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what are shadow registers and what is their purpose? it was asked earlier but the replies were not so clear.... Is it something like a masking register?
At least in my experience shadow registers are typically used where you need need to shadow (copy) a value that you have instant access to, which is otherwise stored in say a RAM that has to be accessed with a read enable, address, etc.
If it makes it easier it's kind of like a FF based cache of some other memory location.
1. for preparing data prior to using it when the bits of the data may not all be updated simultaneously. Typically used when outputting data only when all the bits are ready. For example, when bits in an output port are set by different routines in code and therefore at different times, writing directly to the port would result in it holding each intermediate step as the intended data was constructed and that could cause problems. Building the data in a shadow register until it is complete then transferring it to the port ensures all the bits change simultaneously.
2. during interrupt servicing where registers may be used for other purposes but have to be restored to their original values as the ISR finishes. The 'shadow' is copies of the register contents at the start of the ISR and they are copied back to their original locations before returning to the main program flow.
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