Saturn MPEG card is currently unemulated. Should decrypt and overlay the BIOS, and should do it in a separate bus option than the regular cart (that one is on top of retail Saturn, Video CD "cart" is instead a PCMCIA-like slot lives in the back of the console near the battery).

SMS cart slot bogusly detect any kind of 0x1000 / 4096 length ROM as x-terminator, which obviously crashes because it's a Game Gear only option. I know it's a thing because Super Magic Drive BIOS also has the exact same phenomenon.

In case you're using latest and the new UI, on the left there should be a selector for various options:



Using the mouse and clicking on available should do the trick.

This topic will be dedicated to any external tutorial worth of interest for getting a more accurate emulation. Will be updated from time to time.

Analog Video 101
http://www.ni.com/white-paper/4750/en/#toc1

z80 Assembler Directives
http://www.nongnu.org/z80asm/directives.html