Intel 80287, 80286 manual Inter

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OVERVIEW OF NUMERIC PROCESSING

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I x x x I I C I R cL PC I x I X IPMIUMIOMIZMIDMIIM I

I

(1) PRECISION CONTROL		(2) ROUNDING CONTROL
00	~ 24-BIT SIGNIFICAND	00 ~ ROUND TO NEAREST OR EVEN
o1	~ RESERVED	01	~ ROUND DOWN (TOWARD -co)
10	~ 53-BIT SIGNIFICAND	10	~ ROUND UP (TOWARD +co)
11	~ 64-BIT SIGNIFICAND	11	~ CHOP (TRUNCATE TOWARD ZERO)

EXCEPTION MASKS (1 ~EXCEPTION IS MASKED)

INVALID OPERATION

DENORMALIZED OPERAND

ZERO DIVIDE

OVERFLOW

UNDERFLOW

PRECISION

(RESERVED)

(RESERVED)

PRECISION CONTROL (1)

ROUNDING CONTROL(2)

INFINITY CONTROL (0 ~ PROJECTIVE, 1 ~ AFFINE) (RESERVED)

G30108

Figure 1-5_ 80287 Control Word Format

The infinity control bit (bit 12) determines the manner in which the 80287 treats the special values of infinity. Either affine closure (where positive infinity is distinct from negative infinity) or projective closure (infinity is treated as a single unsigned quantity) may be specified. These two alternative views of infinity are discussed in the section on Computation Fundamentals.

The NPX Tag Word

The tag word indicates the contents of each register in the register stack, as shown in figure 1-6. The tag word is used by the NPX itself in order to track its numeric registers and optimize performance. Programmers may use this tag information to interpret the contents of the numeric registers, The tag values are stored in the tag word corresponding to the physical registers 0-7. Programmers must use the current Stack Top (ST) pointer stored in the NPX status word to associate these tag values with the relative stack registers ST(O) through ST(7).

The NPX Instruction and Data Pointers

The NPX instruction and data registers provide support for programmed exception-handlers. Whenever the 80287 executes a math instruction, the NPX internally saves the instruction address, the operand address (if present), and the instruction opcode. The 80287 FSTENV and FSAVE instructions store this data into memory, allowing exception handlers to determine the precise nature of any numeric exceptions that may be encountered.

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