Using a 64 bit driver in a 32 bit program. Windows

Question

This is only a half-way programming question. First of all I have a PCI-Express card and 32/64 bit drivers. The target operating system has to be a Windows 64 bit system. I read that under Vista64 all drivers have to be certified 64 bit drivers. Is this a general restriction under 64 bit operating systems and does this also apply to "XP 64" or any Linux system?

So for simplicity let's say I use a 64 bit driver for my PCIe card under Vista64 and have a bunch of 64 bit DLLs to use the cards functionality. On the other side there's a large, legacy 32 bit exe program which needs to use the PCIe device. Converting the program to 64 bit would be a really huge effort.

So what can be done to bring that 32 bit program and the 64 bit driver together? I read that mixing 32/64 bit binaries and DLLs is not possible at all but this is hard to believe for me. I'm sure you can print out a document under Vista64 from within a 32 bit app and Windows will somehow wrap this around to a 64 bit printer driver.

Answer 1

I think the whole point of a driver is to abstract away the actually workings of the hardware and present a common interface to the software. In this case, the PCIe driver needs to be 64-bit so that it can act as a go-between for Windows and the hardware, but I would think that a 32-bit application could then access the device without any troubles at all.

What's meant by that incompatibility you read about is that 32 and 64-bit assemblies can't be part of the same application - an application has to target either one or the other, though 32-bit application will generally run fine on Windows x64 using WoW64, which just acts as a translator.

Are you currently experiencing problems, or are you just asking hypothetically?

Answer 2

Does your 32-bit application directly call the driver? (I'm guessing a simulator for the driver!)

The only way to communicate between 32-bit and 64-bit dlls is to write a COM server that manages the communication (read: wrap EITHER the applications calls OR the 64-bit driver responses) in between.

One thing that came back to bite me: When I first wrote this COM server (yes, I too had to bear many sleepless nights before I came to know of this trick) I only built the 32-bit version of the (auto-generated) proxy/stub dll. Another bout of sleepless nights ensued before I came to know of the solution: Build the proxy/stub dll for both 32-bit and 64-bit. The 32-bit side deals with the 32-bit side (in your case the application) and the 64-bit with the 64-bit side (the driver). COM manages how the differnt versions of the proxy/stub talk to each other. And oh, do get the server registered on your system. Easy, right?

Answer 3

64-bit certification is only required under Vista; there is no certifying authority for non-Windows platforms, and I don't believe that XP or Windows Server checks for certification (not sure though, and it may depend on which service pack you're on).

If you're using the driver via the Windows API, then there shouldn't be any problem; Windows will do the 32<->64-bit translations in the kernel. If you're trying to load the driver inside your own process, that probably won't be possible. As Dirk says you'll have to run it inside its own process and communicate through a COM server. I'm not sure what hoops you'll have to jump through if you have to run your driver in a higher-privilege execution level and want to make calls to it from user mode.

Hopefully your 64-bit DLLs offer a 32-bit API, or Windows offers a standard driver interface (if it's a common I/O device like a display or network card).