User's Manual

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OVERVIEW OF NUMERIC PROCESSING
15
I x x x I I C I R cL PC I x I X IPMIUMIOMIZMIDMIIM I
(1) PRECISION CONTROL
00 ~ 24-BIT SIGNIFICAND
o 1 ~ RESERVED
10 ~ 53-BIT SIGNIFICAND
11 ~ 64-BIT SIGNIFICAND
I
(2) ROUNDING CONTROL
00 ~ ROUND TO NEAREST OR EVEN
01 ~ ROUND DOWN (TOWARD -co)
10 ~ ROUND UP (TOWARD +co)
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 FSA VE 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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