Specifications

ARM Vector Floating Point Coprocessor (VFP9)

18.4.4 RunFast Mode

RunFast mode is the combination of the following conditions:

• the VFP9-S coprocessor is in Flush-to-Zero mode

• the VFP9-S coprocessor is in Default NaN mode

• all exception enable bits are cleared.

In RunFast mode the VFP9-S coprocessor:

• processes subnormal input operands as positive zeros

• processes results that are tiny before rounding, that is, between the positive and

negative minimum normal values for the destination precision, as positive zeros

• processes input NaNs as default NaNs

• returns the default result specified by the IEEE 754 standard for overflow,

division-by-zero, invalid, or inexact operations fully in hardware and without

additional latency

• processes all operations in hardware without trapping to support code.

RunFast mode enables the programmer to write code for the VFP9-S coprocessor that

runs in a determinable time without support code assistance, regardless of the

characteristics of the input data. In RunFast mode, no user exception traps are available.

However, the exception flags in the FPSCR register are compliant with the IEEE 754

standard for Inexact, Overflow, Invalid Operation, and Division-by-Zero exceptions.

The underflow flag is modified for Flush-to-Zero mode. Each of these flags is set by an

exceptional condition and can be cleared only by a write to the FPSCR register.