N386SX 데이터 시트보기 (PDF) - Intel

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N386SX

Intel387™ SX MATH COPROCESSOR

Intel 

Intel387TM SX MATH COPROCESSOR

3 2 2 CONTROL WORD (CW) REGISTER

The Math CoProcessor provides the programmer

with several processing options that are selected by

loading a control word from memory into the control

register Figure 3-3 show the format and encoding of

fields in the control word

The low-order byte of the control word register is

used to configure the exception masking Bits 5 – 0

of the control word contain individual masks for each

of the six exceptions that the Math CoProcessor rec-

ognizes See Section 3 5 Exception Handling for

further explanation on the exception control and def-

inition

The high-order byte of the control word is used to

configure the Math CoProcessor operating mode in-

cluding precision rounding and infinity control

 The rounding control (RC) field (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) field (bits 9–8) can be

used to set the Math CoProcessor internal oper-

ating precision of the significand at less than the

default of 64 bits (extended precision) This can

be useful in providing compatibility with early gen-

eration arithmetic processors of smaller preci-

sion PC affects only the instructions FADD

FSUB(R) FMUL FDIV(R) and FSQRT For all

other instructions either the precision is deter-

mined by the opcode or extended precision is

used

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

to the Intel387 SX Math CoProcessor and pro-

grams must ignore its value To maintain compat-

ibility with the 8087 and 80287 (non-387 core)

this bit can be programmed however regardless

of its value the Intel387 SX Math CoProcessor

always treats infinity in the affine sense (b % k

a %) This bit is initialized to zero both after a

hardware reset and after FINIT instruction

All other bits are reserved and should not be pro-

grammed to assure compatibility with future proces-

sors

Precision Control

00 24 bits (single precision)

01 (reserved)

10 53 bits (double precision)

11 64 bits (extended precision)

Rounding Control

00 Round to nearest or even

01 Round down (toward b%)

10 Round up (toward a %)

11 Chop (truncate toward zero)

Figure 3-3 Control Word

240225 – 5

15

15

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