Many modern CPUs today utilize caching, in which a small block from main memory is copied and brought $$closer$$ to the CPU so it can be accessed much faster by working directly on the $$cached copy$.$ Now, cache works great for data, but not so much for devices whose SFRs $($special function registers$)$ are memory$-$mapped. Please thoughtfully and comprehensively explain why this is the case. Please also provide a quick example demonstrate how caching SFRs can be problematic.