N386SX 데이터 시트보기 (PDF) - Intel

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N386SX

Intel387™ SX MATH COPROCESSOR

Intel 

Intel387TM SX MATH COPROCESSOR

A 1 2 EXCEPTIONS

A number of differences exist due to changes in the IEEE standard and to functional improvements to the

architecture of the Intel387 SX Math CoProcessor

1 When the overflow or underflow exception is masked the Intel387 SX Math CoProcessor differs from the

80287 in rounding when overflow or underflow occurs The Intel387 SX Math CoProcessor produces

results that are consistent with the rounding mode

2 When the underflow exception is masked the Intel387 SX Math CoProcessor sets its underflow flag only if

there is also a loss of accuracy during denormalization

3 Fewer invalid-operations exceptions due to denormal operand because the instructions FSQRT FDIV

FPREM and conversions to BCD or to integer normalize denormal operands before proceeding

4 The FSQRT FBSTP and FPREM instructions may cause underflow because they support denormal

operands

5 The denormal exception can occur during the transcendental instruction and the FXTRACT instruction

6 The denormal exception no longer takes precedence over all other exceptions

7 When the denormal exception is masked the Intel387 SX Math CoProcessor automatically normalizes

denormal operands The 8087 80287 performs unnormal arithmetic which might produce an unnormal

result

8 When the operand is zero the FXTRACT instruction reports a zero-divide exception and leaves b % in

ST(1)

9 The status word has a new bit (SF) that signals when invalid-operation exceptions are due to stack

underflow or overflow

10 FLD extended precision no longer reports denormal exceptions because the instruction is not numeric

11 FLD single double precision when the operand is denormal converts the number to extended precision

and signals the denormal operand exception When loading a signaling NaN FLD single double precision

signals an invalid-operation exception

12 The Intel387 SX Math CoProcessor only generates quiet NaNs (as on the 80287) however the Intel387

SX Math CoProcessor distinguishes between quiet NaNs and signaling NaNs Signaling NaNs trigger

exceptions when they are used as operands quiet NaNs do not (except for FCOM FIST and FBSTP

which also raise IE for quiet NaNs)

13 When stack overflow occurs during FPTAN and overflow is masked both ST(0) and ST(1) contain quiet

NaNs The 80287 8087 leaves the original operand in ST(1) intact

14 When the scaling factor is g % the FSCALE instruction behaves as follows

 FSCALE (0 %) generates the invalid operation exception

 FSCALE (finite b%) generates zero with the same sign as the scaled operand

 FSCALE (finite a %) generates % with the same sign as the scaled operand

The 8087 80287 returns zero in the first case and raises the invalid-operation exception in the other

cases

15 The Intel387 SX Math CoProcessor returns signed infinity zero as the unmasked response to massive

overflow underflow The 8087 and 80287 support a limited range for the scaling factor within this range

either massive overflow underflow do not occur or undefined results are produced

A-2

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