Originally Posted by a page found on google

The additional 64-bit general purpose registers are named r8-r15. There are also 8-bit (rXb), 16-bit (rXw), and 32-bit (rXd) subregisters that map to the least significant 8, 16, or 32 bits of the 64-bit register. The original 8 general purpose registers have also been extended to 64-bits: eax, edx, ecx, ebx, esi, edi, esp, and ebp have new 64-bit versions called rax, rdx, rcx, rbx, rsi, rdi, rsp, and rbp respectively. The old 32-bit registers map to the least significant bits of the new 64-bit registers.