// 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