# XSAVEC — Save Processor Extended States With Compaction

Opcode / Instruction | Op/En | 64/32 bit Mode Support | CPUID Feature Flag | Description
--- | --- | --- | --- | ---
NP 0F C7 /4 XSAVEC mem | M | V/V | XSAVEC | Save state components specified by EDX:EAX to mem with compaction.
NP REX.W + 0F C7 /4 XSAVEC64 mem | M | V/N.E. | XSAVEC | Save state components specified by EDX:EAX to mem with compaction.

## Instruction Operand Encoding ¶

Op/En | Operand 1 | Operand 2 | Operand 3 | Operand 4
--- | --- | --- | --- | ---
M | ModRM:r/m (w) | N/A | N/A | N/A

## Description ¶

Performs a full or partial save of processor state components to the XSAVE area located at the memory address specified by the destination operand. The implicit EDX:EAX register pair specifies a 64-bit instruction mask. The specific state components saved 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 13.10, “Operation of XSAVEC,” of Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1 provides a detailed description of the operation of the XSAVEC instruction. The following items provide a highlevel outline:

- Execution of XSAVEC is similar to that of XSAVE. XSAVEC differs from XSAVE in that it uses compaction and that it may use the init optimization.
- XSAVEC saves state component i if and only if RFBM[i] = 1 and XINUSE[i] = 1.1 (XINUSE is a bitmap by which the processor tracks the status of various state components. See Section 13.6, “Processor Tracking of XSAVEManaged State” of Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1.)
- XSAVEC does not modify bytes 511:464 of the legacy region of the XSAVE area (see Section 13.4.1, “Legacy Region of an XSAVE Area” of Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1).
- XSAVEC writes the logical AND of RFBM and XINUSE to the XSTATE_BV field of the XSAVE header.2,3 (See Section 13.4.2, “XSAVE Header” of Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1.) XSAVEC sets bit 63 of the XCOMP_BV field and sets bits 62:0 of that field to RFBM[62:0]. XSAVEC does not write to any parts of the XSAVE header other than the XSTATE_BV and XCOMP_BV fields.
- XSAVEC always uses the compacted format of the extended region of the XSAVE area (see Section 13.4.3, “Extended Region of an XSAVE Area” of Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1).

> 1. There is an exception for state component 1 (SSE). MXCSR is part of SSE state, but XINUSE[1] may be 0 even if MXCSR does not have its initial value of 1F80H. In this case, XSAVEC saves SSE state as long as RFBM[1] = 1.

> 2. Unlike XSAVE and XSAVEOPT, XSAVEC clears bits in the XSTATE_BV field that correspond to bits that are clear in RFBM.

> 3. There is an exception for state component 1 (SSE). MXCSR is part of SSE state, but XINUSE[1] may be 0 even if MXCSR does not have its initial value of 1F80H. In this case, XSAVEC sets XSTATE_BV[1] to 1 as long as RFBM[1] = 1.

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

## Operation ¶

    RFBM := XCR0 AND EDX:EAX;
                        /* bitwise logical AND */
    TO_BE_SAVED := RFBM AND XINUSE;
                        /* bitwise logical AND */
    If MXCSR ≠ 1F80H AND RFBM[1]
        TO_BE_SAVED[1] = 1;
    FI;
    IF TO_BE_SAVED[0] = 1
        THEN store x87 state into legacy region of XSAVE area;
    FI;
    IF TO_BE_SAVED[1] = 1
        THEN store SSE state into legacy region of XSAVE area; // this step saves the XMM registers, MXCSR, and MXCSR_MASK
    FI;
    NEXT_FEATURE_OFFSET = 576;
                        // Legacy area and XSAVE header consume 576 bytes
    FOR i := 2 TO 62
        IF RFBM[i] = 1
            THEN
                IF TO_BE_SAVED[i]
                    THEN save XSAVE state component i at offset NEXT_FEATURE_OFFSET from base of XSAVE area;
                FI;
                NEXT_FEATURE_OFFSET = NEXT_FEATURE_OFFSET + n (n enumerated by CPUID(EAX=0DH,ECX=i):EAX);
        FI;
    ENDFOR;
    XSTATE_BV field in XSAVE header := TO_BE_SAVED;
    XCOMP_BV field in XSAVE header := RFBM OR 80000000_00000000H;

## Flags Affected ¶

None.

## Intel C/C++ Compiler Intrinsic Equivalent ¶

    XSAVEC void _xsavec( void * , unsigned __int64);

    XSAVEC64 void _xsavec64( 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.
#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 or CPUID.(EAX=0DH,ECX=1):EAX.XSAVEC[bit 1] = 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.
#NM | If CR0.TS[bit 3] = 1.
#UD | If CPUID.01H:ECX.XSAVE[bit 26] = 0 or CPUID.(EAX=0DH,ECX=1):EAX.XSAVEC[bit 1] = 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 the memory address is in a non-canonical form.
If a memory operand is not aligned on a 64-byte boundary, regardless of segment.
#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 or CPUID.(EAX=0DH,ECX=1):EAX.XSAVEC[bit 1] = 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).