The MSDN documentation includes information about the x64 registers.
x64 extends x64’s 8 general-purpose
registers to be 64-bit, and adds 8 new
64-bit registers. The 64-bit registers
have names beginning with “r”, so for
example the 64-bit extension of eax is
called rax. The new registers are
named r8 through r15.The lower 32 bits, 16 bits, and 8 bits
of each register are directly
addressable in operands. This includes
registers, like esi, whose lower 8
bits were not previously addressable.
The following table specifies the
assembly-language names for the lower
portions of 64-bit registers.
64-bit register | Lower 32 bits | Lower 16 bits | Lower 8 bits
==============================================================
rax | eax | ax | al
rbx | ebx | bx | bl
rcx | ecx | cx | cl
rdx | edx | dx | dl
rsi | esi | si | sil
rdi | edi | di | dil
rbp | ebp | bp | bpl
rsp | esp | sp | spl
r8 | r8d | r8w | r8b
r9 | r9d | r9w | r9b
r10 | r10d | r10w | r10b
r11 | r11d | r11w | r11b
r12 | r12d | r12w | r12b
r13 | r13d | r13w | r13b
r14 | r14d | r14w | r14b
r15 | r15d | r15w | r15b