Intel 253668-032US Benutzerhandbuch

Vol. 3   9-7

PROCESSOR MANAGEMENT AND INITIALIZATION

 
The EM flag determines whether floating-point instructions are executed by the x87 
FPU (EM is cleared) or a device-not-available exception (#NM) is generated for all 
floating-point instructions so that an exception handler can emulate the floating-
point operation (EM = 1). Ordinarily, the EM flag is cleared when an x87 FPU or math 
coprocessor is present and set if they are not present. If the EM flag is set and no x87 
FPU, math coprocessor, or floating-point emulator is present, the processor will hang 
when a floating-point instruction is executed.
The MP flag determines whether WAIT/FWAIT instructions react to the setting of the 
TS flag. If the MP flag is clear, WAIT/FWAIT instructions ignore the setting of the TS 
flag; if the MP flag is set, they will generate a device-not-available exception (#NM) 
if the TS flag is set. Generally, the MP flag should be set for processors with an inte-
grated x87 FPU and clear for processors without an integrated x87 FPU and without a 
math coprocessor present. However, an operating system can choose to save the 
floating-point context at every context switch, in which case there would be no need 
to set the MP bit. 
Table 2-1 shows the actions taken for floating-point and WAIT/FWAIT instructions 
based on the settings of the EM, MP, and TS flags.
The NE flag determines whether unmasked floating-point exceptions are handled by 
generating a floating-point error exception internally (NE is set, native mode) or 
through an external interrupt (NE is cleared). In systems where an external interrupt 
controller is used to invoke numeric exception handlers (such as MS-DOS-based 
systems), the NE bit should be cleared.

9.2.2 

Setting the Processor for x87 FPU Software Emulation

Setting the EM flag causes the processor to generate a device-not-available excep-
tion (#NM) and trap to a software exception handler whenever it encounters a 
floating-point instruction. (Table 9-2 shows when it is appropriate to use this flag.) 
Setting this flag has two functions:

Table 9-2.  Recommended Settings of EM and MP Flags on IA-32 Processors

EM

MP

NE

IA-32 processor

1

0

1

Intel486™ SX, Intel386™ DX, and Intel386™ SX processors 

only, without the presence of a math coprocessor.

0

1

1 or 0

*

Pentium 4, Intel Xeon, P6 family, Pentium, Intel486™ DX, and 

Intel 487 SX processors, and Intel386 DX and Intel386 SX 

processors when a companion math coprocessor is present.

0

1

1 or 0

*

More recent Intel 64 or IA-32 processors

NOTE:

* The setting of the NE flag depends on the operating system being used.