for the compiler suite, the 32-bit powerpcs are compatible (ignoring the original 601 and not including Altivec)
kernels see bigger differences, where there are three or more possibilities for memory management, and two different schemes for trap handling, and even different instructions for processor control, but generally the differences are fairly small. the surrounding IO structure is usually completely different. 64-bit operation is a fairly small extension to the 32-bit set, as for several other RISCS, compared (say) to x86
