9.13 When to use flush-to-zero mode in VFP

You can change between flush-to-zero mode and normal mode, depending on the requirements of different parts of your code.

You must select flush-to-zero mode if all the following are true:

  • IEEE 754 compliance is not a requirement for your system.
  • The algorithms you are using sometimes generate denormalized numbers.
  • Your system uses support code to handle denormalized numbers.
  • The algorithms you are using do not depend for their accuracy on the preservation of denormalized numbers.
  • The algorithms you are using do not generate frequent exceptions as a result of replacing denormalized numbers with 0.

You select flush-to-zero mode by setting the FZ bit in the FPSCR to 1. You do this using the VMRS and VMSR instructions.

Numbers already in registers are not affected by changing mode.

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