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Vol. 2A 3-423
INSTRUCTION SET REFERENCE, A-M
FXSAVE—Save x87 FPU, MMX Technology, SSE, and SSE2 State
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. But the layout of the 64-bit FXSAVE map
has two flavors, depending on the value of the REX.W bit. The difference of these two
flavors is in the FPU IP and FPU DP pointers. When REX.W = 0, the FPU IP is saved as
CS with the 32 bit IP, and the FPU DP is saved as DS with the 32 bit DP. When REX.W
= 1, the FPU IP and FPU DP are both 64 bit values without and segment selectors.
The IA-32e mode save formats are shown in Table 3-51 and Table 3-52 listed below.
101001Special10
101101Special10
For all legal combinations above. 0 Empty 11
Table 3-51. Layout of the 64-bit-mode FXSAVE Map
with Promoted OperandSize
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
FPU IP FOP FTW FSW FCW 0
MXCSR_MASK MXCSR FPU DP 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
Table 3-50. Recreating FSAVE Format (Contd.)
Exponent
all 1’s
Exponent
all 0’s
Fraction
all 0’s
J and M
bits
FTW valid
bit x87 FTW