CODEWITHSUNDEEP
c++assembly
zhaorufei
zhaorufei

Reputation: 2193

Is it possible to use 64-bit instruction in a 32-bit application on intel/64-bit win7

My environment is 64-bit win7, VC2010.

Of course, the intel inside is 64-bit CPU.

Can I use the 64-bit instruction/native machine word(64-bit) in a 32-bit application? Since most of my code is 32-bit, I do not want to port it to 64-bit.

For some performance critical hot code, I want to manually optimize it by compiler intrinsic or inline assembly(Also 64-bit VC compiler do NOT support inline assembly), is it possible to run 64-bit code fox example mov rax, rbx in a 32-bit mode application?

Upvotes: 5

Views: 1466

Answers (2)

user555045
user555045

Reputation: 64904

Actually, you kind of can. The mechanism used to switch from 32 to 64 bit mode (which clearly must exist on a 64 bit OS capable of running 32 bit code) is not in any way protected, and can be used from user code. On Windows it is called Heaven's Gate, and it's pretty simple: just a far call with a segment selector of 33h.

So, how to run 64bit code?

call 33h:your64bitcode
...
your64bitcode:
; do something
retf

There are, of course, some limitations to what you can do from 64bit code entered in that way, because you're not truly in a 64bit process.

Upvotes: 13

John Zwinck
John Zwinck

Reputation: 249153

If your application uses the 32-bit instruction set, then no, you cannot elect to use 64-bit instructions "sometimes" within the same application. There are some options, however:

  • Use SIMD instructions, such as SSE, AVX, etc. This is documented here: http://msdn.microsoft.com/en-us/library/t467de55(v=vs.90).aspx
  • Port your 32-bit code to 64-bit. Maybe it's not so hard?
  • Split your program in two: a 32-bit process with your legacy code, and a 64-bit process with new code. You could map memory between them, and one could launch the other, so it's a bit like a multithreaded program then. Whether it would perform well depends a lot on details we don't know at this time.

Upvotes: 4

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