I'd guess it's "undocumented", not "undefined". Don't know how the situation is on x86, but on the Z80 there were indeed some slight differences in undocumented

I'd guess it's "undocumented", not "undefined". Don't know how the situation is on x86, but on the Z80 there were indeed some slight differences in undocumented behaviour between CPU vendors, but those are so obscure that it hardly affected any real world code (they affected the undocumented flag bits 3 and 5, and their behaviour was only properly 'decoded' in the 2000's (https://github.com/floooh/emu-info/blob/master/z80/memptr_eng.txt https://github.com/floooh/emu-info/blob/master/z80/memptr_en...). The only CPU I know with actual 'undefined' behaviour is the 6502 for some of the undocumented/illegal opcodes which can yield different results based on things like current CPU temperature (see the ANE/XAA instruction description: https://www.masswerk.at/nowgobang/2021/6502-illegal-opcodes https://www.masswerk.at/nowgobang/2021/6502-illegal-opcodes)