Instantiation Components Compile error

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If you put a sequential (process) block around your code, it still can't work this way. But that's another story.

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Of course you are free to ignore any previous suggestion, but you should at least try to compile the code.

That what Quartus thinks about it:

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Error (10818): Can't infer register for "A" at compt.vhd(17) because it does not hold its value outside the clock edge

Error (10822): HDL error at compt.vhd(17): couldn't implement registers for assignments on this clock edge

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