Push Button on 68K

Question

I want to Write a program to read a value from the push buttons and display that value on the LEDs. The program should run continuously and as the push buttons are changed, the display changes. I tried many ways but it does not show any thing in Could any one help me to know where is the problem.

LEDS       EQU     $E00010                 ;LEDS adress
BUTTON     EQU     $E00014                 ;BUTTON address
           ORG     $400                    ;start of program area

START
Loop        MOVE.B  #2,D0                             
            MOVE.B  BUTTON,D1               ;move the value of button to D1   
            MOVE.B  D2,LEDS 
            NOT.B   D1                      ;take NOT to flip the value in order to present it in LEDS                                    
           MOVE.B  D1,D2                    ;move the value to LEDS                        
           SUB.B    #2,D0                   ; if D0 =0 then loop again
            BEQ     Loop                     



          SIMHALT       
            END     START

Answer 1

A few things are missing from this.

1. A button is normally a single bit, not a whole byte, so there should be some form of mask applied to the button input. Similarly setting an LED normally involves setting a single bit, not a byte, unless it is some form of multicolor LED. I am assuming you have 8 push buttons and 8 corresponding LEDs

2. The code you have illustrated will run continuously because you load D0 with 2 after the LOOP label, and at the end of the loop you subtract 2 from D0 (which has the value 2) and then loop if that result is equal to zero ie always. If you really want a continuous loop, there is no point in using D0 at all.

    LEDS EQU $E00010 ;LEDS address BUTTON EQU $E00014 ;BUTTON address ORG $400 ;start of program area START LOOP MOVE.B BUTTON,D1 ; Read buttons NOT.B D1 ; LEDs are inverse of button MOVE.B D1,LEDS ; write to LEDs BRA.S LOOP ; do continously SIMHALT ; doesn't get here but still END START