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18524B/0-

Mar1996

Hardware

Interrupts

and

the

VIF

and

VIP

Extensions

AMD~

AMD5~6

Processor

Technical

Reference

Manual

In

addition

to

these

performance

problems

caused

by

virtual-

ization

of

the

IF

flag

in

Virtual-8086

mode,

software

interrupts

(those

caused

by

INTn

instructions

that

vector

through

inter-

rupt

gates)

cannot

be

masked

by

the

IF

flag

or

virtual

copies

of

the

IF

flag,

these

flags

only

affect

hardware

interrupts.

Soft-

ware

interrupts

in

Virtual-8086

mode

are

normally

directed

to

the

Real

mode

interrupt

vector

table

(IVT),

but

it

may

be

desirable

to

redirect

interrupts

for

certain

vectors

to

the

Pro-

tected

mode

interrupt

descriptor

table

(IDT).

The

processor's

Virtual-8086

mode

extensions

support

both

of

these

cases-hardware

(external)

interrupts

and

software

interrupts-with

mechanisms

that

preserve

high

performance

without

compromising

protection.

Virtualization

of

hardware

interrupts

is

supported

via

the

Virtual

Interrupt

Flag

(VIF)

and

Virtual

Interrupt

Pending

(VIP) flag

in

the

EFLAGS

regis-

ter.

Redirection

of

software

interrupts

is

supported

with

the

Interrupt

Redirection

Bitmap

(IRB)

in

the

TSS

of

each

Virtual-

8086

program.

When

VME

extensions

are

enabled,

the

IF-modifying

instruc-

tions

that

are

normally

trapped

by

the

operating

system

are

allowed

to

execute,

but

they

write

and

read

the

VIF

bit

rather

than

the

IF

bit

in

EFLAGS.

This

leaves

mask

able

interrupts

enabled

for

detection

by

the

operating

system.

It

also

indicates

to

the

operating

system

whether

the

Virtual-8086

program

is

able

to

or

expecting

to

receive

interrupts.

When

an

external

interrupt

occurs,

the

processor

switches

from

the

Virtual-8086

program

to

the

operating

system,

in

the

same

manner

as

on

a 386

or

486

processor.

If

the

operating

sys-

tem

determines

that

the

interrupt

is for

the

Virtual-8086

pro-

gram,

it

checks

the

state

of

the

VIF

bit

in

the

program's

EFLAGS

image

on

the

stack.

If

VIF

has

been

set

by

the

proces-

sor

(during

an

attempt

by

the

program

to

set

the

IF

bit),

the

operating

system

permits

access

to

the

appropriate

Virtual-

8086

handler

via

the

interrupt

vector

table

(IVT).

If

VIF

has

been

cleared,

the

operating

system

holds

the

interrupt

pend-

ing.

The

operating

system

can

do

this

by

saving

appropriate

information

(such

as

the

interrupt

vector),

setting

the

pro-

gram's

VIP

flag

in

the

EFLAGS

image

on

the

stack,

and

return-

ing

to

the

interrupted

program.

When

the

program

subsequently

attempts

to

set

IF,

the

set

VIP

flag

causes

the

processor

to

inhibit

the

instruction

and

generate

a

general-

Control

Register

4

(CR4)

Extensions

J-lJ

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