In modern processors, 16-bit, 32-bit, and 64-bit x86 all use mostly the same instructions, just with different default operand sizes. In particular, with a few

In modern processors, 16-bit, 32-bit, and 64-bit x86 all use mostly the same instructions, just with different default operand sizes. In particular, with a few big exceptions (like INC/DEC r32 being replaced with the REX prefixes, and segment PUSH/POP being removed), they all have the same encodings. So the set of undocumented instructions, especially in the multibyte encoding ranges, is likely the same. Though their behavior might vary, or be absent altogether.