WAIT/FWAIT (Wait)

Opcodes

Opcode Instruction Op/En 64-Bit Mode Compat/Leg Mode Description
9B WAIT NP Valid Valid Check pending unmasked floating-point exceptions.
9B FWAIT NP Valid Valid Check pending unmasked floating-point exceptions.

Instruction Operand Encoding

Op/En Operand 1 Operand 2 Operand 3 Operand 4
NP NA NA NA NA

Description

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 ensures 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 Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1, for more information on using the WAIT/FWAIT instruction.

This instruction’s operation is the same in non-64-bit modes and 64-bit mode.

Operation

CheckForPendingUnmaskedFloatingPointExceptions;

FPU Flags Affected

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

Floating-Point Exceptions

None.

Protected Mode Exceptions

#NM If CR0.MP[bit 1] = 1 and CR0.TS[bit 3] = 1.
#UD If the LOCK prefix is used.

Real-Address Mode Exceptions

Same exceptions as in protected mode.

Virtual-8086 Mode Exceptions

Same exceptions as in protected mode.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

Same exceptions as in protected mode.