x64 Architecture

x64 Architecture

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The x64 architecture is a backwards-compatible extension of x86.  It provides a legacy 32-bit mode, which is identical to x86, and a new 64-bit mode.

The term "x64" includes both AMD 64 and Intel64. The instruction sets are close to identical.

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

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.

Calling Conventions

Unlike the x86, the C/C++ compiler only supports one calling convention on x64.  This calling convention takes advantage of the increased number of registers available on x64:

• The first four integer or pointer parameters are passed in the rcx, rdx, r8, and r9 registers.

• The first four floating-point parameters are passed in the first four SSE registers, xmm0-xmm3.

• The caller reserves space on the stack for arguments passed in registers.  The called function can use this space to spill the contents of registers to the stack.

• Any additional arguments are passed on the stack.

• An integer or pointer return value is returned in the rax register, while a floating-point return value is returned in xmm0.

• rax, rcx, rdx, r8-r11 are volatile.

• rbx, rbp, rdi, rsi, r12-r15 are nonvolatile.

The calling convention for C++ is very similar: the this pointer is passed as an implicit first parameter.  The next three parameters are passed in registers, while the rest are passed on the stack.

Addressing Modes

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.

Instructions, such as jmp, call, push, and pop, that implicitly refer to the instruction pointer and the stack pointer treat them as 64 bits registers on x64.

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Build date: 2/14/2012

http://msdn.microsoft.com/en-us/library/ff561499(v=vs.85).aspx

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