Intel IA-32 User Manual
Page 68
2-20 Vol. 3A
SYSTEM ARCHITECTURE OVERVIEW
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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 instructions.
Table 2-1 shows the actions taken when the processor encounters an x87 FPU instruc-
tion based on the settings of the TS, EM, and MP flags. Table 11-1 and 12-1 show the
actions taken when the processor encounters an MMX/SSE/SSE2/SSE3 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 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 registers. If the task never
encounters an x87 FPU/MMX/SSE/SSE2/SSE3 instruction; the x87 FPU/MMX/SSE/SSE2/
SSE3 context is never saved.
EM
Emulation (bit 2 of CR0) — Indicates that the processor does not have an internal or
external x87 FPU when set; indicates an x87 FPU is present when clear. This flag also
affects the execution of MMX/SSE/SSE2/SSE3 instructions.
When the EM flag is set, execution of an x87 FPU instruction generates a device-not-
available exception (#NM). This flag must be set when the processor does not have an
internal x87 FPU or is not connected to an external math coprocessor. Setting this flag
forces all floating-point instructions to be handled by software emulation. Table 9-2
shows the recommended setting of this flag, depending on the IA-32 processor and x87
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.
1
1
1
#NM Exception
#NM exception.