@gabrielesvelto yes, it's only a part, but it starts there. The irregular instruction sizes caused problems when instructions straddled cacheline boundaries, etc. Everything after that: superscalar execution, OOOE, all got harder because the simplest case, single instruction in-order, was already non-deterministic.

@gabrielesvelto Intel/AMD had an opportunity to create a clean, easy to decode instruction layout with the transition to 64bit but they failed. http://www.emulators.com/docs/nx05_vx64.htm

@gabrielesvelto x86 instruction complexity alone is unmanageable. A single instruction can be up to 15 bytes long. That's 2^120 possible bit combinations for instructions. So it's already physically impossible to test every instruction individually, let alone test every *sequence* of instructions to find problematic execution sequences.