Hacker's Delight

2-17. Multibyte Add, Subtract, Absolute Value

Some applications deal with arrays of short integers (usually bytes or halfwords), and often execution is faster if they are operated on a word at a time. For definiteness, the examples here deal with the case of four 1-byte integers packed into a word, but the techniques are easily adapted to other packings, such as a word containing a 12-bit integer and two 10-bit integers, and so on. These techniques are of greater value on 64-bit machines, because more work is done in parallel.

Addition must be done in a way that blocks the carries from one byte into another. This can be accomplished by the following two-step method:

1.	Mask out the high-order bit of each byte of each operand and add (there ...