Page 242 - Programming Microcontrollers in C

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port. Does the assembly language generated by the compiler appear
to be efficient? If not, what can be done to improve the code?
2. Write and compile a program that uses Output Compare 3 to gener­
ate a periodic interrupt. This interrupt should occur each millisecond.

3. With the time tick generated by the results of Example 2 above,
create a clock that will keep track of the time of day accurate to the
nearest one-hundredth of a second.

The Compiler Optimizer and Volatile

The compiler optimizer that is a part of the Cosmic C compiler does
an excellent job of reducing code size. One of the steps involved is to
remove code required to change values that are not changed by the pro­
gram. Recall that the volatile qualifier identifies varibles that should not
be optimized. The following code sequence will show this problem:

volatile char able;
char baker

test(void)
{
char dog;

dog = able;
dog = able;
able = 0;
able = 0;
dog = baker;
dog = baker;
baker = 0;
baker = 0;
}
Note that this function makes multiple assignments of dog, able,
and baker. The assignments involving able show how the compiler
responds to a volatile variable, and baker to a nonvolatile variable.
The following listing is the compilation of the above program:
1 ; Compilateur C pour MC68HC11 (COSMIC-France)
32 .include”macro.h11"