Though, for whatever reason, the instruction internally decoded as if it took two parameters, a register destination and a register-or-memory source. Because t

Though, for whatever reason, the instruction internally decoded as if it took two parameters, a register destination and a register-or-memory source. Because the rest of the 0F Fx line also has a ModRM. Ditto for 0F Bx. So if your 0F FF is at the end of a page, and the next page is not present, you sometimes got an invalid opcode exception and you sometimes got an access violation. This reminds me of some related information on instruction length and decoding of "undefined" instructions I have filed away from a long time ago; sadly this stuff is disappearing from the Internet, but both the Archive and I still remember: https://web.archive.org/web/20160721202526/http://pferrie.host22.com/misc/lowlevel2.htm https://web.archive.org/web/20160721202526/http://pferrie.ho... Such information is very important for emulation accuracy and some security contexts.