N386SX 데이터 시트보기 (PDF) - Intel

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N386SX

Intel387™ SX MATH COPROCESSOR

Intel 

Intel387TM SX MATH COPROCESSOR

Table 3-7 CPU Interrupt Vectors Reserved for Math CoProcessor

Interrupt

Number

Cause of Interrupt

7

An ESC instruction was encountered when EM or TS of CPU control register zero (CR0) was

set EM e 1 indicates that software emulation of the instruction is required When TS is set

either an ESC or WAIT instruction causes interrupt 7 This indicates that the current Math

CoProcessor context may not belong to the current task

9

In a protected-mode system an operand of a coprocessor instruction wrapped around an

addressing limit (0FFFFH for expand-up segments zero for expand-down segments) and

spanned inaccessible addresses(1) The failing numerics instruction is not restartable The

address of the failing numerics instruction and data operand may be lost an FSTENV does not

return reliable addresses The segment overrun exception should be handled by executing an

FNINIT instruction (i e an FINIT without a preceding WAIT) The exception can be avoided by

never allowing numerics operands to cross the end of a segment

13

In a protected-mode system the first word of a numeric operand is not entirely within the limit of

its segment The return address pushed onto the stack of the exception handler points at the

ESC instruction that caused the exception including any prefixes The Math CoProcessor has

not executed this instruction the instruction pointer and data pointer register refer to a previous

correctly executed instruction

16

The previous numerics instruction caused an unmasked exception The address of the faulty

instruction and the address of its operand are stored in the instruction pointer and data pointer

registers Only ESC and WAIT instructions can cause this interrupt The CPU return address

pushed onto the stack of the exception handler points to a WAIT or ESC instruction (including

prefixes) This instruction can be restarted after clearing the exception condition in the Math

CoProcessor FNINIT FNCLEX FNSTSW FNSTENV and FNSAVE cannot cause this interrupt

NOTE

1 An operand may wrap around an addressing limit when the segment limit is near an addressing limit and the operand is

near the largest valid address in the segment Because of the wrap-around the beginning and ending addresses of such an

operand will be at opposite ends of the segment There are two ways that such an operand may also span inaccessible

addresses 1) if the segment limit is not equal to the addressing limit (e g addressing limit is FFFFH and segment limit is

FFFDH) the operand will span addresses that are not within the segment (e g an 8-byte operand that starts at valid offset

FFFCH will span addresses FFFC–FFFFH and 0000-0003H however addresses FFFEH and FFFFH are not valid because

they exceed the limit) 2) if the operand begins and ends in present and accessible segments but intermediate bytes of the

operand fall in a not-present page or in a segment or page to which the procedure does not have access rights

Table 3-8 Intel387TM SX Math CoProcessor Exceptions

Exception

Cause

Default Action

(if exception is masked)

Invalid

Operation

Operation on a signalling NaN unsupported format

indeterminate for (0-% 0 0 (a %) a (b %) etc ) or stack

overflow underflow (SF is also set)

Result is a quiet NaN

integer indefinite or

BCD indefinte

Denormalized At least one of the operands is denormalized i e it has the

Operand

smallest exponent but a nonzero significand

Normal processing

continues

Zero Divisor

The divisor is zero while the dividend is a noninfinite nonzero

number

Result is %

Overflow

The result is too large in magnitude to fit in the specified format Result is largest finite

value or %

Underflow

The true result is nonzero but too small to be represented in the

specified format and if underflow exception is masked

denormalization causes the loss of accuracy

Result is denormalized

or zero

Inexact Result The true result is not exactly representable in the specified

Normal processing

(Precision)

format (e g 1 3) the result is rounded according to the rounding continues

mode

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