> Code running in 32-bit protected mode can't access 64-bit instructions or registers, of course, but that's a different issue.
That's what I was referring to - in 16-bit realmode you can use 32-bit registers and addressing modes with prefix bytes just like you can use 16-bit operands or addresses in 32-bit mode, and in protected mode both 16 and 32-bit segments can coexist in the same system. But it looks like 64-bit is completely isolated from that, although they could've used some other prefices, enabling access to 64-bit operands (including the extra registers) and addresses from any mode, and also allow coexistence of 64-bit segments with existing 32- and 16-bit segments. Then x32 wouldn't really need to exist; it would just be a 32-bit segment with instructions that use 64-bit operands.
That's what I was referring to - in 16-bit realmode you can use 32-bit registers and addressing modes with prefix bytes just like you can use 16-bit operands or addresses in 32-bit mode, and in protected mode both 16 and 32-bit segments can coexist in the same system. But it looks like 64-bit is completely isolated from that, although they could've used some other prefices, enabling access to 64-bit operands (including the extra registers) and addresses from any mode, and also allow coexistence of 64-bit segments with existing 32- and 16-bit segments. Then x32 wouldn't really need to exist; it would just be a 32-bit segment with instructions that use 64-bit operands.