Intel 80286, 80287 manual Initializing, NPX Processor State Following Initialization

SYSTEM-LEVEL NUMERIC PROGRAMMING

information on software emulation for the 80287 NPX is described in the "80287 Emulation" section later in this chapter.

In any case, if ESC instructions are to be executed, either the MP or EM bit must be set, but not both.

Initializing the 80287

Initializing the 80287 NPX simply means placing the NPX in a known state unaffected by any activity performed earlier. The example software routine to recognize the 80287 (table 3-1) performed this initialization using a single FNINIT instruction. This instruction causes the NPX to be initialized in the same way as that caused by the hardware RESET signal to the 80287. All the error masks are set, all registers are tagged empty, the ST is set to zero, and default rounding, precision, and infinity controls are set. Table 3-1 shows the state of the 80287 NPX following initialization.

Following a hardware RESET signal, such as after initial power-up, the 80287 is initialized in Real- Address mode. Once the 80287 has been switched to Protected mode (using the FSETPM instruction), only another hardware RESET can switch the 80287 back to Real-Address mode. The FNINIT instruction does not switch the operating state of the 80287.

80287 Emulation

If it is determined that no 80287 NPX is available in the system, systems software may decide to emulate ESC instructions in software. This emulation is easily supported by the 80286 hardware, because the 80286 can be configured to trap to a software emulation routine whenever it encounters an ESC instruction in its instruction stream.

Table 3-1. NPX Processor State Following Initialization

Field	Value	Interpretation
Control Word	0
Infinity Control		Projective
Rounding Control	00	Round to nearest
Precision Control	11	64 bits
Interrupt-Enable Mask	.1	Interrupts disabled
Exception Masks	111111	All exceptions masked
Status Word	0
Busy		Not busy
Condition Code	????	(Indeterminate)
Stack Top	000	Empty stack
Interrupt Request	0	No interrupt
Exception Flags	000000	No exceptions
Tag Word	11	Empty
Tags
Registers	N.C.	Not changed
Exception Pointers
Instruction Code	N.C.	Not changed
Instruction Address	N.C.	Not changed
Operand Address	N.C.	Not changed
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