XGETBVXSETBV

XGETBV and XSETBV are x86 assembly instructions used to read and write the extended control register XCR0, which governs how the processor state is saved and restored by the operating system. They operate in conjunction with the XSAVE family of instructions and are essential for controlling which processor state components are preserved across context switches, such as x87, SSE, AVX, and newer extensions.

XGETBV reads the value of an extended control register specified by the ECX register and places the 64-bit result in EDX:EAX (RDX:RAX in 64-bit mode). XSETBV writes a 64-bit value from EDX:EAX into the XCR register specified by ECX. In practice, XCR0 (ECX = 0) is the commonly used register, with XGETBV and XSETBV enabling or querying the OS’s saved state mask.

The instructions are only usable if the CPU supports the XSETBV/XGETBV mechanism and the operating system supports saving extended processor state. CPU support is indicated by the OSXSAVE feature flag (CPUID.1:ECX bit 27). If OSXSAVE is not present or not enabled, attempts to execute XGETBV/XSETBV may trap or behave unpredictably. The XCR0 register determines which state components are saved by XSAVE; its bitfield enables features such as the legacy x87 state, SSE state, AVX YMM state, and other extended states as defined by the processor.

In practice, software uses XGETBV to query current state-saving settings and XSETBV to enable required states (e.g., enabling AVX by setting the appropriate XCR0 bits). This mechanism is central to maintaining consistent processor state across context switches and for enabling modern vector extensions.