Question

What happens if three CPUs in a multiprocessor attempt to access exactly the same word of memory at exactly the same instant?

Answer 1

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Solution:-

It depends on how the CPUs are connected to the memory, one of which passes first, for example,
catching the bus first. It completes its memory function, then another one will happen. It is unpredictable which
one will go first, but only if the system is designed for simultaneous consistency, it should not matter.

If the memory word in question was already uploaded to all the three CPUs caches and all the three CPUs want to access the same memory word, then it can be accessed from two caches simultaneously.
If the word is not in the cache, then the memory bank containing that word is able to handle one access at a time. The hardware will have an option to select one access to continue, while the other waits. In fact, in most multiprocessor programs this solution only occurs when all the three CPUs try to access the memory bus at the same time.

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