Even if you dislike the REX prefixes of AMD64, you've got to think about the 66/F2/F3 prefixes used for older SIMD instructions. They were introduced by Intel and basically contribute to more opcode bits. There's also the 2E/3E prefixes used for static branch prediction hints in some Pentium 4's (and also in new/upcoming Intel CPUs). VEX is from Intel, EVEX is from Intel, and the upcoming REX2 is also from Intel.
Discussed here: https://news.ycombinator.com/item?id=30409100
I've memorised most of the 1st-page instructions - in octal - and it's easier than it sounds.
Originally, yes. These days, not so much. It's "whatever bit confetti was necessary to squeeze in all the opcode/operand bits".
By "these days" you mean "since AMD messed it up".
Obviously not.
Even if you dislike the REX prefixes of AMD64, you've got to think about the 66/F2/F3 prefixes used for older SIMD instructions. They were introduced by Intel and basically contribute to more opcode bits. There's also the 2E/3E prefixes used for static branch prediction hints in some Pentium 4's (and also in new/upcoming Intel CPUs). VEX is from Intel, EVEX is from Intel, and the upcoming REX2 is also from Intel.