FXSAVE

Save x87 FPU, MMX Technology, and SSE State

참고 사항

Instruction Operand Encoding

Description

Saves the current state of the x87 FPU, MMX technology, XMM, and MXCSR registers to a 512-byte memory loca-tion specified in the destination operand. The content layout of the 512 byte region depends on whether the processor is operating in non-64-bit operating modes or 64-bit sub-mode of IA-32e mode.

Bytes 464:511 are available to software use. The processor does not write to bytes 464:511 of an FXSAVE area.

The operation of FXSAVE in non-64-bit modes is described first.

Non-64-Bit Mode Operation

Table 3-43 shows the layout of the state information in memory when the processor is operating in legacy modes.

Table 3-43. Non-64-bit-Mode Layout of FXSAVE and FXRSTOR Memory Region

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Rsvd FCS FIP[31:0] FOP Rsvd FTW FSW FCW 0

MXCSR_MASK MXCSR Rsrvd FDS FDP[31:0] 16

Reserved ST0/MM0 32

Reserved ST1/MM1 48

Reserved ST2/MM2 64

Reserved ST3/MM3 80

Reserved ST4/MM4 96

Reserved ST5/MM5 112

Reserved ST6/MM6 128

Reserved ST7/MM7 144

XMM0 160

XMM1 176

XMM2 192

XMM3 208

XMM4 224

XMM5 240

XMM6 256

XMM7 272

Reserved 288

Table 3-43. Non-64-bit-Mode Layout of FXSAVE and FXRSTOR Memory Region (Contd.)

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Reserved 304

Reserved 320

Reserved 336

Reserved 352

Reserved 368

Reserved 384

Reserved 400

Reserved 416

Reserved 432

Reserved 448

Available 464

Available 480

Available 496

The destination operand contains the first byte of the memory image, and it must be aligned on a 16-byte boundary. A misaligned destination operand will result in a general-protection (#GP) exception being generated (or in some cases, an alignment check exception [#AC]).

The FXSAVE instruction is used when an operating system needs to perform a context switch or when an exception handler needs to save and examine the current state of the x87 FPU, MMX technology, and/or XMM and MXCSR registers.

The fields in Table 3-43 are defined in Table 3-44.

Table 3-44. Field Definitions

FCW x87 FPU Control Word (16 bits). See Figure 8-6 in the Intel(R) 64 and IA-32 Architectures Software Developer's Manual, Volume 1, for the layout of the x87 FPU control word.

Table 3-44. Field Definitions (Contd.)

FCS x87 FPU Instruction Pointer Selector (16 bits). If CPUID.(EAX=07H,ECX=0H):EBX[bit 13]= 1, the processor deprecates FCS and FDS, and this field is saved as 0000H.

The FXSAVE instruction saves an abridged version of the x87 FPU tag word in the FTW field (unlike the FSAVE instruction, which saves the complete tag word). The tag information is saved in physical register order (R0 through R7), rather than in top-of-stack (TOS) order. With the FXSAVE instruction, however, only a single bit (1 for valid or 0 for empty) is saved for each tag. For example, assume that the tag word is currently set as follows:

R7 R6 R5 R4 R3 R2 R1 R0

11 xx xx xx 11 11 11 11

Here, 11B indicates empty stack elements and "xx" indicates valid (00B), zero (01B), or special (10B).

For this example, the FXSAVE instruction saves only the following 8 bits of information:

R7 R6 R5 R4 R3 R2 R1 R0

0 1 1 1 0 0 0 0

Here, a 1 is saved for any valid, zero, or special tag, and a 0 is saved for any empty tag.

The operation of the FXSAVE instruction differs from that of the FSAVE instruction, the as follows:

• FXSAVE instruction does not check for pending unmasked floating-point exceptions. (The FXSAVE operation in this regard is similar to the operation of the FNSAVE instruction).

• After the FXSAVE instruction has saved the state of the x87 FPU, MMX technology, XMM, and MXCSR registers, the processor retains the contents of the registers. Because of this behavior, the FXSAVE instruction cannot be

used by an application program to pass a "clean" x87 FPU state to a procedure, since it retains the current state. To clean the x87 FPU state, an application must explicitly execute an FINIT instruction after an FXSAVE instruction to reinitialize the x87 FPU state.

• The format of the memory image saved with the FXSAVE instruction is the same regardless of the current addressing mode (32-bit or 16-bit) and operating mode (protected, real address, or system management). This behavior differs from the FSAVE instructions, where the memory image format is different depending on the addressing mode and operating mode. Because of the different image formats, the memory image saved with the FXSAVE instruction cannot be restored correctly with the FRSTOR instruction, and likewise the state saved with the FSAVE instruction cannot be restored correctly with the FXRSTOR instruction.

The FSAVE format for FTW can be recreated from the FTW valid bits and the stored 80-bit FP data (assuming the stored data was not the contents of MMX technology registers) using Table 3-45.

Table 3-45. Recreating FSAVE Format

Exponent J and M FTW valid bitall 1's bits x87 FTW

0 0x 1 Special 10

0 1x 1 Valid 00

0 00 1 Special 10

0 10 1 Valid 00

0 0x 1 Special 10

0 1x 1 Special 10

0 00 1 Zero 01

0 10 1 Special 10

1 1x 1 Special 10

1 1x 1 Special 10

1 00 1 Special 10

1 10 1 Special 10

For all legal combinations above. 0 Empty 11

The J-bit is defined to be the 1-bit binary integer to the left of the decimal place in the significand. The M-bit is defined to be the most significant bit of the fractional portion of the significand (i.e., the bit immediately to the right of the decimal place).

When the M-bit is the most significant bit of the fractional portion of the significand, it must be 0 if the fraction is all 0's.

IA-32e Mode Operation

In compatibility sub-mode of IA-32e mode, legacy SSE registers, XMM0 through XMM7, are saved according to the legacy FXSAVE map. In 64-bit mode, all of the SSE registers, XMM0 through XMM15, are saved. Additionally, there are two different layouts of the FXSAVE map in 64-bit mode, corresponding to FXSAVE64 (which requires REX.W=1) and FXSAVE (REX.W=0). In the FXSAVE64 map (Table 3-46), the FPU IP and FPU DP pointers are 64-bit wide. In the FXSAVE map for 64-bit mode (Table 3-47), the FPU IP and FPU DP pointers are 32-bits.

Table 3-46. Layout of the 64-bit-mode FXSAVE64 Map (requires REX.W = 1)

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

FIP FOP Reserved FTW FSW FCW 0

MXCSR_MASK MXCSR FDP 16

Reserved ST0/MM0 32

Reserved ST1/MM1 48

Reserved ST2/MM2 64

Reserved ST3/MM3 80

Reserved ST4/MM4 96

Reserved ST5/MM5 112

Reserved ST6/MM6 128

Reserved ST7/MM7 144

XMM0 160

XMM1 176

XMM2 192

XMM3 208

XMM4 224

XMM5 240

XMM6 256

XMM7 272

XMM8 288

XMM9 304

XMM10 320

XMM11 336

XMM12 352

XMM13 368

XMM14 384

XMM15 400

Reserved 416

Reserved 432

Reserved 448

Available 464

Available 480

Available 496

Table 3-47. Layout of the 64-bit-mode FXSAVE Map (REX.W = 0)

15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Reserved FCW 0

MXCSR_MASK MXCSR Reserved FDS FDP[31:0] 16

Reserved ST0/MM0 32

Reserved ST1/MM1 48

Reserved ST2/MM2 64

Reserved ST3/MM3 80

Reserved ST4/MM4 96

Reserved ST5/MM5 112

Reserved ST6/MM6 128

Reserved ST7/MM7 144

XMM0 160

XMM1 176

XMM2 192

XMM3 208

XMM4 224

XMM5 240

XMM6 256

XMM7 272

XMM8 288

XMM9 304

XMM10 320

XMM11 336

XMM12 352

XMM13 368

XMM14 384

XMM15 400

Reserved 416

Reserved 432

Reserved 448

Available 464

Available 480

Available 496

Operation

IF 64-Bit Mode
    THEN
          IF REX.W = 1
                THEN
                      DEST <- Save64BitPromotedFxsave(x87 FPU, MMX, XMM15-XMM0,MXCSR);
                ELSE
                      DEST <- Save64BitDefaultFxsave(x87 FPU, MMX, XMM15-XMM0, MXCSR);
          FI;
    ELSE
          DEST <- SaveLegacyFxsave(x87 FPU, MMX, XMM7-XMM0, MXCSR);
FI;

Protected Mode Exceptions

#GP(0)

• For an illegal memory operand effective address in the CS, DS, ES, FS or GS segments.

• If a memory operand is not aligned on a 16-byte boundary, regardless of segment. (See the description of the alignment check exception [#AC] below.)

#SS(0)

• For an illegal address in the SS segment.

#PF(fault-code)

• For a page fault.

#NM

• If CR0.TS[bit 3] = 1.

• If CR0.EM[bit 2] = 1.

#UD

• If CPUID.01H:EDX.FXSR[bit 24] = 0.

#UD

• 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 16-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 align-ment check exception might be signaled for a 2-byte misalignment, whereas a general protec-tion 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 16-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.

• If CR0.EM[bit 2] = 1.

#UD

• If CPUID.01H:EDX.FXSR[bit 24] = 0.

• If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

#PF(fault-code)

• For a page fault.

#AC

• For unaligned memory reference.

#UD

• If the LOCK prefix is used.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#SS(0)

• If a memory address referencing the SS segment is in a non-canonical form.

#GP(0)

• If the memory address is in a non-canonical form.

• If memory operand is not aligned on a 16-byte boundary, regardless of segment.

#PF(fault-code)

• For a page fault.

#NM

• If CR0.TS[bit 3] = 1.

• If CR0.EM[bit 2] = 1.

#UD

• If CPUID.01H:EDX.FXSR[bit 24] = 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 16-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 align-ment check exception might be signaled for a 2-byte misalignment, whereas a general protec-tion exception might be signaled for all other misalignments (4-, 8-, or 16-byte misalignments).

  Implementation Note

The order in which the processor signals general-protection (#GP) and page-fault (#PF) exceptions when they both occur on an instruction boundary is given in Table 5-2 in the Intel(R) 64 and IA-32 Architectures Software Devel-oper's Manual, Volume 3B. This order vary for FXSAVE for different processor implementations.

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