This document describes a MIPS encryption program that uses three arrays: array1 stores the input string, array2 stores the encoded/decoded output, and array3 stores an encryption key. It works by reading each character from array1, determining if it is a letter or number, and then encoding/decoding it based on the encryption key stored in array3 before writing it to array2. If a user inputs 'Y' or 'y' they are asked if they want to terminate the program, otherwise it loops back to the beginning to encode another string. Screenshots of it encoding and decoding messages are also included.

1. Computer Organization and Architecture
Final Project Q1 report
Bruno Diaz
In Thisprojectwe are requiredtowrite anencryptionprogram inMIPS.I designed itbyusing
three arrays array1, array2, and array3. The 1st
array iswhat isread fromthe stringthe user inputted,
the 2nd is the coded/decoded messageandthe third forstoringthe encryptionkeywhichisalsousedto
produce the 2nd
array. Unlike the othertwoarrays insteadof usinganindex i the thirdwill use the mask
h10010100 anda character readfromarray1 to code and decode charactersfromarray1 to array2. The
general software diagramisshowninfigure D-1.
The program firststart by askingforand thenreadinga Stringfromthe use whichis stored in
array1. It thenstepsthrougheveryindexof thisarray(equivalenttoabyte) to readeach character
determiningif itisa letterora number,todetermine if itisaletteror a numberwe lookat table t-1 and
we can see that a character from the stringisa numberif itisbetween48 and57, if it isthe character is
storedinarray2 unchanged. If the character isfoundto be a numberthenthe character needsto
coded/decodedwe dothisbyobservingthatrange 48-57 in decimal isalsothe range 30-39 in
hexadecimal, Iuse thistomy advantage byhavingthe addressof the encryptionkeybe h10010130 this
wayby usingthe mask 10010100 and applyinganOR to character readfrom array1 and determinedto
be a numberto getan addressinarray 3 as can be observedintable T-2

2. Thismeansthat the outputof the operationwill pointthe address of the memorylocationof the value it
needstobe turnedinto.For example: if ahex value 30 (equivalenttoascii 0) isread fromthe array it
getsORed withthe mask h10010100 whichgivesusthe value h10010130 whichif usedas an address
pointstoa hex value 34 (equivalenttoascii 4) whichthengetsstoredintoarray2.
These twooperationsthengivesusastoredstringinarray 2 that is ourcoded/decoded
message.A “/n” andthena null characterisaddedat the endto make a cleanerprintandterminate the
string.Afterthatis done array2 is thenprintedtoscreenandthe user askedif theywishtoterminate
the program. If a y or Y character equivalentto121 or 89 as seenintable T-1 is enteredthe program
loopsto the endwhere the programisended,otherwisethe programloopsbackto the beginningto
beginthe processall overagain.
The code forthe programand screenshotsare shownbelow
Code:
.data
start: .asciiz"Please inputphrase tocode/decode (29characterlimit)n"
ask: .asciiz"wouldyoulike toterminate the program? n"
new_line:.asciiz "n"
.text
.globl my_main
my_main:
li $s0,0x10010100 #load addressmask
li $s1,0x100100A0 #load the base adressof array1(inputstring)
li $s2,0x100100C0 #loadthe base adressof array2(outputstring)

3. li $s3,0x10010130 #load the base adressof array3(encryptioncode)
li $t0,0x35393634 #store the encryptioncode
sw $t0,0($s3)
li $t0,0x38313330
sw $t0,4($s3)
li $t0,0x00003237
sw $t0,8($s3)
addi $s4,$0,48 #lowerlimitfornumber
addi $s5,$0,57 #upperlimitfornumber
addi $s6,$0,89 #Y and
addi $s7,$0,121 #y
LOOP1:
li $v0,4 #print start
la $a0,start
syscall
li $v0, 8 # read string byusingsyscall
add $a0, $0,$s1 # loadfirstarray addressinto$a0
addi $a1, $0,29 #load the character limit
syscall
li $v0,4 #make newline
la $a0,new_line
syscall
addi $t2,$0,0 #offseti
LOOP2:
add $t3,$t2,$s1 #current adressinrespectto array1
add $t4,$t2,$s2 #current adressinrespectto array2
lb $t5,0($t3) #loadelementfromarray1[i]
beq$0,$t5,done #jumpto done if null character ispulledfromarray
slt$t6,$t5,$s4 #is it lessthan48?
bne $t6,$0,continue
slt$t6,$s5,$t5 #is it greaterthan57?
bne $t6,$0,continue
or $t7,$s0,$t5 #decrypt byreadingadressinrespectto the numberoredwiththe maskin array3
lb$t5,0($t7)
continue:
sb $t5,0($t4) #store intoarray 2[i]
addi $t2,$t2,1 #incrementi
j,LOOP2
done:
li $t5,0x5C6D0000 #store n and null chatacterintoendof array
addi $t4,$t4,1
sb $t5,0($t4)
li $v0,4 #printresult
or $a0,$s2,$0

4. syscall
li $v0,4 #make newline
la $a0,new_line
syscall
li $v0,4 #ask to terminate
la $a0,ask
syscall
li $v0,12 #read character
syscall
or $t8,$v0,$0
li $v0,4 #make newline
la $a0,new_line
syscall
beq$t8,$s6,finished#branchto finishif characterisY
beq$t8,$s7,finished#branchto finishif characterisy
j,LOOP1#branch to beginningloopif neither
finished:
li $v0,10 #exitprogram
syscall
Screenshots:

5. Screenshotfor codinga message

6. Screenshotfor decodingamessage
Conclusion:
I was able tocome upwitha MIPS designthatavoidedhavingtouse 10 differentbranchesto
decode numbersbyutilizingakeystoredinmemoryandusinga trick withthe addressandthe ASCII
valuesof numberspointtothe positioninthe keyforthe value itneededtobe turnedintoto. By doing
thisI alsohad the befitof beingable toeasilymodifythe type of encryptionshouldIneedto.Ialso
avoidedhavingtouse a for loopforthe iterationneededforthe characterread byusinga while loop
that keptreadingtill itfoundanull character essentiallyturningfor(i=0;i++;i<29) towhile(array[i]!=00)
thisgivesthe benefitof beingable tomodifythe size of the inputtedstringshouldone needto.Soin
conclusionIcame up witha compact design(77 lineslong) foraMIPS encryptedthatiseasily
modifiable.