12 Oct
2004
12 Oct
'04
6:36 p.m.
ShadowFlare wrote:
--- Mike Nordell <tamlin(a)algonet.se> wrote:
If that was the case Intel would have broken backwards compatibility
which they do not do usually I think... If an OS does not know about a
feature, it does usually not enable it and the processor acts like it
would without the feature in most cases I think. (since it's disabled ;-)
In cr4 (Control Register number 4) is a bit called OSFXSR which means
"Operating System support for FXSAVE and FXRSTOR" - if it's not set SSE
instructions are disabled.
From the IA-32 manual:
"When set, this flag preforms the following functions: (1) indicates to
software
that the operating system supports the use of the FXSAVE and FXRSTOR
instructions,
(2) enables the FXSAVE and FXRSTOR instructions to save and restore the
contents
of the XMM and MXCSR registers along with the contents of the x87 FPU
and MMX
registers, and (3) enables the processor to execute any of the
SSE/SSE2/SSE3 instruc-tions,
with the exception of the PAUSE, PREFETCHh, SFENCE, LFENCE,
MFENCE, MOVNTI, and CLFLUSH.
If this flag is clear, the FXSAVE and FXRSTOR instructions will save and
restore the
contents of the x87 FPU and MMX instructions, but they may not save and
restore the
contents of the XMM and MXCSR registers.
Also, if this flag is clear, the processor will generate an invalid
opcode exception (#UD)
whenever it attempts to execute an SSE/SSE2/SSE3 instruction, with the
exception of
the PAUSE, PREFETCHh, SFENCE, LFENCE, MFENCE, MOVNTI, and
CLFLUSH. The operating system or executive must explicitly set this flag."
You can get the PDFs at
http://developer.intel.com/design/PentiumIII/documentation.htm#manuals
(IA-32 Intel Architecture Software Developer’s Manual Volume 1, 2A, 2B, 3)
IIRC Win95 and NT4 knew nothing but plain FPU.
fxsave & co were introduced
with the introduction of WDM, in Win98 and NT5.
/Mike
If that's the case, would running multiple programs
that try to use these other types of instructions
cause random crashes?
-ShadowFlare