Problem Statement

Implement a   rot-13   function   (or procedure, class, subroutine, or other "callable" object as appropriate to your programming environment). Optionally wrap this function in a utility program   (like tr,   which acts like a common UNIX utility, performing a line-by-line rot-13 encoding of every line of input contained in each file listed on its command line,   or (if no filenames are passed thereon) acting as a filter on its   "standard input."

(A number of UNIX scripting languages and utilities, such as   awk   and   sed   either default to processing files in this way or have command line switches or modules to easily implement these wrapper semantics, e.g.,   Perl   and   Python). The   rot-13   encoding is commonly known from the early days of Usenet "Netnews" as a way of obfuscating text to prevent casual reading of   spoiler   or potentially offensive material. Many news reader and mail user agent programs have built-in rot-13 encoder/decoders or have the ability to feed a message through any external utility script for performing this (or other) actions. The definition of the rot-13 function is to simply replace every letter of the ASCII alphabet with the letter which is "rotated" 13 characters "around" the 26 letter alphabet from its normal cardinal position   (wrapping around from   z   to   a   as necessary). Thus the letters   abc   become   nop   and so on. Technically rot-13 is a   "mono-alphabetic substitution cipher"   with a trivial   "key". A proper implementation should work on upper and lower case letters, preserve case, and pass all non-alphabetic characters in the input stream through without alteration.

Let's start with the solution:

// main registers
p	IS	$255	% text pointer
c	GREG		% char
cc	GREG		% uppercase copy of c
u	GREG		% all purpose

	LOC	Data_Segment
	GREG	@
Test	BYTE	"dit is een bericht voor de keizer",#a,0

	LOC	#100
Main	LDA	p,Test
	TRAP	0,Fputs,StdOut	% show text to encrypt
	LDA	p,Test		% points to text to encrypt
	JMP	4F
// do in place text encryption
				% REPEAT
2H	ADD	cc,c,0		%  copy char
	SUB	cc,cc,' '	%  make uppercase
	CMP	u,cc,'A'
	BN	u,3F		%  IF c < 'A' OR c > 'Z' THEN next char
	CMP	u,cc,'Z'
	BP	u,3F
	CMP	u,cc,'N'	%  ELSE
	BN	u,1F		%    IF c < 'N' THEN encrypt 'up'
	SUB	c,c,26		%    ELSE char ready for encrypt 'down'
1H	INCL	c,13		%  encrypt char
	STBU	c,p		%  replace char with encrypted char
3H	INCL	p,1		%  move to next char
4H	LDBU	c,p		%  get next char
	PBNZ	c,2B		% UNTIL EOT
// print result
	LDA	p,Test
	TRAP	0,Fputs,StdOut	% show encrypted text
	TRAP	0,Halt,0