... ffect, there is a Linux kernel in the FreeBSD kernel; the various underlying functions that implement all of the services provided by the kernel are identical to both the FreeBSD system call table entries, and the Linux system call table entries: file system operations, virtual memory operations, signal delivery, System V IPC, etc... The only difference is that FreeBSD binaries get the FreeBSD glue functions, and Linux binaries get the Linux glue functions (most older OS's only had their own glue functions: addresses of functions in a static global sysent[] structure array, instead of addresses of functions dereferenced off a dynamically initialized pointer in the proc structure of the process making the call).
Which one is the native FreeBSD ABI? It does not matter. Basically the only difference is that (currently; this could easily be changed in a future release, and probably will be after this) the FreeBSD glue functions are statically linked into the kernel, and the Linux glue functions can be statically linked, or they can be accessed via a kernel module.
Yeah, but is this really emulation? No. It is an ABI implementation, not an emulation. There is no emulator (or simulator, to cut off the next question) involved.
So why is it sometimes called “Linux emulation”? To make it hard to sell FreeBSD! Really, it is because the historical implementation was done at a time when there was really no word other than that to describe what was going on; saying that FreeBSD ran Linux binaries was not true, if you did not compile the code in or load a module, and there needed to be a word to describe what was being loaded--hence “the Linux emulator”.
