Most likely a "ROM emulator" that would plug into the ROM socket on the board, replacing it with a few kB of static RAM that you can pop stuff into from a computer.
In the mid-90s I used an already pretty obsolete CP/M machine to drive one of those for developing Z80 embedded code. Although PCs were around - 386s and even early 486s if you had the money, they offered no real advantage because the editor, assembler, and linker only took about a second to load from floppy anyway.
Given how much time I spend looking at 80x25 terminals I wonder why I put up with 20-second boot times now, over 30 years later.
I actually used one of these things[0] during development of the DX9/7 firmware mod. I like your Mirage tools by the way! I've actually got a very battered Ensoniq Mirage I picked up for free that's badly in need of some maintenance. If I can get that running I'd love to give your firmware a try!
What's wrong with it? If it gets as far as loading an OS you ought to be able to run the Forth disk. The big problem they have is sticking on filter calibration, because one or other of the 405x multiplexers fails.
In the mid-90s I used an already pretty obsolete CP/M machine to drive one of those for developing Z80 embedded code. Although PCs were around - 386s and even early 486s if you had the money, they offered no real advantage because the editor, assembler, and linker only took about a second to load from floppy anyway.
Given how much time I spend looking at 80x25 terminals I wonder why I put up with 20-second boot times now, over 30 years later.