wbnoinvd

Write Back and Do Not Invalidate Cache

WBNOINVD

Writes back modified lines but keeps them valid in cache.

Details

Writes back all modified cache lines to memory but keeps them valid in the L3 cache, improving performance compared to WBINVD which invalidates. This instruction is non-privileged and does not modify any flags. It provides write-back semantics without the full cache flush, useful for cache coherency operations while maintaining data locality.

Pseudocode Operation

// Writes back modified lines but keeps them valid in cache

Example

WBNOINVD

Encoding

Binary Layout
F3
+0
0F
+1
09
+2
 
Format Legacy
Opcode F3 0F 09
Extension WBNOINVD

Operands

Reference (Intel® SDM)

Instruction Forms

Opcode Instruction Op/En 64/32-bit Mode CPUID Description
F3 0F 09 WBNOINVD ZO V/V WBNOINVD Write back and do not flush internal caches; initiate writing-back without flushing of external caches.

Description

The WBNOINVD instruction writes back all modified cache lines in the processor’s internal cache to main memory but does not invalidate (flush) the internal caches. After executing this instruction, the processor does not wait for the external caches to complete their write-back operation before proceeding with instruction execution. It is the responsibility of hardware to respond to the cache write-back signal. The amount of time or cycles for WBNOINVD to complete will vary due to size and other factors of different cache hierarchies. As a consequence, the use of the WBNOINVD instruction can have an impact on logical processor interrupt/event response time. The WBNOINVD instruction is a privileged instruction. When the processor is running in protected mode, the CPL of a program or procedure must be 0 to execute this instruction. This instruction is also a serializing instruction (see “Serializing Instructions” in Chapter 9 of the Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 3A). This instruction’s operation is the same in non-64-bit modes and 64-bit mode.

Operation

WriteBack(InternalCaches);
Continue; (* Continue execution *)

Intel C/C++ Compiler Intrinsic Equivalent

WBNOINVD void _wbnoinvd(void);

Flags Affected

None.

Exceptions

Protected Mode Exceptions

#GP(0) If the current privilege level is not 0. #UD If the LOCK prefix is used.

Real-Address Mode Exceptions

#UD If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

#GP(0) WBNOINVD cannot be executed at the virtual-8086 mode.

Compatibility Mode Exceptions

Same exceptions as in protected mode. WBNOINVD—Write Back and Do Not Invalidate Cache Vol. 2D 6-4

64-Bit Mode Exceptions

Same exceptions as in protected mode. WBNOINVD—Write Back and Do Not Invalidate Cache Vol. 2D 6-5