It's not only xor that does this, but most 32-bit operations zero-extend the result of the 64-bit register. AMD did this for backward compatibility. so existing programs would mostly continue working, unlike Intel's earlier attempt at 64-bits which was an entirely new design.

The reason `xor eax,eax` is preferred to `xor rax,rax` is due to how the instructions are encoded - it saves one byte which in turn reduces instruction cache usage.

When using 64-bit operations, a REX prefix is required on the instruction (byte 0x40..0x4F), which serves two purposes - the MSB of the low nybble (W) being set (ie, REX prefixes 0x48..0x4f) indicates a 64-bit operation, and the low 3 bits of low nybble allow using registers r8-r15 by providing an extra bit for the ModRM register field and the base and index fields in the SIB byte, as only 3-bits (8-registers) are provided by x86.

A recent addition, APX, adds an additional 16 registers (r16-r31), which need 2 additional bits. There's a REX2 prefix for this (0xD5 ...), which is a two byte prefix to the instruction. REX2 replaces the REX prefix when accessing r16-r31, still contains the W bit, but it also includes an `M0` bit, which says which of the two main opcode maps to use, which replaces the 0x0F prefix, so it has no additional cost over the REX prefix when accessing the second opcode map.