CPUIDOSXSAVE

CPUIDOSXSAVE, or the OSXSAVE feature, is a processor capability flag exposed by the CPUID instruction. It indicates that the operating system supports saving and restoring the processor’s extended state using the XSAVE family of instructions, and that the system can safely preserve advanced registers such as AVX and AVX-512 across context switches. The flag is reported by CPUID in the standard feature information leaf (EAX=1), specifically in the ECX register; when the bit is set, the processor supports the XSAVE mechanism and the operating system is expected to use it for context switching.

Mechanism and scope. When OSXSAVE is available, the operating system can use the XSAVE and related instructions to save and restore not only x87 and MMX/SSE state but also newer state components such as AVX, AVX-512, and other extended states. The OS determines which components to save by querying the processor’s extended state information and configuring the control register XCR0 accordingly. The XSTATE mechanism requires cooperation between the CPU and the OS: the processor must be able to save the enabled state components, and the OS must ensure those components are preserved during thread or process switches.

Relation to CPUID leaf 0xD. A separate CPUID leaf (0xD) provides details about the size of the save area and which specific state components are supported. This information, together with XCR0, lets the OS decide which states to enable and save. If OSXSAVE is present but the OS does not enable or preserve extended states, performance and correctness penalties can arise when using advanced instruction sets.

Significance. For software and system software, CPUID OSXSAVE signals compatibility with modern saving/restoring mechanisms for processor states. It reflects a CPU capability that, when paired with operating system support, enables efficient use of advanced vector instructions during multitasking.