INTEL387TMDX 데이터 시트보기 (PDF) - Intel

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INTEL387TMDX

Intel387™ DX MATH COPROCESSOR

Intel 

Intel387TM DX MATH COPROCESSOR

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 2 7 Intel387TM DX MCP Control Word

240448 – 4

2 3 5 CONTROL WORD

The MCP provides several processing options that

are selected by loading a control word from memory

into the control register Figure 2 7 shows the format

and encoding of fields in the control word

The low-order byte of this control word configures

the MCP error and exception masking Bits 5– 0 of

the control word contain individual masks for each of

the six exceptions that the MCP recognizes

The high-order byte of the control word configures

the MCP operating mode including precision and

rounding

 Bit 12 no longer defines infinity control and is a

reserved bit Only affine closure is supported for

infinity arithmetic The bit is initialized to zero after

RESET or FINIT and is changeable upon loading

the CW Programs must ignore this bit

 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 MCP internal operating precision

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

2 4 Interrupt Description

Several interrupts of the Intel386 DX CPU are used

to report exceptional conditions while executing nu-

meric programs in either real or protected mode Ta-

ble 2 6 shows these interrupts and their causes

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