All operating system services are provided by the kernel.
The kernel is a bunch of library routines linked to user programs.
The kernel routines are ``encapsulated'' in files.
The user processes and the kernel share a common physical memory.
The set of procedures that can be executed is determined when the OS is started.
An advantage of this organization is that it is simple. Moreover, information to be communicated among user processes and between a user process and the kernel does not have to be copied.
Some disadvantages are:
The kernel routines are encapsulated in files instead of module-like structures.
Address spaces are not protected.
The total address space available is limited to the size of physical memory.
Procedures cannot be dynamically loaded.