Which Dec Emulation is the MOST useful and Versatile?

I've read some VHDL before, but my 6600 gate level model was my first VHDL project. It's actually quite easy, easier than the level of fluency needed to do a good behavioral model. A decent textbook helps a lot. My favorite is The Designer's Guide to VHDL by Peter Ashenden. The point is that modeling small modules (SSI gates, or 6600 "cordwood" modules, or the like) is easy because they are small and have quite simple behavior. Then it's just a matter of wiring them together. Note that you don't need an FPGA to do logic level design; all you need is a VHDL simulator. I use GHDL, which is open source, part of GCC so you can hook in C code if you need it. For example, that allows you to make a model of the I/O channel and connect it to a SIMH style emulation of a peripheral device. The real issue for gate level modeling is the availability of the necessary documentation. If you have schematics, and they includes critical detail such as microcode ROM contents, you're all set. If all you have is functional specs, you can't even start. It helps to have a machine built with sane design principles. Things like RS flops that don't have both inputs active at the same time. And a properly clocked architecture. Neither of these properties holds for the CDC 6600... paul