The MMX technology supports a new arithmetic capability known as saturating arithmetic. Saturation is best defined by contrasting it with wraparound mode.
In wraparound mode, results that overflow or underflow are truncated and only the lower (least significant) bits of the result are returned. That is, the carry is ignored.
In saturation mode, results of an operation that overflow or underflow are clipped (saturated) to a data-range limit for the data type. The result of an operation that exceeds the range of a data-type saturates to the maximum value of the range. A result that is less than the range of a data type saturates to the minimum value of the range. This is useful in many cases, such as color calculations.
For example, when the result exceeds the data range limit for signed bytes, it is saturated to 0x7F (0xFF for unsigned bytes). If a value is less than the data range limit, it is saturated to 0x80 for signed bytes (0x00 for unsigned bytes).
Saturation provides a useful feature of avoiding wraparound artifacts. In the example of color calculations, saturation causes a color to remain pure black or pure white without allowing for an inversion.
MMX instructions do not indicate overflow or underflow occurrence by generating exceptions or setting flags.