I've been using Vivado and been running into issues. The simulator crashes whenever I try to use the std_logic values L and H. They are supposed to be weak pull down and pull up values. I could see how they might not be supported, but the program shouldn't crash. Normally you'd never see those values inside an FPGA, but for simple behavioral simulation like I'm doing it shouldn't be an issue. I don't have access to Modelsim which I know can handle it, so I guess I'll make do.