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Preprocessor Confusion?

Bozo the Clown

Why does Keil's C51 preprocessor work fine with:

#define MULTPLR (unsigned char)((float)9.114584e-5 * (float)18432000 / (float)12)

...but consistently give the wrong result with:

#define MULTPLR (unsigned char)((float)9.114584e-5 * (float)18432000 / (float)6)

???

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  • 0 in reply to Jack Sprat

    Yes, the compiler has some options.

    In this case, the compiler can only generate code with float precision, since the C RTL only supports single precision. But just because the compiler can't generate code that uses double precision doesn't forbids it from supporting additional precision when computing compile-time constants.

    Typecasting everything to float can affect the precision the expression is computed with, compared to just adding a decimal point at the end of the numbers.

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  • 0 in reply to Jack Sprat

    Yes, the compiler has some options.

    In this case, the compiler can only generate code with float precision, since the C RTL only supports single precision. But just because the compiler can't generate code that uses double precision doesn't forbids it from supporting additional precision when computing compile-time constants.

    Typecasting everything to float can affect the precision the expression is computed with, compared to just adding a decimal point at the end of the numbers.

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