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Tuning CPU resources

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

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SCO OpenServer Release 5.0.7 -- 11 February 2003