Apple’s Darwin according to GCC

From gcc’s target definitions for Darwin (Mac OS X) systems

/* The definitions in this file are common to all processor types
running Darwin, which is the kernel for Mac OS X. Darwin is
basically a BSD user layer laid over a Mach kernel, then evolved
for many years (at NeXT) in parallel with other Unix systems. So
while the runtime is a somewhat idiosyncratic Mach-based thing,
other definitions look like they would for a BSD variant. */

/* Although NeXT ran on many different architectures, as of Jan 2001
the only supported Darwin targets are PowerPC and x86. */

/* One of Darwin's NeXT legacies is the Mach-O format, which is partly
like a.out and partly like COFF, with additional features like
multi-architecture binary support. */