Campbell 21X Instrukcja Operatora Pobierz pdf (Strona 43)

lf Then/Else may be

nested

to form

logical

AND

pr

OR branching.

Figure

3.8-2

illustrates

anlAND

construction.

lf

conditions A

and

B

are

trde, the

instructions

included

between

lF B

and thelfirst

End Instruction

will

be

executed. lf either of the

conditions is false,

execution will

jump

to

the corresponding End

Instruction,

qkipping

the

instructions

between.

rF

A

(BB-92

WITH

COMMAND J0)

SECTION 3.

INSTRUCTION SET BASICS

encountered, execution branches

to the END

Instruction

95

which closes the

case test

(see

lnstruction

93).

3.8.2

END,

INSTRUCTION

95

END,

Instruction

95,

is required to

mark

the end

of:

1. A Subroutine

(starts

with lnstruction

87)

2. A

Loop

(starts

with Instruction

85)

3. An |F... THEN DO sequence

(starts

with

one

of

lnstructions 89-93

with the THEN DO

command 30).

4. A case

statement

(starts

with Instruction

93)

The lF instructions

89-93

require

Instruction

95

only

when

the

THEN DO command

30

is used.

lf one of

the above

instructions

is used

without

the

corresponding END, the 21X

will

display

error

22

when compiling the

program.

Error 21

is

displayed if END

is

used

without being

preceded

by one

of these

instructions

(Section

3.10).

An END

instruction

is

always

paired

with the

most recent instruction that requires an END and

does

not

already

have

one.

A

way

of visualizing

this is to

draw

lines

between

each

instruction

requiring

an END and the

END

paired

with it

(as

in Figure

3.8-2).

The lines must

not

cross.

To

debug logic

or

find a missing or

extra

END error,

list the

program

and draw the

lines.

Subroutines

can be

called

from other

subroutines; they cannot be

embedded within

other subroutines.

A

subroutine

must

end before

another subroutine begins

(Enor

20).

Any loops

or lF...THEN DO

sequences

started within a

subroutine must end before the

subroutine.

3.8.3

NESTING

A branching or loop

instruction which

occurs

before a

previous

branch or

loop has

been

closed

with the

END instruction

is nested. The

maximum nesting level

is 9 deep. Error 30 is

displayed when attempting

to compile a

program

which

is nested too

deep.

The Loop Instruction, 87, counts

as

1

level.

lnstructions 86, 88, 89, 91,

and 92

each

count as

one levelwhen used

with the THEN DO

command

30.

Use of Else,

lnstruction

94,

also

counts

as

one nesting

leveleach time it

is

used.

For example, the

AND

construction

above

is

--

rF B

(88-e2

!V|TH

cor/MAND

Jo)

I

I

,

INSTRUCTIONS

TO

.i:

EXECUTE

IF

A

AND

B

ARE BOTH

TRUE

END B

(e5)

END

A

(s5)

FIGURE 3f8-2.

Logical AND Construction

Figure

3.8-3]illustrates

the

instruction

sequence

that will resuft

in

subroutine

X

being executed

if

either

A

or B

is true.

lF A

(88-92

with

command 30)

Calllsubroutine

X

(86,

command=X)

ELSE

(e4)

lF B

(88-92

with command 3O)

Callsubroutine

X

(86,

command=X)

ENq B

(e5)

ENq

A

(es)

FIGURE

$.8-3.

Logical

OR Construction

A logical

Ofl

can also

be constructed by setting

a

flag

if a colnparison

is true.

(The

flag is

cleared

before

making the comparisons.)

After

all comparis0ns have

been made, execute the

desired instructions

if

the

flag

is

set.

The

Begin

Case

Instruction 93

and

lf Case

Instruction 88 allow

a series of

tests

on the

value

in an input ldcation.

The case

test is

started

with

Instruction 90 which specifies

the location to test.

A series

of

Instructions

83 are then used to

compare thQ

value

in

the

location with fixed

values. Wh0n the value

in the

input

location

is

less

than

th4

fixed value specified in

Instruction

83 the

comrhand

in

that

Instruction 83

is

executed; when the

next lnstruction 83

is

3-5

1 2 ... 38 39 40 41 42 43 44 45 46 47 48 ... 190 191

Komentarze do niniejszej Instrukcji

Brak uwag