x86 Instruction Set Reference



Opcode Mnemonic Description
9B WAIT Check pending unmasked floating-point exceptions.
9B FWAIT Check pending unmasked floating-point exceptions.

Causes the processor to check for and handle pending, unmasked, floating-point exceptions before proceeding. (FWAIT is an alternate mnemonic for WAIT.) This instruction is useful for synchronizing exceptions in critical sections of code. Coding a WAIT instruction after a floating-point instruction insures that any unmasked floating-point exceptions the instruction may raise are handled before the processor can modify the instruction's results. See the section titled "Floating-Point Exception Synchronization" in Chapter 8 of the IA-32 Intel Architecture Software Developer's Manual, Volume 1, for more information on using the WAIT/FWAIT instruction.

FPU flags affected

The C0, C1, C2, and C3 flags are undefined.