 Kernel (operating system)

The operating-system kernel is the core of an operating system, controlling access to resources and supplying mechanisms used by operating-system processes. Processor privilege levels let it enforce boundaries that ordinary programs cannot simply overwrite. The operating-system kernel connects process scheduling, virtual memory and file system operations; a system call requests a service across that boundary. A monolithic kernel and a microkernel divide services differently, so the location of every file system service is a design choice.

For the toy file system, opening a resolves its directory entry to inode 42, establishes an open file description, and returns file descriptor 3. Later reads use that file descriptor to reach the existing open file description, without resolving the spelling a again. This explains how unlinking an open file preserves access: the open reference survives the directory entry. The per-operating-system process table interpreting 3 is maintained by the operating-system kernel; 3 itself is the small integer passed by the program, not inode 42. ^[ref]