charlie

@ Joseph Mendoza and
@ Computer Science
@ Assembly language coursework

@ Program function
@ Takes 2 keys and a messsage
@ Formats characters to be processed
@ Either encrypts or decrypts message
@ Prints formatted result


.global  main
.data
.balign 4

letter: .asciz "%c"

.text
.balign 4
.global main


main:
    @ Pushes values neededd onto the stack.
    PUSH {r4, r5, r6, r7, r8, r9, r10, r11, lr}

    @  Loads the number typed in when running program for either decrypt or encrypt.
    LDR r5, [r1,#4]
    LDRB r5, [r5]

    @ For key1
    @ Loads first character of the first key.
    LDR r4, [r1, #8]
    LDRB r6, [r4]
    BL mainloopend
    @ For key2
    @ Loads first character of second key.
    LDR r8, [r1, #12]
    LDRB r9, [r8]
    BL mainloopend

@ This is the main loop in which the program will call lots of other functions from.
@ The values ae: r4=first private char, r5=either encrypt or decrypt, r6=current char from key, r7=counter for key.


mainloop:
    @ Makes changes to the characters if needed.
    BL formatchar

    @ Compare to see if there is a character there to encrypt.
    CMP r0, #-1
    BEQ mainloopend

    @ Moves the character of  key into r1 so it can be processed.
    MOV r0 ,r0
    MOV r1, r6

    @  Encrypts if  r5 is 48 as this is asci number for 0.
    CMP r5, #48
    BLEQ encrypt

    @ Decrypts if r5 is 49 as this is asci number for 1.
    CMP r5, #49
    BLEQ decrypt



    @ Below is for printing the character after it has been processed in the change message function.
    MOV r1, r0
    LDR r0,=letter
    BL printf


    @ Below is for making the key the same length as the message
    ADD r7,r7, #1
    LDRB r6, [r4, r7]

    @ This checks to make sure there is a character there and if there is not then index will be reset.
    CMP r6, #0
    MOVEQ r7, #0

    @ Puts back the character  with index reset.
    LDRB r6, [r4, r7]



mainloopend:
    @ This will be done on loop and will keep getting next character.
    BL getchar

    @ If there is a character in r0 then it will be passed through the main loop.
    CMP r0, #-1
    BNE mainloop
    BL end









@ This section has to be done in order of the asci table.
@ Order: Below A (for characters below  capital a which are not needed.)
@        Below [ (for characters which are capital to be changed to lowercase)
@        Below a (for characters between Z and a in the asci table.
@        Above 122 (for characters above z which are not in alphabet.)
formatchar:
    PUSH {r4,r5,r6,r7,r8,r9,r10, lr}

    @ Removes anything below 65 or A in asci table.
    CMP r0, #65
    MOVLT r0, #-1

    @ Change all characters between 65 and 90 which are capital to lowercase by adding 32.
    CMP r0, #91
    ADDLT r0,r0,#32

    @ Removes anything between 90 and 97 in asci table which are not in alphabet.
    CMP r0, #97
    MOVLT r0, #-1

    @ Removes anything above 122 in asci table which is not in aphabet.
    CMP r0, #122
    MOVGT r0, #-1





@Below is the encryption and decryption section
@ Encrypts message with (r0 and r1 being character of msg and character of key)
encrypt:
    PUSH {r4, r5, r6, r7, r8, r9,r10, lr}

    @ Encryption process stores result in r2.
    @ Does msg - key then adds 2.
    SUB r2, r0, r1
    ADD r2, #2

    @ Below is checking  the chracter after encryption to loop back round to the lowercase alphabet.
    @ This is needed if the key char is bigger than the message char.
    CMP r2, #97
    ADDLT r2, #26
    BL encrypt2


@ Encrypts message again
encrypt2:
    PUSH {r4, r5, r6, r7, r8, r9,r10, lr}
    MOV r1, r9
    SUB r2, r0, r1
    ADD r2, #2

    @ Below is checking the character after encryption  to loop back round to the lowercase alphabet.
    @ This is needed if the key char is bigger than the message char.
    CMP r2, #97
    ADDLT r2, #26

@ Decrypts message
decrypt:
    PUSH {r4,r5,r6,r7,r8,r9,r10, lr}
    MOV r1, r9
    ADD r2, r0, r1
    SUB r2, #2

    @ Below is checking if after decryption the char is within the range of the lowercase alphabet.
    @ If not then it will minus 26 to place it in range.
    CMP r2, #122
    SUBGT r2, r2, #26
    BL decrypt2


@Decrypts message again
decrypt2:
    PUSH {r4,r5,r6,r7,r8,r9,r10, lr}
    MOV r1, r6
    ADD r2, r0, r2
    SUB r2, #2

    @ Below is checking if after decryption  if the char is in the range of the lowercase alphabet.
    @ If not then it will minus 26 to place it in range.
    CMP r2, #122
    SUBGT r2, r2, #26







end:
    @ The pc stands for program counter.
    POP {r4,r5,r6,r7,r8,r9, pc}