271092-005 データシートの表示（PDF） - Intel

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271092-005

80-BIT CHMOS III NUMERIC PROCESSOR EXTENSION

Intel 

M80C287

INSTRUCTION AND DATA POINTERS

Because the NPX operates in parallel with the CPU

any exceptions detected by the NPX may be report-

ed after the CPU has executed the ESC instruction

which caused it To allow identification of the failing

numeric instruction the M80C287 contains registers

that aid in diagnosis These registers supply the op-

code of the failing numeric instruction the address

of the instruction and the address of its numeric

memory operand (if appropriate)

The instruction and data pointers are provided for

user-written exception handlers Whenever the

M80C287 executes a new ESC instruction it saves

the address of the instruction (including any prefixes

that may be present) the address of the operand (if

present) and the opcode CPUs with 32-bit internal

architectures contain 32-bit versions of these regis-

ters and do not use the contents of the NPX regis-

ters This difference is not apparent to programmers

however

The instruction and data pointers appear in one of

four formats depending on the operating mode of

the system (protected mode or real-address mode)

and (for CPUs with 32-bit internal architectures) de-

pending on the operand-size attribute in effect (32-

bit operand or 16-bit operand) (See Figures 6 and 7)

The ESC instructions FLDENV FSTENV FSAVE

and FRSTOR are used to transfer these values be-

tween the registers and memory Note that the value

of the data pointer is undefined if the prior ESC in-

struction did not have a memory operand

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)

271092 – 4

The ‘‘infinity control’’ bit is not meaningful to the M80C287 To maintain compatibility with the M80287 this bit can be

programmed however regardless of its value the M80C287 treats infinity in the affine sense (b% k a %)

Figure 5 Control Word

8

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