MrC is a recent compiler for the PowerPC created by either Motorola or Apple-- which one did it is ambiguous, and it was probably both working together. It will be the final compiler used in Apple's MPW development environment once MPW is left behind in the move to Mac OS X (( apple recently abandoned MrC in favor of gcc, despite claims by some that MrC was a superior compiler, and will apparently be optimising gcc using code from MrC )) and as of this writing is pretty much the compiler you are expected to use if you want support for Alti-vec under Mac OS 9. When compiling C++ this compiler is known as MrCpp.