Operating system vendors face this problem once or twice a decade: They need to migrate their user base from their old operating system to their very different new one, or they need to switch from one CPU architecture to another one, and they want to enable users to run old applications unmodified, and help developers port their applications to the new OS. Let us look at how this has been done in the last 3 decades, looking at DOS/Windows, Macintosh, Amiga and Palm.
The heritage of different operating systems has been discussed many times. Mac OS X includes code from Mach and BSD, AmigaOS is based on TRIPOS, MS-DOS is a CP/M-86 clone and Windows NT is modeled after VMS. But what machines and operating systems were used for cross-compilation and bringup of these systems? In order to find this out about Mac OS X, I talked to a few people working at NEXT and Apple, and people that worked on Mach and BSD.
Here are all three volumes of the original 1985 edition of Inside Macintosh as a searchable PDF:
Everyone and their grandmother builds Linux kernels. Many people build BSD, and some brave men even compile the OS X kernel every now and then. Why not compile your own Solaris kernel for a change?
Imagine you’re an i386 user mode application on a modern operating system, and you want to make a syscall, for example to request some memory or create a new thread. But syscalls can be made in various ways on the i386 family of CPUs (int, call gates, sysenter, syscall), and CPUs tend to support only a subset of them. But hardcoding “int” into the kernel is a waste of resources on modern CPUs, because sysenter is a lot faster.