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.
The combination of time spent waiting for I/O
and time spent idle makes up the total time that the operating
system spends idle.