program reverseString

; Program accepts a string, then reverses it.  It exits when only the ENTER key is pressed.
; Grant Palin
; Created: 2003-01-16
; Last modified: 2003-01-23


        stack   100
CONSOLE EQU     1
GETSTR  EQU     1
PUTCHAR	EQU	2
PUTSTR	EQU	3
EXIT	EQU     0xffff                  ;Code to finish program

        data

input	db	81, [?] * 82		;Define the input size
intro1	db	"Program accepts a string up 80 characters.  Type something, then press ENTER.", 0
intro2	db	"To exit, just press ENTER.", 0
prompt  db      "Enter a string: ", 0	;Define the prompt
newline	db	10, 13, 0		;Define a new line and carriage return

        code

reverseString				;Program start point
	mov	PUTSTR, r1		;Intro line 1
	lea	intro1, r2
	sys	CONSOLE
	
	mov	PUTSTR, r1		;Print a line break
	lea	newline, r2
	sys	CONSOLE

	mov	PUTSTR, r1		;Intro line 2
	lea	intro2, r2
	sys	CONSOLE

	mov	PUTSTR, r1		;Print a line break
	lea	newline, r2
	sys	CONSOLE

START
        mov     PUTSTR, r1		;Print prompt on screen
        lea     prompt, r2
        sys     CONSOLE

;Prompt for string 
	mov	GETSTR, r1		;Accept string from keyboard
	lea	input, r2
	sys	CONSOLE
	
	lodbs	input + 1, , r3		;Load length of string into register 3
	cmp	r3, 0			;Compare the length of the string to zero
	be	Finish			;If they are equal, the user did not type a
					;string, so exit program

;Output result
        mov     PUTSTR, r1		;Print the input string
        lea     input+2, r2
        sys     CONSOLE
         
	mov	PUTSTR, r1		;Print a line break
	lea	newline, r2
	sys	CONSOLE

REVERSE
	cmp	r3, 0			;Compare contents of register 3 with zero
	be	NEW			;If they are equal, jump to NEW

	lodbl	input+1, r3, r2		;Add 1 and length of string and puts result into register 2
	mov	PUTCHAR, r1
	sys	CONSOLE			;Print the last character in the string at position input+1+r3
	dec	r3			;Decrease length value in r3 by 2 bits, so we can access the previous character
	br	REVERSE			;Loops

NEW
	mov	PUTSTR, r1		;Print a line break
	lea	newline, r2
	sys	CONSOLE
	
	br	START			;Jump to the beginning to do it all over again

Finish
	mov	PUTSTR, r1		;Print a line break
	lea	newline, r2
	sys	CONSOLE
	
        sys     EXIT			;Program will end

        end