Operating system states

The operating system can be in one of four states:

executing in user mode

The CPU is executing the text (machine code) of a process that accesses its own data space in memory.

executing in system mode

If a process makes a system call in order to perform a privileged task requiring the services of the kernel (such as accessing a disk), then the operating system places the CPU in system mode (also known as kernel mode).

idle waiting for I/O

Processes are sleeping while waiting for the completion of I/O to disk or other block devices.

idle

No processes are ready-to-run on the CPU or are sleeping waiting for block I/O. Processes waiting for keyboard input or network I/O are counted as idle.