a friend sent this to me

In seamlessly compiling System 6/7 era software in a modern open source
compiler to yield "real" OS X apps. Pretty amazing. The guy
retroactively rebuilt the entire Mac ROM/library calls, from the sounds
of it.

http://blog.steventroughtonsmith.com/post/109040361205/mpw-carbon-and-building-classic-mac-os-apps-in-os

His side by side comparison of the same code compiling on a classic
original Mac and on OS X. Pretty mind blowing.