Block on read/write is the state where process is in block state.
Waiting state and block states are differnet with respect to the CPU allocation.
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A waiting process is waiting for the context switch, where it will get the CPU after a certain time, but a blocked process will not get the CPU untill and unless an particular event occures.
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Eventes can be I/O commands..
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In systems that support virtual memory, a waiting process may be swapped out, that is removed from main memory and placed in virtual memory by the mid-term scheduler. From here the process may be swapped back into the waiting state..
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same case is applied on blocked process also.

thanks