shift4.vhd

来自「一本很好的关于学习VHDL的书,Fundamentals of Digital 」· VHDL 代码 · 共 26 行

LIBRARY ieee ;
USE ieee.std_logic_1164.all ;

ENTITY shift4 IS
	PORT ( 	R 		: IN 		STD_LOGIC_VECTOR(3 DOWNTO 0) ;
			Clock 	: IN 		STD_LOGIC ;
			L, w 	: IN 		STD_LOGIC ;
			Q 		: BUFFER 	STD_LOGIC_VECTOR(3 DOWNTO 0) ) ;
END shift4 ;

ARCHITECTURE Behavior OF shift4 IS	
BEGIN
	PROCESS
	BEGIN
		WAIT UNTIL Clock'EVENT AND Clock = '1' ;
		IF L = '1' THEN
			Q <= R ;
		ELSE
			Q(0) <= Q(1) ;
			Q(1) <= Q(2); 
			Q(2) <= Q(3) ; 
			Q(3) <= w ; 
		END IF ;
	END PROCESS ;
END Behavior ;