The Art of Assembly Language, 2nd Edition

2.11 Bit Fields and Packed Data

Although the 80x86 operates most efficiently on byte, word, and dword data types, occasionally you'll need to work with a data type that uses some number of bits other than 8, 16, or 32. For example, consider a date of the form 04/02/01. It takes three numeric values to represent this date: month, day, and year values. Months, of course, take on the values 1..12. It will require at least 4 bits (maximum of 16 different values) to represent the month. Days range between 1 and 31. So it will take 5 bits (maximum of 32 different values) to represent the day entry. The year value, assuming that we're working with values in the range 0..99, requires 7 bits (that can be used to represent up to 128 different values). 4 ...