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Vol. 3 2-21
SYSTEM ARCHITECTURE OVERVIEW
delayed until an x87 FPU/MMX/SSE/SSE2/SSE3/SSSE3/SSE4 instruction is
actually executed by the new task. The processor sets this flag on every task
switch and tests it when executing x87
FPU/MMX/SSE/SSE2/SSE3/SSSE3/SSE4 instructions.
• If the TS flag is set and the EM flag (bit 2 of CR0) is clear, a device-not-
available exception (#NM) is raised prior to the execution of any x87
FPU/MMX/SSE/ SSE2/SSE3/SSSE3/SSE4 instruction; with the exception
of PAUSE, PREFETCHh, SFENCE, LFENCE, MFENCE, MOVNTI, CLFLUSH,
CRC32, and POPCNT. See the paragraph below for the special case of the
WAIT/FWAIT instructions.
• If the TS flag is set and the MP flag (bit 1 of CR0) and EM flag are clear, an
#NM exception is not raised prior to the execution of an x87 FPU
WAIT/FWAIT instruction.
• If the EM flag is set, the setting of the TS flag has no affect on the
execution of x87 FPU/MMX/SSE/SSE2/SSE3/SSSE3/SSE4 instructions.
Table 2-1 shows the actions taken when the processor encounters an x87
FPU instruction based on the settings of the TS, EM, and MP flags. Table 12-1
and 13-1 show the actions taken when the processor encounters an
MMX/SSE/SSE2/SSE3/SSSE3/SSE4 instruction.
The processor does not automatically save the context of the x87 FPU, XMM,
and MXCSR registers on a task switch. Instead, it sets the TS flag, which
causes the processor to raise an #NM exception whenever it encounters an
x87 FPU/MMX/SSE /SSE2/SSE3/SSSE3/SSE4 instruction in the instruction
stream for the new task (with the exception of the instructions listed above).
The fault handler for the #NM exception can then be used to clear the TS flag (with
the CLTS instruction) and save the context of the x87 FPU, XMM, and MXCSR regis-
ters. If the task never encounters an x87 FPU/MMX/SSE/SSE2/SSE3//SSSE3/SSE4
instruction; the x87 FPU/MMX/SSE/SSE2/ SSE3/SSSE3/SSE4 context is never saved.
Table 2-1. Action Taken By x87 FPU Instructions for Different
Combinations of EM, MP, and TS
CR0 Flags x87 FPU Instruction Type
EM MP TS Floating-Point WAIT/FWAIT
0 0 0 Execute Execute.
0 0 1 #NM Exception Execute.
0 1 0 Execute Execute.
0 1 1 #NM Exception #NM exception.
1 0 0 #NM Exception Execute.
1 0 1 #NM Exception Execute.
1 1 0 #NM Exception Execute.