# What is the Kernel?

## General

The kernel has three main jobs: to manage resources, to interface with
hardware, and to provide an API that offers a useful level of abstraction
to user space programs, as summarized in the following diagram:
![OS Diagramm](../img/os_diagramm.jpg)

## System Spaces

Applications running in *user space* run at a low CPU privilege level.
They can do very little other than make library calls.
The primary interface between the *user space* and the *kernel space* is
the **C library**, which translates user-level functions, such as those
defined by POSIX, into kernel system calls. The system call interface uses
an architecture-specific method, such as a trap or a software interrupt,
to switch the CPU from **low-privilege user mode** to
**high-privilege kernel mode,** which allows access to all memory addresses
and CPU registers.

*source:* Mastering Embedded Linux Programming - Third Edition By Frank Vasquez, Chris Simmonds