#INCLUDE p16F877A.inc	; Include the standard definitions

__CONFIG _CP_OFF & _DEBUG_ON & _WRT_OFF & _CPD_OFF & _LVP_OFF & _BODEN_OFF & _PWRTE_OFF & _WDT_OFF & _HS_OSC
	
		NOP
		clrf	PORTB		; Initialize PORTB so that all LEDs are off by default初始化PORTB，將所有LED關閉
		BANKSEL	TRISB		; BANKSEL is used to get to bank with TRISB in it (bank 1)
		clrf	TRISB		; Clear TRISB to make all PORTB pins outputs
		movlw	B'00010000'	; Make pin RA4 an input (rest of PORTA pins are outputs)
		movwf	TRISA		; This is redundant as TRISA bits are 1抯 by default (inputs)
							; (the rest of the pins are set low)

		movlw	H'07'		; Writing 7 into ADCON1 causes all analog pins to allow
		movwf	ADCON1		; digital input.  You must set ADCON1 when using pins with 
							; A/D functions as they are analog mode by default.
							; This code isn抰 needed in this example - do you know why?

		BANKSEL	PORTA		; BANKSEL is used to return to bank 0 (for PORTA, PORTB)
Loop
		btfss	PORTA,4		; Test RA4 on PORTA, skip if pin is high
		goto	On			; go to "On" code if button is pressed (pressed button is low)

Off		
		movlw	B'00000010'	; "On" code is skipped so "Off" code runs - it writes a value to
		movwf	PORTB		; PORTB to turn on RB1 (high) and sets rest of PORTB low
		goto	Loop		; The test of the button happens again by branching to 揕oop?
On		
		movlw	B'00001000'	; "On" code writes a value to PORTB to turn on RB3 (high)
		movwf	PORTB		; and sets rest of PORTB low, including RB1.
		goto	Loop		; The test of the button happens again by branching to "Loop"

Here 	
		goto	Here		; This code is not needed either.  Why?
		END					; All programs must have an END directive