Supporting 64bit OS which currently works in 32 bit OS

I have an application which has many services and one UI module. All these are developed in VC++ 6.0. The total KLOC would be 560 KLOC.

It uses Mutltithreading,MFC and all datatypes like word,int, long.

Now we need to support 64bit OS. What would be the changes we would need to make to the product.

By support i mean both like running the application on a 64bit OS and also making use of the 64bit memory.

Edit: I am ruling out migration to VS2005 or anything higher than VC6.0 due to time constraints.

So what changes need to be done.

64bit Windows includes 32bit via WOW. Any 32bit application should just continue to work.

(It is only drivers that have to match the bitness of the OS.)

[Note to commenters: plugins—of whatever type—are not separate applications but dlls used by other applications which do need to match the host. In that case you also get the same problem where 64bit extensions are incompatible with 32bit hosts.]

As Richard says, the 32-bit version should continue to work unless you've got a driver or a shell extension or something.

However if you do need to upgrade the code you're going to have to upgrade the compiler too: I don't think MFC got good 64-bit support until VS2005 or later. I'd suggest you get the 32-bit code building in VS2010 - this will not be trivial - and then start thinking about converting it to 64-bit. You can of course leave the production 32-bit builds in VC6 but then you add maintainership burden.

You'll probably get most of the way converting by flipping the compiler to 64-bit and turning on full warnings - particularly given the size of your code it may be impractical to review it all. One thing to watch out for is storing pointers in ints, dwords, etc. which may now be too short to hold the pointer - you need DWORD_PTR etc. now - but I think the warnings do catch that.

Or if this is in many components then you might get away with only migrating a few components to 64-bit. Then, unfortunately, you've got data length issues for communication between the two versions.

You must convert to a newer compiler. Time constraints are pretty much irrelevant. The VC6 compiler simply cannot generate 64 bits code. Every pointer it generates is 32 bits, for starters. If you need to access "64 bit memory", i.e. memory above 0x00000000FFFFFFFF, then 32 bits is simply not enough.

If you're ruling out changing your IDE to one that intrinsically supports 64-bit compiling and debugging, you're making your job unnecessarily more complex. Are you sure it's not worth the hit?

Just for running on a 64bit OS, you won't need to make any changes. That's what WOW64 is for.

If, however, you want to run 64bit natively (i.e., access the 64bit memory space) you will have to compile as 64bit. That means using an IDE that supports 64bit. There is no way around this.

Most programs should have no problem converting to 64bit if they are written with decent coding standards (mainly, no assumptions about the size of a pointer, like int-pointer conversions). You'll get a lot of warnings about things like std::size_t conversions, but they will be fairly meaningless.