In Performance Mode, the hardware and operating system are relieved of the requirements to precisely trap floating-point exceptions and to compute using denormalized operands. This mode is defined in such a way that it is adequate for a majority of application programs in use today, yet it can also be used in conjunction with compiler and library support to fully implement the Standard in the future.
Performance Mode improves the floating-point execution speed of processors. On the R4000, Performance Mode enables flushing operands to zero, thus avoiding the software emulation overhead of denormalized computations. On the R8000, Performance Mode enables floating-point instructions to execute out-of-order with respect to integer instructions, improving performance by a factor of two or more.
Performance Mode is the standard execution environment on R8000 based Power Challenge systems.
In Precise Exception Mode the responsibility for compliance lies entirely with the hardware and operating system software; no compiler support is assumed. Since there is no information about the application, the hardware must assume the most restrictive features of the Standard applies at all times. The result is lost performance opportunities on applications that utilize only a subset of the features called for by the Standard.