For 3d engines the most important part is system independent, unless you do bad coding that requires the use of lots of dlls. If you use only plain C is also machine independent, if you use C and Assembler for improving speed, the assembler code dependents on the CPU (Intel class for PC, and Motorola for Mac). The parts of the code that dependends of the operating system are: - One part (very small) that deals with requesting and allocating memory, and file I/O can be done with standart C libraries. - For the video you only need that the operating system returns you the video memory pointer. - The last part that depend completely on the operating system is which deals with the user interface (GUI). This part is a very easy one (compared to the 3d processing). So with good software programming is very little overhead in making different versions for different platforms.

Stupidity also evolves!