Vol. 1 13-19
MANAGING STATE USING THE XSAVE FEATURE SET
XMM registers) even if XINUSE[1] = 0. Unlike the XSAVE instruction, RFBM[2] does not determine whether
XSAVEC saves MXCSR and MXCSR_MASK.
13.11 OPERATION OF XSAVES
The operation of XSAVES is similar to that of XSAVEC. The main differences are (1) XSAVES can be executed only
if CPL = 0; (2) XSAVES can operate on the state components whose bits are set in XCR0 | IA32_XSS and can thus
operate on supervisor state components; and (3) XSAVES uses the modified optimization (see Section 13.6). See
Section 13.2 for details of how to determine whether XSAVES is supported.
The XSAVES instruction takes a single memory operand, which is an XSAVE area. In addition, the register pair
EDX:EAX is an implicit operand used as a state-component bitmap (see Section 13.1) called the instruction
mask. EDX:EAX & (XCR0 | IA32_XSS) (the logical AND the instruction mask with the logical OR of XCR0 and
IA32_XSS) is the requested-feature bitmap (RFBM) of the state components to be saved.
The following conditions cause execution of the XSAVES instruction to generate a fault:
•
If the XSAVE feature set is not enabled (CR4.OSXSAVE = 0), an invalid-opcode exception (#UD) occurs.
•
If CR0.TS[bit 3] is 1, a device-not-available exception (#NM) occurs.
•
If CPL > 0 or if the address of the XSAVE area is not 64-byte aligned, a general-protection exception (#GP)
occurs.
1
If none of these conditions cause a fault, execution of XSAVES writes the XSTATE_BV field of the XSAVE header
(see Section 13.4.2), setting XSTATE_BV[i] (0 ≤ i ≤ 63) as follows:
•
If RFBM[i] = 0, XSTATE_BV[i] is written as 0.
•
If RFBM[i] = 1, XSTATE_BV[i] is set to the value of XINUSE[i] (see below for an exception made for
XSTATE_BV[1]). Section 13.6 defines XINUSE to describe the processor init optimization and specifies the
initial configuration of each state component. The nature of that optimization implies the following:
— If state component i is in its initial configuration, XSTATE_BV[i] may be written with either 0 or 1.
— If state component i is not in its initial configuration, XSTATE_BV[i] is written with 1.
XINUSE[1] pertains only to the state of the XMM registers and not to MXCSR. However, if RFBM[1] = 1 and
MXCSR does not have the value 1F80H, XSAVES writes XSTATE_BV[1] as 1 even if XINUSE[1] = 0.
(As explained in Section 13.6, the initial configurations of some state components may depend on whether the
processor is in 64-bit mode.)
The XSAVES instructions sets bit 63 of the XCOMP_BV field of the XSAVE header while writing RFBM[62:0] to
XCOMP_BV[62:0]. The XSAVES instruction does not write any part of the XSAVE header other than the XSTATE_BV
and XCOMP_BV fields.
Execution of XSAVES saves into the XSAVE area those state components corresponding to bits that are set in RFBM
(subject to the optimizations described below). State components 0 and 1 are located in the legacy region of the
XSAVE area (see Section 13.4.1). Each state component i, 2 ≤ i ≤ 62, is located in the extended region; the XSAVES
instruction always uses the compacted format for the extended region (see Section 13.4.3).
See Section 13.5 for specifics for each state component and for details regarding mode-specific operation and
operation determined by instruction prefixes; in particular, see Section 13.5.6 for some special treatment of PT
state by XSAVES. See Section 13.13 for details regarding faults caused by memory accesses.
Execution of XSAVES performs the init optimization to reduce the amount of data written to memory. If
XINUSE[i] = 0, state component i is not saved to the XSAVE area (even if RFBM[i] = 1). However, if RFBM[1] = 1
and MXCSR does not have the value 1F80H, XSAVES writes saves all of state component 1 (SSE — including the
XMM registers) even if XINUSE[1] = 0.
Like XSAVEOPT, XSAVES may perform the modified optimization. Each execution of XRSTOR and XRSTORS estab-
lishes XRSTOR_INFO as a 4-tuple
w,x,y,z
(see Section 13.8.3 and Section 13.12). Execution of XSAVES uses the
modified optimization only if the following all hold:
1. If CR0.AM = 1, CPL = 3, and EFLAGS.AC =1, an alignment-check exception (#AC) may occur instead of #GP.