8位计数器作业中的behavioral描写，没有带testbench，已经通过-1.　　Construct VHDL models for 74-139 dual 2-to-4-line decoders using three descr iption types, i.e., behavioral, dataflow and structural descr iptions. Synthesize and simulate these models respectively in the environment of Xilinx ISE with the ModelSim simulator integrated. When simulating these models, test vector(s) are required to stimulate the units under test (UUT). Reasonable test vectors are designed and created by your own as sources added to your VHDL project.