CPU state in top

[Meni Shapiro <meni.shapiro@gmail.com>]

hi guys,
ok, I'm a little confused about 'top' results...

I get a lot of CPU wait %:
eg: Cpu(s): 71.3% us,  6.7% sy,  0.0% ni, 10.0% id, 11.7% wa,  0.0% hi,  0.3% si

can anyone explain simply why the CPU goes to wait state??? and not back to idle state?
Lets examine the example above...
The CPU is 71.3% used by user process and 6.7 by system. why then it is 11.7% in wait state and not idle? and if CPU resource is still available (71.3+6.7=78% means 22% free....) why it doesn't use it???
Out of 22% free (as i see it) , it could handle those 11.7% in wait state, couldn't it??

and what are those hi & si states??

Thanks,

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Meni Szapiro