Hardware Changes

Hardware Changes

The R8000 microprocessor includes four floating point multiply/add /
subtract instructions which allow two floating point computations to be performed with one instruction. The intermediate result is calculated to infinite precision and is not rounded prior to the addition or subtraction. The result is then rounded according to the rounding mode specified by the instruction. This can yield slightly different calculations than a multiply instruction (which is rounded) and an add instruction (which is rounded again).

The R8000 microprocessor architecture also defines two execution environments which can affect your application if it generates floating point exceptions such as underflow. Performance Mode enhances the execution speed of floating point applications, by rounding denormalized numbers to zero and allowing the hardware to trap exceptions imprecisely. Precise Exception Mode, on the other hand, is fully compatible to the existing MIPS floating point architecture.

It should be emphasized that running in Performance Mode does not affect those applications which don't cause floating point exceptions.

A program, fpmode, allows you to run your application in either Performance (imprecise) or Precise Mode. Its usage is as follows:

%fpmode precise commandargs

%fpmode imprecise commandargs

A full discussion of the Extended MIPS Floating Point Architecture is provided as a reference.