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EVERYTHING CHANGES

Using the Const Modifier
by George Martin

Start ý Define and Qualify ý In Other Words ý Sources and PDF

IN OTHER WORDS

While weýre here, what other keywords might be of the same sort of use? Volatile comes to mind. The volatile modifier warns the compiler that there may be some special hardware behind the variable causing it to unexpectedly change (see Listing 5).

In Listing 5, two variables, a and b, are declared. a is set to 7 and then tested. A good compiler would know that a is in fact equal to 7, so no code would be generated for the test and the print() statement would always be executed. The variable b is declared as volatile, so even though it was just set to 9, b would be loaded and tested before the print() statement is reached. If b was the serial port status register and a was the serial port control register, in a memory-mapped scheme they could both be at the same addressýone a read variable and one a write variable. There is hardware behind that common address that could change the values, so always reload it.

Although this example may be obvious, perhaps you can go up another level of optimization on the compiler now and it will produce stable code!

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