80C187 datasheet(8/30 Pages) INTEL

8 / 30 page

80C187

The low-order byte of this control word configures

exception masking Bits 5 – 0 of the control word

contain individual masks for each of the six excep-

tions that the 80C187 recognizes

The high-order byte of the control word configures

the 80C187 operating mode including precision

rounding and infinity control

The ‘‘infinity control bit’’ (bit 12) is not meaningful

to the 80C187 and programs must ignore its val-

ue To maintain compatibility with the 8087 this

bit can be programmed however regardless of

its value the 80C187 always treats infinity in the

affine sense (b % k a %) This bit is initialized

to zero both after a hardware reset and after the

FINIT instruction

The rounding control (RC) bits (bits 11–10) pro-

vide for directed rounding and true chop as well

as the unbiased round to nearest even mode

specified in the IEEE standard Rounding control

affects only those instructions that perform

rounding at the end of the operation (and thus

can generate a precision exception) namely

FST FSTP FIST all arithmetic instructions (ex-

cept FPREM FPREM1 FXTRACT FABS and

FCHS) and all transcendental instructions

The precision control (PC) bits (bits 9–8) can be

used to set the 80C187 internal operating preci-

sion of the significand at less than the default of

64 bits (extended precision) This can be useful in

providing compatibility with early generation arith-

metic processors of smaller precision PC affects

only the instructions ADD SUB DIV MUL and

SQRT For all other instructions either the preci-

sion is determined by the opcode or extended

precision is used

Table 3 Condition Code Interpretation after FPREM and FPREM1 Instructions

Condition Code

Interpretation after

FPREM and FPREM1

Incomplete Reduction

Further Iteration Required

for Complete Reduction

Q MOD 8

000

Complete Reduction

C0 C3 C1 Contain Three Least

Significant Bits of Quotient

001

011

101

111

Table 4 Condition Code Resulting from Comparison

Order

TOP l Operand

TOP k Operand

TOP e Operand

Unordered

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