[aseipp]

Right, this is the kind of information I was looking for (maybe some actual ISA documentation; the instruction encoding method itself, etc).

But for this specific point -- I thought all of the Xtensa series used register windows? Or did they both have them but the ESP8266 never actually use them (somehow?)

[MrBuddyCasino]

The xtensa series is highly configurable, afaik the ESP8266 doesn't have them in hardware.

This is second hand knowledge though.