Operating System Design/Kernel Architecture/Monolithic Kernel

The monolithic approach is to define a high-level virtual interface over the hardware, with a set of primitives or system calls to implement operating system services such as process management, concurrency, and memory management in several modules that run in supervisor mode.

Even if every module servicing these operations is separate from the whole, the code integration is very tight and difficult to do correctly, and, since all the modules run in the same address space, a bug in one module can bring down the whole system. However, when the implementation is complete and trustworthy, the tight internal integration of components allows the low-level features of the underlying system to be effectively utilized, making a good monolithic kernel highly efficient. Proponents of the monolithic kernel approach make the case that if code is incorrect, it does not belong in a kernel, and if it is, there is little advantage in the microkernel approach. More modern monolithic kernels such as Linux, FreeBSD and Solaris can load executable modules at runtime, allowing easy extension of the kernel's capabilities as required, while helping to keep the amount of code running in kernel-space to a minimum.