Here’s a test of the SIMM CART switch on SW2 at the front of the Sega Saturn Sophia Programming Box.

My first interpretation was that it simply selected between the internal SIMM memory and the Saturn cartridge slot.

Sega’s development documentation shows that it is more complicated than that.

What SIMMCART actually changes

The Programming Box can map its internal SIMM memory into address space associated with the Saturn’s external cartridge area.

Sega’s development FAQ documents these SIMM mappings:

  • SIMMCART OFF: SIMM mapped at 24000000h to 24FFFFFFh, giving a 16 MB continuous region
  • SIMMCART ON: SIMM mapped at 22000000h to 23FFFFFFh, giving a 32 MB continuous region

Sega’s extended RAM documentation separately warns that installed Programming Box SIMM memory can overlap the extended RAM cartridge address space.

It also states that when DIP switch 2, SIMMCART, is OFF, the system cannot read the external cartridge ID.

So the behaviour I observed was real, but my original “SIMM versus cartridge slot” explanation was too simple.

SIMMCART is fundamentally involved in how the Programming Box maps its SIMM memory, and that mapping can interfere with access to retail extended-RAM cartridges.

Retail RAM cartridge test candidates

I was looking at a few games to help test what the Sophia could actually see:

I still don’t trust “auto” Action Replay cartridges as a clean diagnostic tool for this sort of thing.

Better to test known hardware against known configurations.

More information can be found in the archived ASSEMblergames discussion.

Sources