Ultrasim doesn't simulate a mixed-signal design (well, it's not a mixed-signal simulator; it's a FastSPICE simulator). It's also not called MMSIM...

MMSIM (Multi-Mode Simulation) is a flexible set of simulation technology that gives access to circuit simulation (Spectre/APS), RF (Spectre RF), FastSPICE (XPS, and also the older UltraSim) and Mixed-Signal (AMS Designer, using Spectre, APS or UltraSim as the solver for the analog parts). There's a flexible token-based license model which means you can easily use the licenses you have to access any of the overall set of simulation capabilities, or you can have "a la carte" licenses for the specific simulators you want.

You can of course use any of these (including UltraSim) to simulate a mixed-signal design, but you'd probably be better off using a mixed-signal simulator (such as AMS Designer) if there's significant digital content or you have behavioural models for the digital parts.

Not sure if any of this answered your question, but then again it was pretty open-ended...

Regards,

Andrew.

I don't understand your question. Are you expecting me to explain precisely how SPICE and FastSPICE simulators work? That's a topic for a book, not a forum reply.

There's a fair bit of background in the documentation, but you're going to have to ask something a bit more specific, I think.

Regards,

Andrew.