The point I made was that the way a simulator simulates is how the programmer of the simulator interpreted the function of the '51.
If this is not in the data sheet the simulator will simulate as the simulator programmer guessed/found out for a given derivative. It can be:
a) totally unpredictable for a given variant
b) manufacturer dependent
c) derivative dependent
d) changed when the manufacturer update the silicon masks for whatever reason

Thus, If you want to know how something NOT in the data sheet functions, a simulator is NOT the way to find out.

Erik

PS some of us remember how often, in the 256k days, PC programs ( especially "freeware") failed becuse someone had used an "undocumented instruction" of the 86 to save a bit of code.