O

The O constant specifies the subdirectory into which build objects will be put.

It has the value

obj$(BUILD_ALT_DIR)\Cpu 

Cpu represents the processor. For an x86 processor, this is i386. For an Itanium-based processor, this is ia64. For an x64-based processor, this is amd64.

The benefit of using $(O) is that any files that you have built and put in the objects subdirectory will be deleted on the next clean build. This guarantees that no collisions will occur between two builds running on the same computer at the same time. They will never override each other's files if you follow the convention that everything that you build goes in $(O).

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Build date: 5/3/2011