ADDR B1 B2 B3 B4 X:\public_html\EEL4746\programs\updown.asm       PAGE       1

                   count   EQU     $1004   ; Put count in port B
                   dir     EQU     $1003   ; Put dir in port C

  D000 4F          start:  CLRA            ; Set the current state to S0.
  D001 B7 10 04    output: STAA    count   ; Output current state # as count.
  D004 BD D0 1B            JSR     delay   ; Delay so we can see what's
.                  happening
  D007 7D 10 03    loop:   TST     dir     ; Test the "dir" input
  D00A 27 07               BEQ     down    ; Go down if dir=0
  D00C 81 05       up:     CMPA    #5      ; Already 5?
  D00E 27 F7               BEQ     loop    ; If so, no change.
  D010 4C                  INCA            ; Increment state number.
  D011 20 EE               BRA     output  ; Update output and repeat.
  D013 7D 10 04    down:   TST     count   ; Check current count
  D016 27 EF               BEQ     loop    ; If count=0, keep it.
  D018 4A                  DECA            ; Decrement state number.
  D019 20 E6               BRA     output  ; Update output and repeat.

  D01B CE 03 E8    delay:  LDX     #1000
  D01E 09          dloop:  DEX
  D01F 26 FD               BNE     dloop
  D021 39                  RTS

  Symbol Table 

  UP                 D00C
  DIR                1003
  DELAY              D01B
  DLOOP              D01E
  COUNT              1004
  START              D000
  DOWN               D013
  LOOP               D007
  OUTPUT             D001