Intel 80287, 80286 manual Control Word, Status Word

OVERVIEW OF NUMERIC PROCESSING

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I B I Co I ST I c" I c, IeoJEsl X I PE IUEIOEIZEIDEIIE I

I	EXCEPTION FLAGS (1 ~ EXCEPTION HAS OCCURRED)
	INVALID OPERATION'
	DENORMALIZED OPERAND'
	ZERO DIVIDE'
	OVERFLOW'
	UNDERFLOW'
	PRECISION'
	(RESERVED)
	ERROR SUMMARY STATUS(1)
	CONDITION CODE(2)
	STACK TOP POINTER(3)
	NEU BUSY

(1)ES IS SET IF ANY UNMASKED EXCEPTION BIT IS SET, CLEARED OTHERWISE.

(2)SEE TABLE 1-4 FOR CONDITION CODE INTERPRETATION.

(3)ST VALUES

000 ~ REGISTER 0 IS TOP OF STACK

00 1 ~ REGISTER 1 IS TOP OF STACK

111 ~ REGISTER 7 IS TOP OF STACK

'FORDEFINITIONS, SEE THE SECTION ON EXCEPTION HANDLING

G30108

Figure 1-4. 80287 Status Word

The Busy bit (bit 15) and the BUSY pin indicate whether the 80287's execution unit is idle (B=O) or is executing a numeric instruction or signalling an exception (B= 1). (The instructions FNSTSW, FNSTSW AX, FNSTENV, and FNSAVE do not set the Busy bit themselves, nor do they require the Busy bit to be clear in order to execute.)

The four NPX condition code bits (CO-C3)are similar to the flags in a CPU: the 80287 updates these bits to reflect the outcome of arithmetic operations. The effect of these instructions on the condition code bits is summarized in table 1-4. These condition code bits are used principally for conditional branching. The FSTWAX instruction stores the NPX status word directly into the CPU AX register, allowing these condition codes to be inspected efficiently by 80286 code.

Bits 12-14 of the status word point to the 80287 register that is the currentStack Top (ST). The significance of the stack top has been described in the section on the Register Stack.

Figure 1-4 shows the six error flags in bits 0-5 of the status word. Bit 7 is the error summary status (ES) bit. ES is set if any unmasked exception bits are set, and is cleared otherwise. If this bit is set, the ERROR signal is asserted. Bits 0-5 indicate whether the NPX has detected one of six possible

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Control Word

The NPX provides the programmer with several processing options, which are selected by loading a word from memory into the control word. Figure 1-5 shows the format and encoding of the fields in the control word.

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