CS280 Lab 3: LCD Scrolling Text
Jonathan Dunder and Derek Bressler

We both spent approximately three hours on this lab assignment.  We programmed 
the microprocessor to scroll the text "Sixteen Characters" on the LCD screen 
repeatedly.  The microprocessor also counts the length of the string in memory 
and sends this length to the LCD control algorithm.

; Hello World Demo
; Written by Dr. H. Welch, 3/18/2005
; Updated by Dr. E. Durant, 3/20/2005 (latest tools,
;		more documentation, 8.3 WBUG11 limit)
; $Id: hello.s,v 1.2 2005/03/21 06:11:27 durant Exp durant $

; Assemble with   : as -o hello.o hello.s
; Link with       : ld -T ../fox11w.x -o hello.elf hello.o
; Convert to S19  : objcopy -O srec hello.elf hello.s19
; Create a listing: objdump -d hello.elf > hello.rst

; DER - Modified to use lcdout.s, and relocate stack pointer to
; Buffalo friendly address.

MAXLEN  = 35

.section .page0
linelength: .short 1 ; = does not allocate memory; it is like a #define in C++

;;;;;;;;;; Data (read-only, ROMable) ;;;;;;;;;;
	.section    .rodata
; read-only data, this is put in the text section, after executable code
; Describe strings to display.  The trailing NULs are not needed by
; LCD_LILNE?, but they might be useful for calculating the string length
; at runtime.

line1: .asciz  "Sixteen charactersSixteen character"
LINE1LEN = 35


; Note: OUTSTRG00 (see buffalo.s) sends data back to the monitor (e.g., WBUG11)
; over the serial port.  It might be useful for debugging.

;;;;;;;;;; Executable code, main entry point ;;;;;;;;;;
	.section .text
	.global _start      ; exported entry point expected by GNU as
_start:

; Set up string pointer and counter
        ldx     #line1  ; pointer to string
        ldy     #0      ; count of non-null characters

; Advance through string, calculating length, and minding MAXLEN.
1:      tst     0,x     ; Have we found a null in the string?
        beq     2f      ; If so, we're almost done -- go to final code (2f = forward to 2).
        iny             ; Count the character we just saw.
        ;cpy     MAXLEN  ; Have we reached the maximum length?
        ;beq     2f      ; If so, we're almost done -- go to final code.
        inx             ; Otherwise, point to the next character...
        bra     1b      ; ...and repeat. (1b = backward to 1)

; Finally, store the length in memory and come to a stop (sort of).
2:      sty     linelength
		bra		3f

3:  ldy		#18
	jsr     LCD_INI     ; Initialize the display
	ldx		#line1
	lds     #0x8fff     ; Initialize stack pointer - Buffalo friendly
4:	ldab    #linelength
	jsr     LCD_LINE1	; LCD_LINE1 destroys D (A:B) and X.  Y is untouched.
	bsr		wait
	inx
	dey
	bne		4b
	beq		3b

	bra     .

  .section .text
; subroutine wait - waits 250 ms - modifies nothing


wait:   psha                ; save A
        tpa                 ; save condition codes
        psha                ; "
        pshx                ; save X
        ldx     #250        ; wait 250 milliseconds
1:      bsr     wait1ms     ; local label
        dex
        bne     1b
        pulx                ; restore X
        pula                ; restore condition codes
        tap                 ; "
        pula                ; restore a
        rts

wait1ms:psha                ; 1 ms wait
        tpa
        psha
        pshx
        ldx     #200
2:      dex
        nop
        nop
        bne     2b
        pulx
        pula
        tap
        pula
        rts


; Get LCD routines
; include after code so it doesn;t assemble at beginning of
; code segment
.include "lcdout.s"