It is interesting to think about the 'last value' written. The definition of 'last' cannot be based on time, because the processors would take some time to communicate with each other, and hence it would be impossible to come up with the same 'time' for all processors. So, it is easier and probably more correct to think about this from the memory's point of view. We can imagine that there is only a single path to memory and only one processor can read or write from it at any point. So 'last' can be thought to be the last time some processor wrote to that memory location.
amg
I don't think that gets at the whole of it either, as the value may not make it all the way to memory. In a coherent system, the last value written to address X would somewhat circularly be defined as the value the system agrees was most recently written to. There will be some agreed upon timeline when writes occur.
It is interesting to think about the 'last value' written. The definition of 'last' cannot be based on time, because the processors would take some time to communicate with each other, and hence it would be impossible to come up with the same 'time' for all processors. So, it is easier and probably more correct to think about this from the memory's point of view. We can imagine that there is only a single path to memory and only one processor can read or write from it at any point. So 'last' can be thought to be the last time some processor wrote to that memory location.
I don't think that gets at the whole of it either, as the value may not make it all the way to memory. In a coherent system, the last value written to address X would somewhat circularly be defined as the value the system agrees was most recently written to. There will be some agreed upon timeline when writes occur.