MONITORX instruction (MONITORX)

Forms

Every encoding of MONITORX that Intel XED knows, one row per XED instruction form (iform). How to read the notation.

Instruction	Encoding	Requires
`MONITORX` MONITORX	`NP 0F 01 FA` `67 0F 01 FA`	MONITORX

Operation

What the forms do, in pseudocode that our tests run against results recorded on real processors. How to read.

if ecx != 0:
    fault(GP, 0)                 # no extension is defined
a = low(rax, ASIZE)              # DS:rAX, EAX with a 67 prefix
if not canonical(a):
    fault(GP, 0)
if not readable(a):
    fault(PF)                    # the checks of a 1-byte read
# arms the monitor on the range that holds a, for MWAITX, at any
# privilege level; EDX holds hints, which no processor defines

Not run on a processor: the Zen 5 guest that ran the probe does not report MONITORX in CPUID.

Operands

What each form reads and writes, as Intel XED records it: the operands of its syntax, in order, and the implicit ones that the syntax does not name. How to read.

Operand	Kind	Action
`MONITORX`
`rAX`	implicit	read
`ECX`	implicit	read
`EDX`	implicit	read

Flags

None: XED records no flag that the forms test or change.

Intrinsics

The C intrinsics of Clang 23.1.2 that compile to MONITORX, each with the form it compiles to and the Clang options it needs. The forms come from compiling every intrinsic and decoding the result with Intel XED; all intrinsics A–Z explains the method.

Intrinsic	Compiles to
`_mm_monitorx` `void _mm_monitorx(void *__p, unsigned int __extensions, unsigned int __hints)`	`MONITORX` Legacy -mmwaitx

CPUID and processors

Each form belongs to an XED ISA set. A form can be used when the processor reports every CPUID bit of one of its ISA set's alternatives.

CPUID requirements and processors of the XED ISA sets above. Alternatives are separated by “or”; processors follow XED’s chip model, whose AMD processors end with Zen 2, and the CPU data of this site for Zen 3 to Zen 6.

ISA set	CPUID	Processors
`MONITORX` MONITORX	MONITORX `CPUID.80000001H:ECX[29]` MONITORX	Zen 2, Zen 3, Zen 4, Zen 5, Zen 6 Note: XED gives MONITORX and MWAITX to its Zen 2 chip alone; GCC's -march=bdver4 (Excavator) and -march=znver1 and later enable them.

More MONITORX instructions

MWAITX