list p=18F1220 ; Set processor type
    radix hex ; Sets the default radix for data exp.
    config WDT=OFF, LVP=OFF, OSC = INTIO2 ; Disable Watchdog timer, Low V. Prog, and RA6 as a clock


#define ADCON1 0xFC1
#define PORTB 0xF81
#define TRISB 0xF93
#define WREG 0xFE8


LAST equ 0x80
CURR_A equ 0x81
RESULT equ 0x82

    org 0x00


    ; this is just a bunch of dumb setup
    CLRF PORTB
    CLRF TRISB

    ; what does this do? nobody knows
    MOVLW 0x7F
    MOVWF ADCON1

    ; in binary this is 0001 1111
    ; means that pins RB0-RB5 are inputs, rest are outputs
    ; why 5 pins? it works
    MOVLW 0x1F
    MOVWF TRISB

    CLRF LAST
    CLRF CURR_A

Loop:
    ; Put the value from PORTB into WREG
    MOVF PORTB, 0

    ; Check and see if it's different from the last value
    XORWF LAST, 0

    ; If it is different, branch over to CalcSum
    BNZ CalcSum

    ; Redo the loop
    GOTO Loop

CalcSum:

    ; Grab the value of A and put it into CURR_A
    MOVF PORTB, 0
    ANDLW 0x3 ; W = A
    MOVWF CURR_A

    ; Grab the value of B and put it in WREG
    ; (we have to shift it to the right and AND it
    ; to ensure we have ONLY the bits we want)
    MOVF PORTB, 0
    RRNCF WREG
    RRNCF WREG
    ANDLW 0x3 ; W = B

    ; Add the value in WREG (aka B) with the value
    ; in CURR_A (aka A), and put the result in WREG
    ADDWF CURR_A, 0

    ; Put the sum of A and B in RESULT for easy debuggin'
    MOVWF RESULT

    ; Shift WREG to the left a bunch so that it fits
    ; correctly in PORTB
    RLNCF WREG
    RLNCF WREG
    RLNCF WREG
    RLNCF WREG
    MOVWF PORTB

    ; Set the last PORTB to this PORTB. This way,
    ; we can tell in the future if the inputs change.
    MOVFF PORTB, LAST

    ; Go back to the main loop
    GOTO Loop

    END