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Tip of the Day
Language: Algorithms
Expertise: Beginner
Dec 4, 1998

Extracting bits.

Question:
How do I extract the value from a certain number of bits of an unsigned type?


ex: date is stored as unsigned date = 0x26cf;
    bits         meaning
    0-4          Day
    5-8          Month
    9-15         Year (1980-2099)

How do I get the Day, Month and Year?

Answer:
There are two steps to extracting bit values this way. First, you must shift those bits down to the zero bit so that the value starts with 0. Second, you must clear any bits you are not interested in so you are testing the value of only the intended bits.

Use C's shift operator (>>), and a bitwise AND (&) to clear unwanted bits, to extract the values you want.

nDay = (nDate & 0x1F);
nMonth = ((nDate >> 5) & 0xF);
nYear = 1980 + ((nDate >> 9) & 0x7F);
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