User blog comment:ヌヌハラ/alice-tools v0.2.0/@comment-45214281-20200313112813/@comment-44256294-20200314152831

It seems to take a struct/object reference and (presumably) does some kind of check on it. I'm guessing that the "UDO" stands for "undefined object" since it seems to be used with uninitialized local variables a lot. It's hard to say whether it has any other semantics, but it doesn't look like it from what I've seen.

For code reading purposes, I think you can basically ignore code like the following:

PUSHLOCALPAGE PUSH n REF CHECKUDO (The CHECKUDO instruction consumes the value pushed onto the stack by the previous three instructions.)