The spec is very large, not particularly well written, and is not “total” (in the sense that AMD64 and IA32e and other x86-64 flavors are all subtly different).

The spec is very large, not particularly well written, and is not “total” (in the sense that AMD64 and IA32e and other x86-64 flavors are all subtly different). There are a lot of ways to get it wrong; even XED (the reference decoder from Intel) has bugs. If I remember correct, the Intel SDM alone is over 3000 pages long.