format 	ELF 	executable 3
entry 	start
	
segment	readable writeable
buf	rb	1
	
segment	readable executable
start:	mov	eax, 3		; syscall "read"
	mov	ebx, 0		; stdin
	mov	ecx, buf	; buffer for read byte
	mov	edx, 1		; len (read one byte)
	int	80h

	cmp	eax, 0		; EOF?
	jz	exit

	xor 	eax, eax	; load read char to eax
	mov	al, [buf]
	cmp	eax, "A"	; see if it is in ascii a-z or A-Z
	jl	print
	cmp	eax, "z"
	jg	print
	cmp	eax, "Z"
	jle	rotup
	cmp	eax, "a"
	jge	rotlow
	jmp	print

rotup:	sub	eax, "A"-13	; do rot 13 for A-Z
	cdq
	mov	ebx, 26
	div	ebx
	add	edx, "A"
	jmp	rotend
	
rotlow:	sub	eax, "a"-13	; do rot 13 for a-z
	cdq
	mov	ebx, 26
	div	ebx
	add	edx, "a"

rotend:	mov	[buf], dl
	
print: 	mov	eax, 4		; syscall write
	mov	ebx, 1		; stdout
	mov	ecx, buf	; *char
	mov	edx, 1		; string length
	int	80h

	jmp	start

exit: 	mov     eax,1		; syscall exit
	xor     ebx,ebx		; exit code
	int     80h