下方那个移动的滑块的设计

PROCESS(MDR,MDL) BEGIN
--底部滑块超左极限位置的监测及处理和移动速度的设置
IF(rising_edge(MDL))THEN 
IF(0<X1 and X2<640)THEN
X1<=X1-30;
X2<=X2-30;
X3<=X3-30;
X4<=X4-30;
ELSIF(X1<0 )THEN
X1<=0;Y1<=400;X2<=0;Y2<=460;
X3<=30;Y3<=480;X4<=30;Y4<=480;
END IF ;
END IF;
--底部滑块超右极限位置的监测及处理和移动速度的设置
IF(rising_edge(MDR))THEN 
IF(0<X1 and X2<640)THEN
X1<=X1+30;
X2<=X2+30;
X3<=X3+30;
X4<=X4+30;
ELSIF(X3>640)THEN
X1<=610;Y1<=400;X2<=610;Y2<=460;
X3<=640;Y3<=480;X4<=640;Y4<=480;
END IF ;
END IF ;
END PROCESS;