There is only 2 reasons to run 32 bit cad on a 64 bit machine and those are 1) you have an old copy of cad and it wasn't available in 64 bit when you got it. 2) You have a plugin or shell program for cad that is only works with 32 bit acad.
There are several major reasons to run 64 bit cad. 1) 32 bit on a 64 bit machine is an unsupported configuration 2) there exists the large possibility that 32 bit cad could expect certain behavior in your system and or drivers that doesn't apply when running on a 64 bit machine. 3) No support or solutions to problems will be available. 4) The larger memory space allows you to manipulate much larger files. 5) OS may occupy parts of the first 32 bit memory space that autocad expects to be free and that could limit your available memory to less than might be available if you ran 32 bit on a 32 bit OS or result in instabilities.
The fact is that Autodesk didn't set up one program that would recognize the memory space and adjust its behavior. They set up 2 discrete programs that each work within the specific hardware/os configurations that they were designed to use. but set up 1 installer that would choose which one to install.
Some additional input that wasn’t quite made apparent:
It isn't entirely true that VBA is not compatible with 64-bit AutoCAD. There are some project files that will compile and others that will not on 64-bit VBA, it just depends on the properties and methods of AutoCAD objects that you are invoking. There is a list of properties and methods that, if used, will cause a compile error on 64-bit AutoCAD. And fortunately, Autodesk has added replacement properties and methods named with the number "32" on the end (e.g. ObjectID property becomes ObjectID32) in order to compile the project in 64-bit AutoCAD. I believe most, if not all, of these can be found by searching "64-bit" under the AutoCAD ActiveX and VBA Reference. The bottom line is that you can make it work by doing a little find and replace.
My problem is that my office has just begun a transition to 64-bit machines, making a large library of VBA programs unusable for these new machines. I can’t rewrite the programs yet, since they then would not work on the 32-bit machines. And to make a duplicate VBA project file for 64-bit would be a major pain, since I constantly update and make changes to the programs....
You can make VBA work on 64bit, that's right - however, aside from the extract work needed to make it work, on 64bit AutoCAD VBA runs in a separate 32bit exe, so it runs out of process making it 1000 times slower!! Nice :-S
Also, some points made earlier about 64bit against 32bit are generally right - you definitely have access to much more RAM, except the points about a 32bit app running differently on 64bit - take Mircosoft's own software, maybe Word or Excel - they are both 32bit applications and they run fine on 64bit Windows?
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