; initialize a PIC assembly program
LIST P=16F84A       ; Tells the assembler what chip we are using
    INCLUDE "p16f84a.inc"   ; Include the defaults for the chip
    __CONFIG _CP_OFF & _WDT_OFF & _PWRTE_ON & _HS_OSC ; Configuration settings

    ORG 0x00            ; Start at location 0
    GOTO MAIN            ; Skip over the interrupt service routine

    ORG 0x04            ; Interrupt service routine location
    RETFIE               ; Return from interrupt

    MAIN
    BANKSEL TRISA        ; Select TRISA to configure inputs/outputs
    MOVLW b'00001111'    ; Set RA3-RA0 as inputs
    MOVWF TRISA          ; Write to TRISA
    BANKSEL TRISB        ; Select TRISB to configure inputs/outputs
    CLRF TRISB           ; Set all port B as outputs

    BEGIN
    BANKSEL PORTA        ; Select PORTA to read inputs
    MOVF PORTA,W         ; Move PORTA to W register
    ANDLW b'00001111'    ; Keep only the last 4 bits (A, B, C, and Y)

    ; Evaluate logical expression !C || (A xor B)

    ; First, check if C is 0 (!C)
    BTFSS PORTA,2        ; Skip the next instruction if C (bit 2) is set
    GOTO SET_OUTPUT      ; If C is 0, go to set output on PORTB

    ; If not, evaluate A xor B
    MOVF PORTA,W         ; Move PORTA to W register
    ANDLW b'00000011'    ; Keep only bits A and B
    XORWF PORTA,F        ; XOR A and B
    BTFSC STATUS,Z       ; Skip the next instruction if Z flag is set (A xor B == 0)
    GOTO CLEAR_OUTPUT    ; If A xor B is 0, go to clear output on PORTB

    ; If none of the conditions are met, set output on PORTB
    SET_OUTPUT
    BSF PORTB,0          ; Set bit 0 of PORTB
    GOTO END             ; End of program

    CLEAR_OUTPUT
    BCF PORTB,0          ; Clear bit 0 of PORTB

    END
    GOTO BEGIN           ; Loop back to the beginning

    END                  ; End of program