks, you need to write a MARIE program, which once executed, will print
your name string on the display. You may use the following template to begin with.

Load myStringAddr / Load string start address
Store PrintStringAddr / Pass as an argument
/ Your code may begin here
………
………
/Your MARIE program ends here

/Variables or labels used in the program
PrintStringAddr, Dec 0 / variable or label to store the
/ start address of a string
……
myStringAddr, ADR myString
myString, HEX ??? /First
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Monash College MCD4700 Assignment 1
HEX ??? /Name
HEX ??? /
……………
HEX ??? /
HEX 020 /’Space’
HEX ??? /Last
HEX ??? /Name
HEX ??? /
……………
HEX ??? /
HEX 02E /’.’
/end of the list

2.2 A subroutine for printing a string ​(15 marks)
Convert the MARIE program done in section 2.1, into a subroutine called
subP​rintString that can print any ASCII ‘.’ terminated string (using the “Output”
instruction).

To receive full marks, your code needs to be in the form of a ​subroutine that can be called
using the JnS instruction. You need to modify the MARIE main program (from section 2.1) to
call this subroutine. You may use the following template to begin with.

Load myStringAddr / Load string start address
Store PrintStringAddr / Pass as an argument
Jns subPrintString / Call printing subroutine
Halt

/Your subroutine​ “subPrintString ​” begins here
PrintStringAddr, Dec 0 / variable or label to store the
/ start address of a string
subPrintString, Hex 0 /label (subroutine name) to begin the
/ subroutine
/ Your code may begin here
………
………
/Your subroutine ends here
2.3 Implementation of Caesar Cipher Substitution ​(20 marks)
In this section, we are going to implement Caesar Cipher (a type of substitution cipher)
encryption that we have learned in the Cryptography lab. It replaces each character in a
string by a different character of the alphabet defined by a simple shift. An encryption “Key”
using a shift of 3 positions, shown in the table below.

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Monash College MCD4700 Assignment 1
a b c d e f g h i j k l m n o p q r s t u v w x y z
d e f g h i j k l m n o p q r s t u v w x y z a b c

This is a very simple form of encryption. ​Please note the use of lower case alphabets only,
and we will encrypt or decrypt only “alphabets”. One more example “Key” below is showing a
Caesar Cipher with a shift of 5 positions.

a b c d e f g h i j k l m n o p q r s t u v w x y z
f g h i j k l m n o p q r s t u v w x y z a b c d e

To encrypt, each letter in the top row is replaced by the corresponding letter in the bottom
row, and to decrypt, each letter in the bottom row is replaced by the corresponding letter in
the top row.

In this part, your task is to implement ​subroutines ​that perform Caesar cipher substitution on
a string, either performing encryption or decryption (cipher or decipher). You will implement
the mapping in MARIE by a lookup table, storing the result of the substitution for each letter.
The lookup table below maps an encryption key using a shift of 3 positions.

myEncKey1Addr, ADR myEncKey1
myEncKey1, HEX 064 / a -> d
HEX 065 / b -> e
HEX 066 / c -> f
HEX 067 / d -> g
HEX 068 / e -> h
HEX 069 / f -> i
HEX 06A / g -> j
HEX 06B / h -> k
HEX 06C / i -> l
HEX 06D / j -> m
HEX 06E / k -> n
HEX 06F / l -> o
HEX 070 / m -> p
HEX 071 / n -> q
HEX 072 / o -> r
HEX 073 / p -> s
HEX 074 / q -> t
HEX 075 / r -> u
HEX 076 / s -> v
HEX 077 / t -> w
HEX 078 / u -> x
HEX 079 / v -> y
HEX 07A / w -> z
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Monash College MCD4700 Assignment 1
HEX 061 / x -> a
HEX 062 / y -> b
HEX 063 / z -> c
A subroutine to perform encryption on a string
In this task, you will implement a MARIE ​subroutine ​called ​subCaesarEnc that can perform
encryption using Caesar cipher substitution on a string whose start address is passed as an
argument in the ​EncryptStr ​label. You need to use the string referred in section 2.1
(​myString containing your first name and last name) to encrypt. You need to display the
string before and after the encryption process.

You can assume that the string only contains lower case characters. For each letter in the
string, you will need to load the correct value for its substitution from the table “myEncKey1”,
and then store that value back into memory (overwriting the original letter).

Think about how to get “a value” from a table. E.g., assume that the original letter is ‘k’,
which is the 11th letter in the alphabet. So we need to load the 11th entry in the table– in
other words, the entry at address ​myEncKey1Addr + 10 (which is HEX 06E or DEC 110, or
the letter ‘n’).

The letter ‘k’ has an ASCII value of HEX 06B (DEC 107). So, how do we determine to add
‘10’ to the address “​myEncKey1Addr​”? We simply subtract the ASCII value for the letter ‘a’,
which is HEX 061 (DEC 97). So, in decimal notation, “117 - 97 = 10” or in HEX notation,
“HEX 06B - HEX 061 = HEX A.” The following illustrated example explains the encryption
process further. Here, the first name is “patrick”, and the last name is “lee.” And, it will be
stored in the memory as shown below.

070 061 074 072 069 063 06B 020 06C 065 065 02E
p a t r i c k l e e .

During the encryption process, the following transformation (in the memory) will occur.

p a t r i c k l e e
s d w u l f n o h h

And, the new encrypted string will be stored in the memory as shown below.

073 064 077 075 06C 066 06E 020 06F 068 068 02E
s d w u l f n o h h .

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Monash College MCD4700 Assignment 1
Please note that, during the encryption process, ASCII value for “SPACE” and ‘.’ remains
unchanged.

To receive full marks, your code needs to be in the form of a ​subroutine that can be called
using the JnS instruction. You need to write a MARIE main program to call this subroutine.
You may use the following template to begin with. Here, we are (re)using the
subP​rintString​ subroutine from section 2.2.

/printing the string before encryption
Load myStringAddr / Load string start address
Store PrintStringAddr / Pass as an argument
Jns subPrintString / Call printing subroutine

/calling the subroutine to perform encryption
Load myStringAddr / Load string start address
Store EncryptStr / Pass as an argument
Jns subCaesarEnc / Call the cipher subroutine

/printing the string after encryption
Load myStringAddr / Load string start address
Store PrintStringAddr / Pass as an argument
Jns subPrintString / Call printing subroutine
Halt

/Your subroutine “​subCaesarEnc​” begins here.
EncryptStr, ​Dec 0 / variable or label to store the start
/ address of a string
subCaesarEnc, ​HEX 0 / label (subroutine name) to begin the
/ subroutine
/ Your codes…..
………
………
/Your subroutine ends here.
2.4 A subroutine to perform decryption on a string ​(20 marks).
In this part, we want to decrypt a string. To do that, we need a second table (in addition to
the encryption table “​myEncKey1 ​” in section 2.3) that has the reverse mapping. Add a table
“myDecKey1” to your code that is exactly the inverse of the “​myEncKey1​” table above.

myDecKey1Addr, ADR myDecKey1
myDecKey1, HEX ??? / a -> x
HEX ??? / b -> y
……..
HEX ??? / z -> w

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Monash College MCD4700 Assignment 1

Modify your subroutine in section 2.3 to create a new subroutine performing decryption. You
can assume that the string only contains lower case characters. For each letter in the string,
you will need to load the correct value for its substitution from the table, and then store that
value back into memory (overwriting the original letter).
The following illustrated example explains the decryption process further. Here, you will
encrypt the string first, storing the encrypted string back in the memory, and then, decrypt
this string back to the original form. In this example, the first name is “patrick”, and the last
name is “lee.” And, it will be stored in the memory as shown below.

070 061 074 072 069 063 06B 020 06C 065 065 02E
p a t r i c k l e e .

During the encryption process, the following transformation (in the memory) will occur.

p a t r i c k l e e
s d w u l f n o h h

And, the new encrypted string will be stored in the memory as shown below.

073 064 077 075 06C 066 06E 020 06F 068 068 02E
s d w u l f n o h h .

Now, you will decrypt the above string “sdwulfn ohh.” to its original form “patrick lee.”


070 061 074 072 069 063 06B 020 06C 065 065 02E
p a t r i c k l e e .


To receive full marks, your code needs to be in the form of a ​subroutine that can be called
using the JnS instruction. You need to write a MARIE main program to call this subroutine.
Please note that, as you need to encrypt the string first, so that you can decrypt an already
encrypted string, there is a series of activities involved here ​[Print the original string->
Encryption -> Print the encrypted string-> Decryption -> Print the original string]. You
may use the following template to begin with.

/To print the original string
Load ​myStringAddr / Load string start address
Store ​PrintStringAddr / Pass as an argument
Jns ​subPrintString / Call printing subroutine
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Monash College MCD4700 Assignment 1

/Encrypting the string
Load myStringAddr / Load string start address
Store ​EncryptStr / Pass as an argument
Jns subCaesarEnc / Call the Encryption cipher subroutine
/print the encrypted string
Load ​myStringAddr / Load string start address
Store ​PrintStringAddr / Pass as an argument
Jns ​subPrintString / Call printing subroutine

/Decrypting the string
Load ​myStringAddr / Load string start address
Store ​DecryptStr / Pass as an argument
Jns ​subCaesarDec / Call the Decryption cipher subroutine

/print the decrypted string
Load ​myStringAddr / Load string start address
Store ​PrintStringAddr / Pass as an argument
Jns ​subPrintString / Call printing subroutine
Halt

/Your subroutine “subCaesarDec” begins here.
DecryptStr, ​DEC 0 / variable or label to store the start
/ address of a string
subCaesarDec, ​HEX 0 / label (subroutine name) to begin the
/ subroutine
/ Your codes…..
………
………
/Your subroutine ends here.

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Monash College MCD4700 Assignment 1
Files to be submitted:
One folder named “YourFirstNameLastNameStudentID” containing the following files:
1. Report for the written tasks (One Word file called YourFirstNameLastName
StudentID.doc / docx). The report should include your Full name, your student ID,
your class number and your tutor’s name.
2. Two Logisim files, one for task 1.2 and one for 1.3, name them LogicalCircuit.circ and
OptimizedCircuit.circ respectively.
3. MARIE files for tasks 2.1 to 2.4 name them as below:
● 2_1_PrintString.mas
● 2_2_SubPrintString.mas
● 2_3_SubCaesarEncrypt.mas
● 2_4_SubCaesarDecrypt.mas

Zip the folder under the same name and submit it to moodle. You need to make sure there
are no spaces in any of the filenames.
NOTE! ​Your submitted files must be correctly identified (as described above).
Any submission that does not comply will receive an automatic 10 marks
penalty (applied after marking).
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