Microkernels

• Who here uses an apple device (iOS, watchOS, TvOS, Mac OS X)

• Or an Intel x86 computer?

• If you said yes to either, you’re using a microkernel.

• XNU/Darwin is a hybrid kernel + BSD-like userspace for Apple products.

• Minix is used on Intel chips.

What’s a Monolithic Kernel?

• A monolithic kernel (Linux, the BSDs, old Windows, classic Mac OS) is one program (pid 0) that manages the underlying hardware for you.
• Interfacing is done with system calls; you issue a system call and then the OS will do what’s required for you.
• Thus, any user space program (e.g. ls, cat, firefox) on Linux is coupled to Linux via its system call interface.
• Linux can thus never “remove” a system call, since userspace needs backwards compatibility. Thus we have dup, dup2, dup3, preadv, preadv2, fstat, fstat64, etc.

What’s a Microkernel?

• A Microkernel manages as little code as possible. For Minix, the kernel is ~6000 loc.
• As much as possible is put into user space, and thus, can be treated as processes.
• In a Monolithic Kernel, drivers can be externally loaded (with root), but are linked to the kernel’s address space – any malicious or improperly written drivers can easily crash your computer. For a Microkernel, devices are run in user space, and thus crash like a normal process does.
• Message passing is used between user space components. Messages are sent and then actions are taken on them, rather than interfacing with the system call library.

Microkernels