Intel® 64 and IA-32 Software Developer's Manual: Document Changes
This document is an update to the specifications contained in the Affected Documents table. This document is a compilation of device and documentation errata, specification clarifications and changes. It is intended for hardware system manufacturers and software developers of applications, operating systems, or tools....In 64-bit mode, there are 16 general purpose registers and the default operand size is 32 bits. However, general-purpose registers are able to work with either 32-bit or 64-bit operands. If a 32-bit operand size is specified: EAX, EBX, ECX, EDX, EDI, ESI, EBP, ESP, R8D - R15D are available. If a 64-bit operand size is specified: RAX, RBX, RCX, RDX, RDI, RSI, RBP, RSP, R8-R15 are available. R8D-R15D/R8-R15 represent eight new general-purpose registers. All of these registers can be accessed at the byte, word, dword, and qword level. REX prefixes are used to generate 64-bit operand sizes or to reference registers R8-R15.
Registers only available in 64-bit mode (R8-R15 and XMM8-XMM15) are preserved across transitions from 64-bit mode into compatibility mode then back into 64-bit mode. However, values of R8-R15 and XMM8-XMM15 are undefined after transitions from 64-bit mode through compatibility mode to legacy or real mode and then back through compatibility mode to 64-bit mode.
Read the full Intel® 64 and IA-32 Software Developer's Manual: Document Changes.