Inside Macintosh: PowerPC Numerics / Part 1 - The PowerPC Numerics Environment

Chapter 4 - Environmental Controls

Rounding Precision

A rounding precision mode specifies a precision to which all numeric operation results are rounded. For example, if the rounding precision mode were set to single, the results of all operations would be rounded to single precision until the rounding precision mode changed.

The IEEE standard requires rounding precision modes only on systems that always deliver results to double or extended format destinations. With a rounding precision mode set to a narrower format, such systems round to the precision of that format regardless of the destination. Thus, they can use rounding precision modes to emulate other systems that deliver results in narrower formats.

Because PowerPC Numerics delivers results in any of its three data formats, it does not support dynamic rounding precision modes. Instead, a PowerPC Numerics implementation may support static narrowing of rounding precision at translation time through pragmas, compiler options, or narrower expression evaluation methods.