這程式是使用PIC16F877
PORTD是8顆LED做跑馬燈，
PORTB是4顆選擇按鈕，
按下B1.左移B2.右移B3.延遲變大B4.延遲變小，

左右移沒問題，但是延遲卻無法變化，
撿查許久都不知道問題所在
程式碼如下:

LIST P=16F877, R=DEC
INCLUDE <P16F877.INC>
CBLOCK H'20'
A,X,X1,X2,X3,X4
ENDC
ORG 0
NOP
BANKSEL TRISD
MOVLW 0
MOVWF TRISD
BANKSEL PORTD
MOVLW B'11111110'
MOVWF A
MOVWF PORTD
CALL DLY
MOVLW 100
MOVWF X
MOVLW 10
MOVWF X4

S1:
RLF A,W
RLF A,F
MOVF A
MOVWF PORTD
CALL DLY
BTFSC PORTB,2
GOTO S1
GOTO S2

S2:
RRF A,W
RRF A,F
MOVF A
MOVWF PORTD
CALL DLY
BTFSC PORTB,1
GOTO S2
GOTO S1

DLY:
MOVF X
MOVWF X1
BTFSS PORTB,4
GOTO A1
BTFSS PORTB,3
GOTO A2
GOTO AA

A1:
MOVF X1
ADDWF X4
MOVWF X1
GOTO AA

A2:
MOVF X1
SUBWF X4
MOVWF X1
GOTO AA

AA:
MOVF X1
MOVWF X

LL:
MOVF 10
MOVWF X2

L1:
MOVLW 50
MOVWF X3

L2:
DECFSZ X3
GOTO L2
DECFSZ X2
GOTO L1
DECFSZ X1
GOTO LL
RETURN

END