Compare microkernel and monolithic kernel architectures.

Monolithic kernel (Linux, BSD): all OS services in kernel space - fast (no context switches between services), but large attack surface, any bug can crash system, harder to maintain. Microkernel (QNX, MINIX, seL4): minimal kernel with IPC, scheduling; services run in user space - more stable/secure (service crash doesn't crash system), easier to extend, but IPC overhead. Hybrid kernels (Windows NT, macOS XNU) combine approaches. Modern microkernels minimize IPC cost; seL4 is formally verified.