ENTITY and_or IS PORT(A,B:IN bit; C,D:OUT bit);END and_or;ARCHITECTURE rtl OF and_or ISBEGIN C <= A AND B; D <= A XOR B;END rtl;是个1位半加器。虽然图中的逻辑关系是D <=( A OR B) AND (A NAND B),但经过逻辑化简之后,就变成D <= A XOR B了。