N386SX データシートの表示（PDF） - Intel

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N386SX

Intel387™ SX MATH COPROCESSOR

Intel 

Intel387TM SX MATH COPROCESSOR

16-BIT REAL-ADDRESS MODE AND VIRTUAL 8086 MODE FORMAT

15

7

0

CONTROL WORD

0

STATUS WORD

2

TAG WORD

4

INSTRUCTION POINTER 15 0

6

IP19 16 0

OPCODE 10 0

8

OPERAND POINTER 15 0

A

DP 19 16 0 0 0 0 0 0 0 0 0 0 0 0 C

Figure 3-8 Instruction and Data Pointer Image in Memory 16-Bit Real-Mode Format

3 3 Data Types

Table 3-6 lists the seven data types that the Math

CoProcessor supports and presents the format for

each type Operands are stored in memory with the

least significant digit at the lowest memory address

Programs retrieve these values by generating the

lowest address For maximum system performance

all operands should start at physical-memory ad-

dresses that correspond to the word size of the

CPU operands may begin at any other addresses

but will require extra memory cycles to access the

entire operand

The data type formats can be divided into three

classes binary integer decimal integer and binary

real These formats however exist in memory only

Internally the Math CoProcessor holds all numbers

in the extended-precision real format Instructions

that load operands from memory automatically con-

vert operands represented in memory as 16 32 or

64-bit integers 32 or 64-bit floating point numbers

or 18 digit packed BCD numbers into extended-pre-

cision real format Instructions that store operands in

memory perform the inverse type conversion

In addition to the typical real and integer data values

the Intel387 SX Math CoProcessor data formats en-

compass encodings for a variety of special values

These special values have significance and can ex-

press relevant information about the computations

or operations that produced them The various types

of special values are denormal real numbers zeros

positive and negative infinity NaNs (Not-a-Number)

Indefinite and unsupported formats For further in-

formation on data types and formats see the In-

tel387 Programmer’s Reference Manual

3 4 Interrupt Description

CPU interrupts are used to report errors or excep-

tional conditions while executing numeric programs

in either real or protected mode Table 3-7 shows

these interrupts and their functions

3 5 Exception Handling

The Math CoProcessor detects six different excep-

tion conditions that occur during instruction execu-

tion Table 3-8 lists the exception conditions in order

of precedence showing for each the cause and the

default action taken by the Math CoProcessor if the

exception is masked by its corresponding mask bit in

the control word

Any exception that is not masked by the control

word sets the corresponding exception flag of the

status word sets the ES bit of the status word and

asserts the ERROR signal When the CPU at-

tempts to execute another ESC instruction or WAIT

exception 16 occurs The exception condition must

be resolved via an interrupt service routine The re-

turn address pushed onto the CPU stack upon entry

to the service routine does not necessarily point to

the failing instruction nor to the following instruction

The CPU saves the address of the floating-point in-

struction that caused the exception and the address

of any memory operand required by that instruction

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