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.
x64 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 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|
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.
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.
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.