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Vol. 2A 3-713
INSTRUCTION SET REFERENCE, A-M
MWAIT—Monitor Wait
EDX = 0 (* Hints *)
IF ( !trigger_store_happened) {
MONITOR EAX, ECX, EDX
IF ( !trigger_store_happened ) {
MWAIT EAX, ECX
}
}
The above code sequence makes sure that a triggering store does not happen
between the first check of the trigger and the execution of the monitor instruction.
Without the second check that triggering store would go un-noticed. Typical usage of
MONITOR and MWAIT would have the above code sequence within a loop.
Numeric Exceptions
None
Protected Mode Exceptions
#GP(0) If a memory operand effective address is outside the CS, DS,
ES, FS, or GS segment limit.
If the DS, ES, FS, or GS register is used to access memory and it
contains a NULL segment selector.
If ECX = 0.
#SS(0) If a memory operand effective address is outside the SS
segment limit.
#PF(fault-code) For a page fault.
#UD If CPUID.01H:ECX.MONITOR[bit 3] = 0.
If current privilege level is not 0.
Real Address Mode Exceptions
#GP If any part of the operand in the CS, DS, ES, FS, or GS segment
lies outside of the effective address space from 0 to FFFFH.
If ECX
≠ 0.
#SS If any part of the operand
in the SS segment lies outside of the
effective address space from 0 to FFFFH.
#UD If CPUID.01H:ECX.MONITOR[bit 3] = 0.
Virtual 8086 Mode Exceptions
#UD The MONITOR instruction is not recognized in virtual-8086 mode
(even if CPUID.01H:ECX.MONITOR[bit 3] = 1).