Section 18.104.22.168 of the VICE manual mentions that cartridge slot 1 (of the four slots 0, 1, Main and I/O) in the emulator is special in that it's designed to support "mostly RAM-based cartridges" (of which there seem to be just four that are officially supported in this slot).
The cartridges are:
- Double Quick Brown Box (DQBB), on which I've found no information.
- Expert Cartridge, which seems like a fairly standard fast loader and freezer except that it "held its system software in an 8 KB RAM that could be reprogrammed."
- ISEPIC, another freezer, this one with 2 KB of RAM, "banked into a 256-byte page at 0xDF00 – 0xDFFF." (This is the IO2 area, not used by internal devices except RAM, when mapped in.)
- RamCart, a memory expansion with battery backup, also banking the RAM into the $D000-FF IO2 area (and using $DE00 and $DE01 in the IO1 area to control which page is banked in), used as a RAM disk.
What was special about these cartridges that they need a separate slot in the emulator? Were there cartridges with RAM that don't need the special emulated slot? (Here I'm speaking of contemporary cartridges, rather than modern hardware like the MMC64.)
The two above that I have details about use the standard external I/O areas, but there's already an I/O slot that most I/O cartridges seem to use. It looks as if these cartridges probably all had ROM as well, which would likely be mapped into the usual areas so that it could be run in the usual way or via MAX mode when the C64 is started. And two or three of these seem to be freezer cartridges, but, as with I/O cartridges, other freezer cartridges don't get a special slot.
Note that I'm not asking about VICE in particular; I'm assuming there that there's some good reason related to the actual hardware itself that made the emulator developers decide things like, "yes, this cartridge and this other one can be used at the same time, but these two can't, and perhaps "this cartridge needs special emulation code." (Note that the C64 itself never prevented anyone from using any pair of cards at the same time; you could just plug in a bus expander and plug in both cards. Whether they would work together is another question, of course, and one similar to this one.)