*-----------------------------------------------------------
* Title      : Ceasar chipher
* Written by : Manuel V?gele, Simon W?rner (Team 1)
* Date       : 12.05.2015
* Description: This program encrypts a string using ROT3
*-----------------------------------------------------------
    ORG    $3000    ; Start at address 3000
START:              ; This is where the program starts - obviously

* Put program code here
    
    ; Load string address
    move.l  #input, A0     ; Store the address of the varialbe input into address register A0
    move.l  #output, A1    ; Store the address of the varialbe output into address register A1
    
LOOP:
    ; Load char to register
    move.b  (A0), D0    ; Store the value at the address stored in address register A0 into data register D0
    
    ; If end of string is reached end loop
    cmp.b   #0, D0      ; Compare the constant 0 and the value in the register D0
    beq     LOOP_END    ; Jump to label LOOP_END if the value in the register D0 == the constant 0
    
    ; Apply the encryption key
    add.b   #key, D0    ; Add the constant key and the value in the register D0 and store the result into data register D0
    
    ; Make the char lower case
    move.b  D0, D1          ; Store the value in the register D0 into data register D1
    or.b    #casebit, D1    ; Bitwise or the constant casebit and the value in the register D1 and store the result into data register D1

    ; Check if char is greater than 'z'
    cmp.b   #z, D1        ; Compare the constant z and the value in the register D1
    ble     WRITE_CHAR    ; Jump to label WRITE_CHAR if the value in the register D1 <= the constant z
    
    ; Cycle to A after Z
    sub.b   #alphabetLength, D0    ; Subtract the constant alphabetLength from the value in the register D0 and store the result into data register D0
    
WRITE_CHAR:
    ; Write the encrypted char to the output string
    move.b  D0, (A1)    ; Store the value in the register D0 into the memory at the address stored in address register A1
    
    ; Move pointers to next char
    add.l   #1, A0    ; Increase the address in the register A0 by 1
    add.l   #1, A1    ; Increase the address in the register A1 by 1
    bra     LOOP      ; Jump to label LOOP

LOOP_END:

    ; Add '\0' at output string end
    move.b  #0, (A1)    ; Store the constant 0 into the memory at the address stored in address register A1

    SIMHALT    ; halt simulator

* Put variables and constants here

input           dc.b    'Zykluszeit',0
output          ds.b    11

key             EQU     3

z               EQU     'z'
casebit         EQU     $20
alphabetLength  EQU     26


    END    START    ; last line of source




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