XSAVEXRSTOR

XSAVEXRSTOR is a low‑level system instruction used in certain processor architectures to save and restore the extended state of a CPU. The operation typically handles the contents of extended registers that go beyond the traditional general‑purpose set, such as SIMD, floating‑point, and control registers introduced by modern instruction set extensions. By bundling the saving and restoring of these registers into a single atomic action, XSAVEXRSTOR helps maintain consistency during context switches, virtualization, and power‑state transitions, reducing the overhead associated with managing each register group individually.

The instruction is commonly associated with the Intel x86 family, where it extends the functionality of earlier state‑management instructions like XSAVE, XRSTOR, and their version‑specific variants (XSAVEOPT, XRSTOROPT). The “X” prefix indicates the inclusion of additional feature bits defined by the processor, allowing software to specify which components of the extended state should be processed. The operation uses a memory operand that points to a state‑save area organized according to the processor’s layout, which includes a header describing the supported features and a series of contiguous blocks for each register group.

Support for XSAVEXRSTOR depends on the CPU’s feature flags, and software must check these flags (e.g., via the CPUID instruction) before invoking the operation to avoid illegal instruction traps. Operating systems and hypervisors often employ the instruction in their scheduler and virtualization layers to efficiently manage multiple execution contexts. Incorrect use or misaligned state‑save areas can lead to data corruption or performance degradation, so proper alignment and size calculations are essential for reliable operation.