MARIE Assembly Program Help!

Need help with this program that inputs a sequence of characters from the set A-Z (capital letters only), stores each character in memory after it is transformed by the trivial ROT13 cipher, and then, after character input completes, outputs the transformed characters.

Current Code:

ORG 100 / Start the program at location 100 hexadecimal

/ -----

/ Input characters, transform, and store in memory until '.' is input

/ -----

Load Start / Initialize character pointer to start of block

Store Ptr

/>>>>> Add code to accomplish the input and output phases. <<<<<

/>>>>> Here's an example of how subroutine ROT13 is called. <<<<<

/>>>>> We'll just transform 'A' in this example then halt. <<<<<

Load ChA / Put 'A' in AC

Store InVal / Store value to be transformed into InVal

Jns ROT13 / Jump to the ROT13 subroutine

/ Upon return, the transformed character is in AC

Halt

/ -----

/ Rotate-13 subroutine: Apply ROT13 to input character in location InVal and return in AC

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/>>>>> WARNING: This subroutine *almost* works. You need to fix a bug.

ROT13, HEX 0

Load InVal / Get character

Add Val13 / Add 13

Store Hold / Save it

Subt ChZ / Check if modulo adjust is needed (past 'Z')

Skipcond 800 / No adjust needed if past 'Z'

Jump NoAdj

Add ChA / Add 'A' back to difference to perform modulo

Jump Done / Result is in AC

NoAdj, Load Hold / No adjust needed, get result

Done, JumpI ROT13 / Return with result in AC

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/ Constants (the program should not write to these locations)

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ChA, HEX 0041 / Constant value 'A' for modulo adjust in subroutine

ChZ, HEX 005A / Constant value 'Z' for modulo check in subroutine

ChPe, HEX 2E / Constant period character that marks end of input

Val13, DEC 13 / Constant rotate value of 13 for subroutine

One, HEX 1 / Constant value 1

Start, HEX 200 / Constant address for start of character block

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/ Data area (these locations are for reading and writing)

/ -----

InVal, HEX 0 / Reserved for subroutine input value

Hold, HEX 0 / Reserved for temporary variable for subroutine

Ptr, HEX 0 / Reserved for character pointer