It is a little add-on board with a PSRAM (specific for the ESP32) and a microSD socket. I've called it uHat (micro Hat) as it is fitted over the uTerm2-S board using the AUX (J6) connector like an hat:

The uHat add-on allows to run an interesting application of the FabGL library that requires the SD and the PSRAM.

PC XT EMULATION

There is a FabGL library application (inside the FabGL library examples) that emulates a PC XT. There are a few options, but the most interesting is probably that one with Windows 3.0.

Because the SD on the uHat uses different GPIO, before compiling the PCEmulator.ino application you have to change the parameters at the line containing the "FileBrowser::mountSDCard" call as shown in the following image (yellow line):

And here the emulator in action:

Z80-MBC2 EMULATION

Tanks to Michel Bernard a Z80-MBC2 can be completely emulated with the uTerm2-S + uHat (this emulator is based on the FabGL lib too). You have to use the same SD of the real one. The User key of the Z80-MBC2 here is emulated with the DFLT key.

In the Files section I've uploaded the ready to use version for the uTerm2-S (changes for the uTerm2-S + uHat are marked with the "J4F:" string inside comments):

NOTE: All the files, updates and the details to build it are in the uTerm2-S page here:

https://hackaday.io/project/181583-uterm2-s-a-multi-emulation-color-rs232-terminal