Difference between "My Mac 32-bit" and "My Mac 64-bit" in Xcode 4?

Question

I have a Cocoa project that I recently imported into Xcode 4. I notice two Schemes for each of my specific build targets, one for 32-bit, and one for 64-bit. My Project settings are set to "Standard (32/64-bit Intel)".

When I select the "My Mac 32-bit" Scheme, does this compile it for 32-bit machines, and selecting "My Mac 64-bit", then compiles it for 64-bit machines? If I select the "My Mac 64-bit" Scheme, will it no longer run on 32-bit machines at all?

Answer 1

The Scheme setting changes what happens in Xcode when you click Run, Build, etc. The architecture settings are changed inside the project settings, visible in your screen shot. Only there would you be able to chose "32-bit only" or "64-bit only". And it is likely that if you publish a binary with 64-bit only it wouldn't run on 32-bit machines. You would have to build for both platforms and include both binaries in a single Universal binary to get backwards compatibility with 32-bit-only Macs.