微机第六章大学课程

Chapter

6

Program

Control

Instruction1¨

Direct

the

flow

of

a

program

and

allow

the

flow

tchange¨

Jumps,

calls,

returns,

interrupts,

and

machinecontrol

instructions¨

.If,

.ELSE,

.ELSEIF,

.ENDIF,

.WHILE,

.ENDW,REPEAT,

and

.UNTIL

relational

assemblylanguage¨

MASM

6.*

and

TASM

5.*The

Jump

Group2¨

JMP

instruction

allows

the

programmer

to

skipsections

of

a

program

and

branch

to

any

of

thememory

for

the

next

instruction¨

A

conditional

jump

instruction

allows

theprogrammer

to

make

decisions

baseduponnumerical

tests.¨

The

results

of

numerical

tests

are

held

in

the

fbits,

which

are

then

tested

by

conditional

jumpinstructions¨

The

conditional

setJMP

(Unconditional

Jump)3¨

Short

jump,

near

jump,

and

far

jump¨

Short

jump:

a

two-byte

instruction

that

allowsjumps

or

branches

to

memory

location

within

+12and

–128

bytes

from

the

address

following

thejump¨

Near

jump:

a

three-byte

instruction

that

allowjumps

or

branches

to

memory

location

within

+/-32K

bytes

from

the

address

following

the

jump,

oanywhere

in

the

current

real

mode

code

segment¨

Short

jump,

near

jump,

and

far

jump¨

In

the

80386

and

above

processors,

the

displaceis32

bits

and

the

Near

jump

is

five

bytes

long¨

In

the

80386

and

above

processors,

the

protectemode

code

segment

can

be

4G

bytes

long,

so

the

32bit

displacement

allows

a

near

jump

to

any

locatwithin

+/-2G

bytes4JMP

(Unconditional

Jump)JMP

(Unconditional

Jump)5¨

Short

jump,

near

jump,

and

far

jump¨

Far

jump:

a

five-byte

instruction

that

allows

jto

any

memory

location

within

the

real

memorysystem¨

A

new

segment

and

offset

address

is

obtained

toaccomplish

the

jump¨

If

the

microprocessor

(80286

through

Pentium

4operated

in

the

protected

mode,

the

segmentaddress

accesses

a

descriptor

that

contains

theaddress

and

the

offset

location

within

the

new

csegmentJMP

(Unconditional

Jump)6¨

Short

jump,

near

jump,

and

far

jump¨

The

short

and

near

jumps

are

often

calledintrasegment

jumps,

and

the

far

jumps

are

oftencalled

intersegment

jumps78¨

Example

1¨

Show

how

short

jump

instructions

passcontrol

from

one

section

of

the

program

toanother

andillustrate

the

use

of

a

label

(asymbolic

name

for

a

memory

address)910¨

Example

211¨

Develop

a

same

basic

program

as

the

example

1,except

that

the

jump

distance

is

greater¨

Notice

that

the

letter

R

denotes

a

relocatable

jaddress

of

0200H¨

The

relocatable

address

of

0200H

is

for

theassembler

program’s

internal

use

only¨

The

actual

machine

language

instruction

assembas

an

E9

F6

01,

which

does

not

appear

in

theassembler

listing¨

The

actual

displacement

is

a

01F6H

for

this

jumpinstruction1213¨

Example

314¨

The

far

jump

instruction

sometimes

appears

witthe

FAR

PTR

directives¨

Another

way

to

obtain

a

farjump

is

to

define

alabel

as

a

far

label¨

In

this

example,

the

label

UP

is

defined

as

a

farlabel

by

the

EXTRN

UP:

FAR

directive¨

When

the

program

files

are

jointed,

the

linkerinserts

the

address

for

the

UP

label

into

the

JMPUP

instruction.

It

also

inserts

address

in

the

JSTART

instruction15Jumps

with

Register

Operands16¨

The

jump

instruction

can

also

use

a

16-

or

32-biregister

as

an

operand.¨

This

automatically

sets

up

the

instruction

as

aindirect

jump¨

The

address

of

the

jump

is

in

the

register

specibythe

jump

instruction¨

Unlike

the

displacement

associated

with

the

nejump,

the

contents

of

the

register

are

transferdirectly

into

the

instruction

pointer¨

The

JUMP

AX

instruction,

for

example,

copies

thcontents

of

the

AX

register

into

the

IP

when

thejump

occurs¨

Example

4¨

Show

how

the

instruction

accesses

a

jump

table

ithe

code

segment¨

Read

a

key

from

the

keyboard

and

then

modifiesthe

ASCII

code

to

a

00H

in

AL

for

a

‘1’,

a

01Hfor

a

‘2’,

and

a

02H

for

a

‘3’¨

If

a

‘1’,

‘2’,

or

‘3’

is

typed,

AH

is

clearto

00H¨

Because

the

jump

table

contains

16-bit

offsetaddress,

the

contents

of

AX

are

doubled

to

0,

2,4,

so

a

16-bit

entry

in

the

table

can

be

accessed¨

Next,

the

offset

address

of

the

start

of

the

jumtable

is

loaded

to

SI,

and

AX

is

added

toform

thereference

to

the

jump

address¨

The

MOV

AX,

[SI]

instruction

then

fetches

anaddress

from

the

jump

table,

so

the

JMP

AXinstruction

jumps

to

the

address

(ONE,

TWO,

or17¨

Example4

(continued)¨

THREE)

stored

in

the

jumptable181920Indirect

Jumps

Using

an

Index21¨

Example

5¨

Repeat

the

Example

4

using

JMP

TABLE

[SI]

insteaof

JMP

AX2223Conditional

Jumps

and

Conditional

Sets24¨

In

the

8086~80286

microprocessors,

conditionajumps

are

always

short

jumps,

within

the

jumpinrange

between

+127

and

–128

bytes¨

In

the

80386

and

above,

conditional

jumps

areeither

short

or

near

jumps

which

allows

to

jump

tany

location

within

the

current

code

segment25¨

Notes26¨

The

conditional

jump

instructions

test

the

flasign

(S),

zero

(Z),

carry

(C),

parity

(P),

andoverflow

(O)¨

An

FFH

(255)

is

above

the

00H

in

the

set

ofunsigned

numbers,

but

an

FFH

(-1)

is

less

than

00for

signed

numbers¨

When

signed

numbers

are

compared,

use

the

JG,JL,

JGE,

JE

and

JNE

instructions¨

When

unsigned

numbers

are

compared,

use

the

JA,JB,

JAE,

JBE,

JE,

and

JNE

instructions27¨

Example

6¨

Use

JCXZ

conditional

jumps

instruction

to

searctable

for

a

0AH2829The

Conditional

SET

Instructions30¨

In

the

80386

and

above,

the

conditional

setinstructions

set

a

byte

to

either

a

01H

or

clearbyte

to

00H,

depending

on

the

outcome

of

thecondition

under

test¨

For

example,

the

SETNC

MEM

instruction

placesa

01H

into

memory

location

MEM

if

carry

iscleared,

and

a

00H

into

MEM

if

carry

is

set¨

The

contents

of

MEM

can

be

tested

at

a

later

poinin

the

program

to

determine

if

carry

is

cleared

athe

point

where

the

SETNC

MEM

instructionexecuted31LOOP32¨

Be

a

combination

of

a

decrement

CX

And

the

JNZconditional

jump¨

In

the

8086

through

the

80286

microprocessors,LOOP

decrements

CX;

if

CX<

>0,

it

jump

to

theaddress

indicated

by

the

label.

If

CX

becomes

a

0the

next

sequential

instruction

executes¨

In

the

80386

through

Pentium

4,

LOOPdecrements

either

CX

or

ECX¨

Example

7¨

Look

at

the

ExampleNo.

6-7

on

the

Page

No.

19233Conditional

Loops34¨

LOOPE

(loop

while

equal):jump

if

CX<

>

0

whilean

equal

condition

exists¨

LOOPNE

(loop

while

not

equal):

jump

if

CX<

>0while

an

not-equal

condition

existsControlling

the

flow

of

an

Assembly

language

Program35¨

.IF,

.ELSE,

.ELSEIF,

and

.ENDIFB¨

.REPEAT-.UNTIL,

and

.WHILE-.ENDW¨

These

statements

always

indicate

a

specialassembly

language

command

to

MASM6.*¨

Example

836¨

Look

at

theExample6-8

on

the

Page

193¨

Look

at

theExample6-9

on

the

Page

194¨

Look

at

the

Table

6-3¨

Look

at

theExample6-10

on

the

Page

195DO-WHILE

Loops37¨

Example

9¨

Look

at

theExample6-11

on

the

Page

196¨

Look

at

theExample6-12

on

the

Page

197¨

Look

at

theExample6-13

on

the

Page

198REPEAT-UNTIL

Loops38¨

Example

10¨

Look

at

theExample6-14

on

the

Page

199¨

Look

at

theExample6-15

on

the

Page

199Procedure39¨

The

procedure

or

subroutine

is

a

reusable

sectiof

the

software

that

is

stored

in

memory

once,

buused

as

often

as

necessary¨

The

CALL

instruction

links

to

the

procedure,

anthe

RET

(return)

instruction

returns

from

theprocedure¨

The

stack

stores

the

return

address

whenever

aprocedure

is

called

during

the

execution

of

aprogram¨

The

CALL

instruction

pushes

the

address

of

theinstruction

following

the

CALL

(return

address¨

the

stack¨

The

RET

instruction

removes

an

address

from

thestack

so

the

program

returns

to

the

instructionfollowing

the

CALL¨

A

procedure

begins

with

the

PROC

directive

andends

with

the

ENDP

directive¨

Each

directive

appears

with

the

name

of

theprocedure¨

The

procedure

directive

is

followed

by

the

typeprocedure:

NEAR

or

FAR¨

The

USES

statement

following

the

NEAR

or

FARallows

any

number

of

registers

to

be

automaticapushed

to

the

stack

and

popped

from

the

stackwithin

the

procedure40Procedure

(continued)the

Difference

Between

the

Near

and

the

Far

Procedures41¨

The

difference

in

the

RET

instruction:

the

nearreturn

instruction

uses

opcode

C3H,

and

the

farreturn

uses

opcode

CBH¨

A

near

return

removes

a

16-bit

number

from

thestack

and

places

it

into

the

instruction

pointereturn

from

the

procedure

in

the

current

codesegment¨

A

far

return

removes

a

32-bit

number

from

thestack

and

places

it

into

both

IP

and

CS

to

returnfrom

the

procedure

to

any

memory

location¨

Example

11¨

Look

at

theExample6-16

on

the

Page

20042CALL43¨

Transfer

the

flow

of

the

program

to

the

procedur¨

Differ

from

the

JUMP

instruction

because

a

CALLsaves

a

return

address

on

the

stack¨

The

return

instruction

returns

control

to

theinstruction

that

immediately

followings

the

CALin

a

program

when

a

ret

instruction

executes¨

Near

CALL:

be

three-byte

long,

with

a

+/-32Kbytes

displacement

or

distance

in

the

8086~8028processors;

with

32-bit

displacement,

I.

E.

+/-bytes

distance

in

80386

through

Pentium

4processors

when

operating

in

the

protected

modeCALL

(Continued)44¨

Far

CALL:

be

five-byte

long

instruction,

with

tnext

value

for

the

IP

and

CS

registers¨

The

far

CALL

instruction

places

the

contents

ofboth

IP

and

CS

onthe

stack

before

jumping

to

theaddress

indicated

by

bytes

2~5

of

the

instructio4546CALLs

with

Register

Operands47¨

Take

CALL

BX

instruction

as

an

example¨

Push

the

contents

of

IP

onto

the

stack,

then

jumpto

the

offset

address

located

in

register

BX,

incurrent

code

segment¨

This

type

of

CALL

always

uses

a

16-bit

offsetaddress,

stored

in

any

16-bit

register

except

thsegment

registers¨

Example

12¨

Look

at

theExample6-17

on

the

Page

20248CALLs

with

indirect

Memory

Addresses49¨

A

CALL

with

an

indirect

memory

address

isparticularly

useful

whenever

different

subroutneed

to

be

chosen

in

a

program¨

Example

13¨

Look

at

theExample6-18

on

the

Page

203RET50¨

The

return

instruction

(RET)

removes

a

16-bitnumber(near

return)

from

the

stack

and

places

iinto

IP¨

The

return

instruction

(RET)

removes

a

32-bitnumber(far

return)

from

the

stack

and

places

itinto

IP

and

CS¨

With

the

80386

through

the

Pentium

4

processorsoperating

in

the

protected

mode,

the

far

returnremoves

six

bytes

from

the

stack.

The

first

fourbytes

contain

the

new

value

for

EIP,

and

the

lasttwo

contain

the

new

value

for

CS¨

With

the

80386

through

the

Pentium

4

processorsoperating

in

the

protected

mode,

the

near

returnremoves

four

bytes

from

the

stack

and

places

theinto

EIP51¨

Example

14¨

Look

at

theExample6-19

on

the

Page

20552Instruction

to

Interrupts53¨

An

interrupt

is

either

a

hardware-generated

CAL(externally

derived

from

a

hardware

signal)

or

asoftware-generated

CALL

(internally

derived

frthe

execution

of

an

instruction

or

by

some

otherinternal

event)Interrupt

Vectors54¨

There

are

totally

256

different

interrupt

vectoeach

containing

the

address

of

an

interrupt

servprocedure¨

When

the

microprocessor

operates

in

the

real

moan

interrupt

vector

is

a

four-byte

number

storethe

first

1024

bytes

of

the

memory¨

In

the

protected

mode,

the

vecto