Additional Notes: The x64 architecture extends x86'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 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.
* Operations that output to a 32-bit subregister are automatically zero-extended to the entire 64-bit register.
* Operations that output to 8-bit or 16-bit subregisters are *not* zero-extended (this is compatible x86 behavior).
* The high 8 bits of **ax**, **bx**, **cx**, and **dx** are still addressable as **ah**, **bh**, **ch**, **dh**, but cannot be used with all types of operands.
* The instruction pointer, **eip**, and **flags** register have been extended to 64 bits (**rip** and **rflags**, respectively) as well.
The x64 processor also provides several sets of floating-point registers:
* Eight 80-bit x87 registers.
* Eight 64-bit MMX registers. (These overlap with the x87 registers.)
* The original set of eight 128-bit SSE registers is increased to sixteen.
The addressing modes in 64-bit mode are similar to, but not identical to, x86.
* Instructions that refer to 64-bit registers are automatically performed with 64-bit precision. (For example **mov rax, \[rbx\]** moves 8 bytes beginning at **rbx** into **rax**.)
* A special form of the **mov** instruction has been added for 64-bit immediate constants or constant addresses. For all other instructions, immediate constants or constant addresses are still 32 bits.
* x64 provides a new **rip**-relative addressing mode. Instructions that refer to a single constant address are encoded as offsets from **rip**. For example, the **mov rax, \[***addr***\]** instruction moves 8 bytes beginning at *addr* + **rip** to **rax**.
Note: Instructions, like **jmp**, **call**, **push**, and **pop**, that implicitly refer to the instruction pointer and the stack pointer treat them as 64 bits registers on x64.
