x86 32 bit opcodes that differ in x86-x64 or entirely removed

Question

I\'ve looked up Wikipedia for x86 backward compatibility in x86-x64 and it says:

x86-64 is fully backwards compatible with 16-bit and 32-bit x86 code.Bec

Answer 1

The statement is true, but somewhat misleading. The x86-64 architecture is backward compatible with x86, but the 32-bit instruction set is not compatible with the 64-bit instruction set.

You can run x86 code on a x86-64 CPU by using a compatibility mode. Actually, since the CPU should be transparently x86 to x86 code, it's the opposite : you enter 64-bit mode (long mode) when you want to run x86-64 code. This means you can't run both at the same time, though it is possible to switch from one mode to another.