- Monolithic
- Entire OS runs in kernel space (privileged level)
- Easier to debug issues than a microkernel
- Are often faster than a microkernel
- Hybrid
- Attempts to combine the best features of monolithic and microkernels
- Microkernel
- Only the tiny microkernel operates at a privileged level
- Everything else (drivers, protocol stacks, file systems) run in user space
- Are often smaller than monolithic kernels
- May be more inherently secure than monolithic
- Loadable modules allow a monolithic kernel to load and unload execuatable modules during runtime, avoiding the need to reboot the system.
- BSD Kernel - 1978 Monolithic
- Procto - 1982 Microkernel
- QNX RTOS
- Mach - 1985 Microkernel
- Carnegie Mellon University
- Used today by Apple, and no one else
- MINIX - 1987 Microkernel
- For educational use
- GNU Hurd - 1990 Microkernel
- Has never been fully operational
- Linux - 1991 Monolithic
- Used in Linux, ChromeOS, and Android operating systems
- L4 Microkernel - 1993 Microkernel
- For the L4 family of operating systems
- XNU - 1996 Microkernel
- Apple MacOS (originally from NeXT)
- Derived from CMU Mach
- Is a mostly-monolithic executive built on top of the Mach microkernel
- GNU Mach - 1997 Microkernel
- Continuation of CMU Mach
- 2016 first year it was stable
- DragonflyBSD Kernel - 2004 Hybrid
- Less complicated than the GNU Mach kernel
- Illumos - 2010 Monolithic
- Solaris Kernel modernized to be completely FOSS
- Zircon - 2021 Microkernel
- Google project for Fuchsia OS
- Maestro - 2023 Monolithic
- Unix-like (Linux-compatible) kernel written in Rust