XRSTOR — Restore Processor Extended States

Opcode / Instruction	Op/En	64/32 bit Mode Support	CPUID Feature Flag	Description
NP 0F AE /5 XRSTOR mem	M	V/V	XSAVE	Restore state components specified by EDX:EAX from mem.
NP REX.W + 0F AE /5 XRSTOR64 mem	M	V/N.E.	XSAVE	Restore state components specified by EDX:EAX from mem.

Instruction Operand Encoding ¶

Op/En	Operand 1	Operand 2	Operand 3	Operand 4
M	ModRM:r/m (r)	NA	NA	NA

Description ¶

Performs a full or partial restore of processor state components from the XSAVE area located at the memory address specified by the source operand. The implicit EDX:EAX register pair specifies a 64-bit instruction mask. The specific state components restored correspond to the bits set in the requested-feature bitmap (RFBM), which is the logical-AND of EDX:EAX and XCR0.

The format of the XSAVE area is detailed in Section 13.4, “XSAVE Area,” of Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1. Like FXRSTOR and FXSAVE, the memory format used for x87 state depends on a REX.W prefix; see Section 13.5.1, “x87 State” of Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1.

Section 13.8, “Operation of XRSTOR,” of Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1 provides a detailed description of the operation of the XRSTOR instruction. The following items provide a highlevel outline:

Execution of XRSTOR may take one of two forms: standard and compacted. Bit 63 of the XCOMP_BV field in the XSAVE header determines which form is used: value 0 specifies the standard form, while value 1 specifies the compacted form.
If RFBM[i] = 0, XRSTOR does not update state component i.¹
If RFBM[i] = 1 and bit i is clear in the XSTATE_BV field in the XSAVE header, XRSTOR initializes state component i.
If RFBM[i] = 1 and XSTATE_BV[i] = 1, XRSTOR loads state component i from the XSAVE area.
The standard form of XRSTOR treats MXCSR (which is part of state component 1 — SSE) differently from the XMM registers. If either form attempts to load MXCSR with an illegal value, a general-protection exception (#GP) occurs.
XRSTOR loads the internal value XRSTOR_INFO, which may be used to optimize a subsequent execution of XSAVEOPT or XSAVES.
Immediately following an execution of XRSTOR, the processor tracks as in-use (not in initial configuration) any state component i for which RFBM[i] = 1 and XSTATE_BV[i] = 1; it tracks as modified any state component i for which RFBM[i] = 0.

1. There is an exception if RFBM[1] = 0 and RFBM[2] = 1. In this case, the standard form of XRSTOR will load MXCSR from memory, even though MXCSR is part of state component 1 — SSE. The compacted form of XRSTOR does not make this exception.

Use of a source operand not aligned to 64-byte boundary (for 64-bit and 32-bit modes) results in a general-protection (#GP) exception. In 64-bit mode, the upper 32 bits of RDX and RAX are ignored.

See Section 13.6, “Processor Tracking of XSAVE-Managed State,” of Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1 for discussion of the bitmaps XINUSE and XMODIFIED and of the quantity XRSTOR_INFO.

Operation ¶

RFBM ← XCR0 AND EDX:EAX; /* bitwise logical AND */
COMPMASK ← XCOMP_BV field from XSAVE header;
RSTORMASK ← XSTATE_BV field from XSAVE header;
IF COMPMASK[63] = 0
    THEN
        /* Standard form of XRSTOR */
        TO_BE_RESTORED ← RFBM AND RSTORMASK;
        TO_BE_INITIALIZED ← RFBM AND NOT RSTORMASK;
        IF TO_BE_RESTORED[0] = 1
            THEN
                load x87 state from legacy region of XSAVE area;
                XINUSE[0] ← 1;
        ELSIF TO_BE_INITIALIZED[0] = 1
            THEN
                initialize x87 state;
                XINUSE[0] ← 0;
        FI;
        IF RFBM[1] = 1 OR RFBM[2] = 1
            THEN load MXCSR from legacy region of XSAVE area;
        FI;
        IF TO_BE_RESTORED[1] = 1
            THEN
                load XMM registers from legacy region of XSAVE area; // this step does not load MXCSR
                XINUSE[1] ← 1;
        ELSIF TO_BE_INITIALIZED[1] = 1
            THEN
                set all XMM registers to 0; // this step does not initialize MXCSR
                XINUSE[1] ← 0;
        FI;
        FOR i ← 2 TO 62
            IF TO_BE_RESTORED[i] = 1
                THEN
                    load XSAVE state component i at offset n from base of XSAVE area;
                        // n enumerated by CPUID(EAX=0DH,ECX=i):EBX)
                    XINUSE[i] ← 1;
            ELSIF TO_BE_INITIALIZED[i] = 1
                THEN
                    initialize XSAVE state component i;
                    XINUSE[i] ← 0;
            FI;
        ENDFOR;
    ELSE
        /* Compacted form of XRSTOR */
        IF CPUID.(EAX=0DH,ECX=1):EAX.XSAVEC[bit 1] = 0
            THEN /* compacted form not supported */
                #GP(0);
        FI;
        FORMAT = COMPMASK AND 7FFFFFFF_FFFFFFFFH;
        RESTORE_FEATURES = FORMAT AND RFBM;
        TO_BE_RESTORED ← RESTORE_FEATURES AND RSTORMASK;
        FORCE_INIT ← RFBM AND NOT FORMAT;
        TO_BE_INITIALIZED = (RFBM AND NOT RSTORMASK) OR FORCE_INIT;
        IF TO_BE_RESTORED[0] = 1
            THEN
                load x87 state from legacy region of XSAVE area;
                XINUSE[0] ← 1;
        ELSIF TO_BE_INITIALIZED[0] = 1
            THEN
                initialize x87 state;
                XINUSE[0] ← 0;
        FI;
        IF TO_BE_RESTORED[1] = 1
            THEN
                load SSE state from legacy region of XSAVE area; // this step loads the XMM registers and MXCSR
                XINUSE[1] ← 1;
        ELSIF TO_BE_INITIALIZED[1] = 1
            THEN
                set all XMM registers to 0;
                MXCSR ← 1F80H;
                XINUSE[1] ← 0;
        FI;
        NEXT_FEATURE_OFFSET = 576;
                                // Legacy area and XSAVE header consume 576 bytes
        FOR i ← 2 TO 62
            IF FORMAT[i] = 1
                THEN
                    IF TO_BE_RESTORED[i] = 1
                        THEN
                            load XSAVE state component i at offset NEXT_FEATURE_OFFSET from base of XSAVE area;
                            XINUSE[i] ← 1;
                    FI;
                    NEXT_FEATURE_OFFSET = NEXT_FEATURE_OFFSET + n (n enumerated by CPUID(EAX=0DH,ECX=i):EAX);
            FI;
            IF TO_BE_INITIALIZED[i] = 1
                THEN
                    initialize XSAVE state component i;
                    XINUSE[i] ← 0;
            FI;
        ENDFOR;
FI;
XMODIFIED_BV ← NOT RFBM;
IF in VMX non-root operation
    THEN VMXNR ← 1;
    ELSE VMXNR ← 0;
FI;
LAXA ← linear address of XSAVE area;
XRSTOR_INFO ← CPL,VMXNR,LAXA,COMPMASK;

Flags Affected ¶

None.

Intel C/C++ Compiler Intrinsic Equivalent ¶

XRSTOR: void _xrstor( void * , unsigned __int64);

XRSTOR: void _xrstor64( void * , unsigned __int64);

Protected Mode Exceptions ¶

#GP(0)	If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit.
	If a memory operand is not aligned on a 64-byte boundary, regardless of segment.
	If bit 63 of the XCOMP_BV field of the XSAVE header is 1 and CPUID.(EAX=0DH,ECX=1):EAX.XSAVEC[bit 1] = 0.
	If the standard form is executed and a bit in XCR0 is 0 and the corresponding bit in the XSTATE_BV field of the XSAVE header is 1.
	If the standard form is executed and bytes 23:8 of the XSAVE header are not all zero.
	If the compacted form is executed and a bit in XCR0 is 0 and the corresponding bit in the XCOMP_BV field of the XSAVE header is 1.
	If the compacted form is executed and a bit in the XCOMP_BV field in the XSAVE header is 0 and the corresponding bit in the XSTATE_BV field is 1.
	If the compacted form is executed and bytes 63:16 of the XSAVE header are not all zero.
	If attempting to write any reserved bits of the MXCSR register with 1.
#SS(0)	If a memory operand effective address is outside the SS segment limit.
#PF(fault-code)	If a page fault occurs.
#NM	If CR0.TS[bit 3] = 1.
#UD	If CPUID.01H:ECX.XSAVE[bit 26] = 0.
	If CR4.OSXSAVE[bit 18] = 0.
	If the LOCK prefix is used.
#AC	If this exception is disabled a general protection exception (#GP) is signaled if the memory operand is not aligned on a 64-byte boundary, as described above. If the alignment check exception (#AC) is enabled (and the CPL is 3), signaling of #AC is not guaranteed and may vary with implementation, as follows. In all implementations where #AC is not signaled, a general protection exception is signaled in its place. In addition, the width of the alignment check may also vary with implementation. For instance, for a given implementation, an alignment check exception might be signaled for a 2-byte misalignment, whereas a general protection exception might be signaled for all other misalignments (4-, 8-, or 16-byte misalignments).

Real-Address Mode Exceptions ¶

#GP	If a memory operand is not aligned on a 64-byte boundary, regardless of segment.
	If any part of the operand lies outside the effective address space from 0 to FFFFH.
	If bit 63 of the XCOMP_BV field of the XSAVE header is 1 and CPUID.(EAX=0DH,ECX=1):EAX.XSAVEC[bit 1] = 0.
	If the standard form is executed and a bit in XCR0 is 0 and the corresponding bit in the XSTATE_BV field of the XSAVE header is 1.
	If the standard form is executed and bytes 23:8 of the XSAVE header are not all zero.
	If the compacted form is executed and a bit in XCR0 is 0 and the corresponding bit in the XCOMP_BV field of the XSAVE header is 1. If the compacted form is executed and a bit in the XCOMP_BV field in the XSAVE header is 0 and the corresponding bit in the XSTATE_BV field is 1. If the compacted form is executed and bytes 63:16 of the XSAVE header are not all zero. If attempting to write any reserved bits of the MXCSR register with 1.
#NM	If CR0.TS[bit 3] = 1.
#UD	If CPUID.01H:ECX.XSAVE[bit 26] = 0.
	If CR4.OSXSAVE[bit 18] = 0.
	If the LOCK prefix is used.

Virtual-8086 Mode Exceptions ¶

Same exceptions as in protected mode

Compatibility Mode Exceptions ¶

Same exceptions as in protected mode.

64-Bit Mode Exceptions ¶

#GP(0)	If a memory address is in a non-canonical form.
	If a memory operand is not aligned on a 64-byte boundary, regardless of segment.
	If bit 63 of the XCOMP_BV field of the XSAVE header is 1 and CPUID.(EAX=0DH,ECX=1):EAX.XSAVEC[bit 1] = 0.
	If the standard form is executed and a bit in XCR0 is 0 and the corresponding bit in the XSTATE_BV field of the XSAVE header is 1.
	If the standard form is executed and bytes 23:8 of the XSAVE header are not all zero.
	If the compacted form is executed and a bit in XCR0 is 0 and the corresponding bit in the XCOMP_BV field of the XSAVE header is 1.
	If the compacted form is executed and a bit in the XCOMP_BV field in the XSAVE header is 0 and the corresponding bit in the XSTATE_BV field is 1.
	If the compacted form is executed and bytes 63:16 of the XSAVE header are not all zero.
	If attempting to write any reserved bits of the MXCSR register with 1.
#SS(0)	If a memory address referencing the SS segment is in a non-canonical form.
#PF(fault-code)	If a page fault occurs.
#NM	If CR0.TS[bit 3] = 1.
#UD	If CPUID.01H:ECX.XSAVE[bit 26] = 0.
	If CR4.OSXSAVE[bit 18] = 0.
	If the LOCK prefix is used.
#AC	If this exception is disabled a general protection exception (#GP) is signaled if the memory operand is not aligned on a 64-byte boundary, as described above. If the alignment check exception (#AC) is enabled (and the CPL is 3), signaling of #AC is not guaranteed and may vary with implementation, as follows. In all implementations where #AC is not signaled, a general protection exception is signaled in its place. In addition, the width of the alignment check may also vary with implementation. For instance, for a given implementation, an alignment check exception might be signaled for a 2-byte misalignment, whereas a general protection exception might be signaled for all other misalignments (4-, 8-, or 16-byte misalignments).