只是在行为上编码计数器(a 以下为原文
What you're missing is that the synthesis tools have become a good deal smarter since these library elements were made. Just coding a counter behaviorally (a <= a + 1) will in fact create a counter with carry logic, and the placement tools will use the correct relative placement to allow the carry to use the dedicated routes. There's really no need to use these macros for more recent versions of ISE.
-- Gabor