Vector Graphic Extended Systems Monitor Manual de usuario descargar pdf (Pagina 23)

vector

RAID

Reference

Manual

.8

Stack

pointer

and

stack

-

Along

the

right

side

of

the

display

is

a colllIlll

with

the

syrrt:cl "SP"

at

the

top,

st.an:ling

for

"stack

J;X'inter".

The

contents

of

the

stack

pointer

appear

immediately

below,

in

hexadecimal

notation.

Belew

this

is

found

the

contents

of

the

stack

itself,

as

nuch

as

will

fit

in

the

colunn:

13

hexadecimal

words

(2

bytes

in

each.

word).

RAID

keeps

track

of

a

value

knc:wn

as

the

"start

of

the

stack".

The

start

of

the

stack

is

the

address

just

al:ove

the

location

of

the

first

word

pushed

on

to

the

stack.

For

example,

if

the

stack

pointer

is

pointing

at

AOO2H

when a

program

pushes

BBBBH

onto

the

stack

as

the

first

word

on

the

stack,

then

the

start

of

the

stack

is

at

A002H

and

BBBBH

will

be

stored

at

AOOOH

and

AOOlH.

(The

processor

of

ccurse

decrenents

the

stack

pointer

by

2

before

pushing

a

word

onto

the

stack,

because

it

always

};X)ints

to

the

last

word

p,IShed

onto

the

stack.

)

How

does

RAID

know

where

the

start

of

the

stack

is?

When

RAID

is

first

loaded,

RAID

a.ssuIteS

the

stac::X

is

empty.

In

other

words,

it

assunes

the

start

of

the

stack

is

EqUal

to

the

initial

setting

of

the

stack

IX>inter.

The

hexadecirral

word

at

the

top

of

the

stack

colunn

is

always

the

contents

of

the

two

locations

just

below

the

start

of

the

stack.

Using

the

above

example,

if

the

start

of

the

stack

is

at

AOO2H,

then

the

word

stored

at

AO

0

OH

and

AOOrn

will

be

the

tcp

v.m:d

in

the

display.

The

botton

tNOrd

in

the

colLnml

is

always

the

contents

of

the

lOcation

r;ointed

to

by

the

stack

pointer.

When

the

stack

pointer

decreases

by

2,

due

to

pepping

the

stack,

the

botton

word

disapt=ears.

When

the

lX>inter

increases

by

2,

due

to

pushing

a

word

onto

the

stack,

the

new word

is

added

on

the

bottan.

Hence

the

display

really

does

shew

the

stack.

As

words

are

p,IShed

onto

aOO

t=OPr:eJ

off

the

stack,

the

column

will

grow

longer

am

shorter

respectively.

Note.

that

there

is

a

Stack

conunand'

used

for

changing

the

setting

of

the

"start

of

the

stack,"

which

in

turn

alters

what

part

of

rrem::>t:y

is

displayed

in

the·

stack

column. .

Stack

camnands

are

cllsOJSSed

in

Olapter

3.

Rev.

l.l-A

5/30/80

2-9

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