os8 basic reference

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CHAPTER 6

BASIC

6.1

INTRODUCTION

OS/78

BASIC* is an interactive program ming language used in scientific and business environments t o solve mathe-

matical problems with a minimum o f programming effor t. It also is used by educators and stud ents as a problem-

solving to ol and as an aid to learning through programmed in struction and simulation.

In m any respects the BASIC language is similar to oth er programm ing languages (such as FOR TRA N IV), but BASIC

is aimed at facilitating commu nication between the user and the com puter. BASIC simply requires th at yo u type in

the comp utational procedure as a series of numbered statements, making use of co mm on English words and familiar

mathematical notations. Because of the small num ber of comm ands necessary and its easy application in solving

problems, BASIC is an easy com puter language to learn . With experience, the advanced techniques available can be

added in the language to perform m ore intricate manipulations or to express a problem m ore efficiently and

concisely.

6.2 MAJOR COMPONENTS OF

OS/78

BASIC

The BASIC subsystem has four major components

1. BASIC Editor (BASICS V)

2 .

Compiler (BCOM PSV)

3 . Loader (BLOADSV)

4.

BASIC Run Time System (BR TS SV , BASIC.AF, BASIC SF, BASIC.FF)

Th e BASIC editor is used to create and edit program source files. During this process, the editor creates a file called

BASIC.WS containing the current program.

Once the program has been prepared for execution, entering a RUN command cm ses the editor to chain to the

BASIC compiler. The com piler converts the statements in BASIC.WS into relocatable binary in structions.

Following compilation, the BASIC loader is automatically requested. The loade : onverts the relocatable binary

data outp ut by the compiler into executable form and loads the result into mem ory.

The BASIC loader then chains to the BASIC Run Time System (BRTS) w h ch executes the program. The modules

BASIC.AF, BASIC.FF, and B ASIC SF are overlays to BRT SSV .

6.3 BASIC INSTRUCTION REPERTOIRE

BASIC instructions and commands can be g rouped in th ree categories as follows:

1. BASIC Editor comm ands that allow you t o

a. Create or mod ify a program,

b. Execute a program,

c. Retrieve a program from d iskette, and

d. Save a program .

*BASIC is a registered trademark

of

the trustees of Dartmouth College.

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BASIC

2 .

3 .

BASIC statements, comprising the BASIC language, tha t are th e building blocks used t o create and struc-

ture BASIC programs.

BASIC Fu nctio ns, represented b y subroutines, that are built into BASIC primarily t o facilitate problem

solving activities.

6.4

CALLING BASIC

To enter the BASIC subsystem, type

e

B A S

C

C

-

in response to the Monitor do t. This command invokes the BASIC editor.

6.5

BASIC EDITO R COMMANDS

6.5.1

Using the BASIC Editor

The BASIC editor incorp orates all the too ls and capabilities necessary to create , correct, mod ify, execu te, save and

retrieve BASIC programs. After calling BASIC, the BASIC editor respond s with the displayed que ry

Editing is now co ntinued in one of two ways:

1.

Typing NEW with a file name instru cts the system to initiate the creation

of

a new

file.

2 .

Typing OL D with a file name in structs the system to retrieve an old file containing a previously-generated

program.

Figure 6-1 summ arizes user actions implem ented by the BASIC editor. This figure shows two arbitrarily selected

file names (MAKER/ALTER). File names may contain no more than six alphanumeric symbols.

The left side of Figure

6-1

shows the types and patterns o f activities tha t yo u ordinarily pursue after typ ing the NEW

comm and. The right side of the illustration shows activities freque ntly carried ou t after typing the OLD com mand .

Note tha t mo st of the BASIC editor com mands appear on both sides of the figure. Only the sequence in which they

are used differs.

6.5.2

BASIC Editor C ommands

This section summ arizes the BASIC editor commands. Letters that are not required in naming the comm and are

shown as lower case in the Com mand/Param eter d escription. Fo r exam ple, only NE is required t o be recognized by

the BASIC editor as the NEW co mm and .

NOTE

The R ETU RN key must be pressed following each BASIC

editor command.

6.5.2.1

NEW Command

The NEW co mm and clears the mem ory w orksp rce and sp ecifies the nam e of the pro -

gram that is to be inp ut.

Command Parameters

NEW file [.ex]

file .ex is the new file name and extension of the program ab ou t to be typed in. If the extension

is om itte d, the ed itor assigns .BA.

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BASIC

Correction

Keys

J

NEW

F I LE

NAME

/

I

Add BASIC Statements

I

PRESERVE

Your Program Using

SAVE

8 M t C

/ \

\

QLES

A R E A D Y

CREATE

Your Program

u;i;;;;\

MODIFY

f

our Program Using

REVIEW

Your Program Using

PRESERVE

Your Program Using

SJIV-15

EXECUTE

Your Program Using

{

zH

RROR MESSAGE

1

\

F I LE NAME

\

ALTER

t

Miscellaneous BASIC

Editor Commands

BYE

SCRATCH

CTRL/C

EXECUTE

REVIEW

Your Program Using

Your Program Using

MODIFY

Your Program Using

~ ~~

End of BASIC Session

1. *Keyboard Monitor Command

2. Shaded areas indicateuser

action at the console

Figure 6-1 BASIC Editor Commands and Related Uses

6

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BASIC

An

alternate metho d is to type NEW w ithout a file name, followed by the R ETUR N key. BASIC displays

F I L E

NAME I

in response to w hich the file name and extension is typ ed .

For exam ple, to clear the workspace and name the new program TEST.BA, type

NEW TEST

or

NE

TEST+RA

6.5.2.2

OLD Command

The OLD com mand clears the mem ory w orkspace, and causes the editor to find a

program on a diskette and place it into th e workspace.

Command Parameters

OLd dev:file

[

ex]

dev: file ex is the device, file nam e, and extens ion of the program on the disk. If the extension

is om itted , BASIC assumes .BA.

Another m ethod is to ty pe O LD without a file name. BASIC displays

F I L E

NAME

--

in response to w hich the dev ice, file name, and extension is typ ed . When no device is specified, the BASIC editor

defaults to

DSK

(usually

= SYS).

For example,

to

bring TEST.BA in to the workspace from RXAl, type

OLKI

R X A i :TEST

B A

or

6.5.2.3

LIST/LISTNH

Commands

The LIST command displays the current program along with

a

header line,

containing the program name, da te, and the revision num ber of BASIC. The d ate

is

displayed only if the current

date has been entered into the system.

Command Parameters

LIst

b l

If n

is

om itted all program statem ents in the workspa ce are displayed. When n

is

specified, line n and all subsequen t

lines are displayed. Ty pe CTRL/O t o terminate a listing.

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BASIC

For example, typing LIST (or LI) displays the program PROG.BA:

LIST

PROG B A 5 A

26- JUL-77

LO FOR A = I TO 5

20

FRXNT

A

30

NEXT

A

40

END

REASIY

Typing LI

30

displays line

30

and all subsequent lines:

1-1

30

P R O G B A 5 A 26.- U L - 7 7

30

NEXT A

4Q

END

R E A D Y

The LISTNH comm and also displays the program statem ents in the workspace but withou t the header.

Command Parameters

LISTNH bl

n

has the same effects as specified for the LIST co mm and, but does not display the header

6.5.2.4

SAVE

Command

The SAVE command writes the program in the workspace on to the diskette as a

permanently saved file. Do no t confuse this comman d with the Monitor SAVE command.

Comm and Parame ers

SAve [dev:file.ex]

dev

is the device on which you want t o store your program.

file .ex

is

the file name and ex tension that the program will have on the diskette. If bot h are left

ou t , BASIC will use the cu rrent file name and extension o f the p rogram in the wo rkspace.

If only the exten sion is omitted

BASIC

assigns

.BA. If DEV:

is omitted ,

BASIC

assumes the

device is

DSK.

In

the following example, the program TEST.BA is in the workspace. To store it

on

RXAl under th e same file

name and extension, type

or

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BASIC

6.5.2.5

RUN lRUNN H Commands The RUN command executes the program in the workspace. N ote that

this command differs from the monitor RUN co mm and.

Command Parameters

Run (none)

Prior to program ex ec utio n, this comm and d isplays a heading consisting of the file name and exten sion, BASIC

version numb er and system dat e, assuming tha t the curren t date has been en tered in the system. When program

execution is com pleted, BASIC displays

READY

Th e following example shows the program PROG .BA in the workspace displaying the result of a calculation :

R U N

FROG Bc)

5 A 26-JAN-76

3.179

The RUNNH command executes the program in the w orkspace, but does not display the header line.

Command Parameters

RUNNH (none)

Thus, the only difference between the RUNNH and RUN commands is that RUN prints the header and RUNNH

does not display the header.

6.5.2.6

NAME Comm and

The NAME co mm and allows the user to renam e the program in the w orkspace.

Command Parameters

NAme newfil[ .ex]

newfil.ex

is the new file name and ex tension of the program in the workspace. If the extension

is om itte d, the editor assigns .BA.

To change the nam e of the program in the workspace to PROG.BA, type

N A M E

F'ROG,BA

or

NA PROG

6.5.2.7

SCRATCH Comm and The SCRATCH command erases all statements from the workspace, tha t is,

it clears the workspace.

Command Parameters

Scratch (none)

6.5.2.8 BYE

Command

The BYE command ex its from BASIC and returns con trol to the M onitor from

BASIC.

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BASIC

Command Parameters

BYE (none)

Typing BYE before SA VEing a newly created prog ram will delete the p rogram . Whenever the BASIC editor is

waiting for user inp ut, typing CTR L/C performs the same function as BYE .

6.5.3

BASIC

Control Keys

This section describes the c ontro l keys that are used to correct errors, eliminate and sub stitute program links, and

con trol program listings.

6.5.3.1

Correcting Typing and Form at Errors (DELE TE, CTRL/U)

Errors made while typin g programs at the

terminal are easily corrected. Pressing the DE LETE key causes deletion of the last character typ ed . One charac ter is

deleted each time th e key is pressed.

Som etimes it is easier to delete a line being typed and retyp e the line rather tha n attem pt a correction using a series

of DELETEs. Typing CTRL/U will delete the entire line currently being worked on and echoe DELETED and a

carriage return-line feed. Use of the CTR L/U key is equivalent to typ ing DELE TES back t o the beg inning of the line.

6.5.3.2

Eliminating and Substituting Program Lines (RETURN )

To delete a program line that h as already been

entered into the com puter, simply type the line number and then press the RETU RN key . Both the line numb er

and the statement(s) are removed from the program.

Change individual lines by simply retyping them in again. Whenever a line is entered , it replaces any existing line

having the sam e line numb er. New lines may be inserted anyw here in th e program by giving them uniqu e line

numbers.

6.5.3.3 Interrupting Program Execution Program execution may be terminated by typing CTRL/C. BASIC

responds by displaying the READY message allowing you to correct or add statements to the program.

NOTE

BASIC responds to C TRL/C with a REA DY message

on ly if you have already given the RUN com man d.

Typin g CTRL /C when the BASIC Editor is operational

causes a return t o the Monitor.

6.5.3.4 Controlling Program Listings at the Terminal Console (CTRL/S, CTRL/C an CTRL/O) For programs

exceeding a single display fram e 24 lines) the user may w ish to s top the scrolling effect that occurs after typing the

LISTlLISTNH com man d. Three sets of control keys are provided to do this. They are as follows:

1,

CTR L/S keys. Simu ltaneously pressing these keys suspends listing (scrolling) of the progra m. However

it leaves the program in a state where yo u m ay resume listing.

2. CTR L/Q key s. Simu ltaneous ly pressing these keys (after having suspended scrolling with C TRL /S),

resumes the listing process.

3.

CTR L/O keys. Simu ltaneously pressing these keys ab orts the listing process and causes the BASIC editor

to display READY.

6.5.4 Resequencing Programs (RESEQ )

If a program is extensively mod ified, you may find that some portions of th e program have line num bers spaced

so

closely together that the y d o no t permit any further addition of stateme nts. Renum bering the lines in the program

to provide

a

practical increment between line numbers can be done by using the RESEQ program. Note tha t the

RESEQ program mo difies the line numb ers in GOSUB and IF-THEN statemen ts to agree with the new line num bers

assigned to program statements by RESEQ. Line lengths must no t exceed

80

characters and programs may n ot

exceed

350

lines.

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BASIC

Typically, the program wo uld be used as follows:

SAVE DSK:SAMPLE BA

READY

OLD D SK:RESEQ

READY

RUNNH

FILE :DSK:SAMPLE BA

START,STEP:

100,lO

READY

OLD

DSK:SAMPLE.BA

READY

LISTNH

User saves program SAMPLE which requires renumb ering.

BASIC is ready for next comm and.

User calls for program RES EQ.

BASIC is ready for next com man d.

User runs RESEQ program.

Program asks for filename. User responds with dev ice, nam e, and exten sion of

program t o be renumbered.

Program asks for a starting line numb er (START) and for the increm ent

between line numbers (STEP). User requests that SAM PLE start with line

number

100

and each line be incremented by

10.

Renum bering is accomplished. BASIC ready for next co mm and .

User calls back his program .

BASIC ready for next com man d.

User gets listing of program SAMPLE for further mo dification.

6.6

DATA FOR MATS ACCEPTABLE TO

BASIC

6.6.1

Numeric Information

6.6.2

Numbers

Num bers are expressed in decimal or E (exponential) for m at. Exam ples of numbers in both categories are as follows:

Decim al E TYpe

0

10.23E27

7 6.21Et27

t 6 9

-

7.232E6

-

52 2.2

1

1

E-

3

-

3.9265 -2.2 1 14E-4

0.123

- 0 . 7 6 9

In the decimal format, the decimal point is optional for integers. That is, BASIC assumes a decimal point after the

rightmost digit of an integer. In E type form at, BASIC assumes a positive expoh ent when no plus ( t ) or minus (-)

sign follows the E. Substitute th e words times ten t o the power o f for the letter E w hen reading

E

type format

numbers.

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BASIC

Num eric da ta may be inp ut in either format. Results of comp utations with an absolute value outside the range

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BASIC

Examples:

10 LET 1=53721

20 LET B3=456.9

30

LET X=20E9

40

INPUT

Q

50

DATA 5,6,7

6.6.4 Subscripted Variables

In addition

to

simple variables, BASIC accep ts an oth er class of variables called su bsc ripte d variables. Sub scripted

variables provide ad ditional co mp uting capabilities for handling array s, lists, tables, matrices, or an y set of re lated

variables. Variables are allowed o ne or tw o sub scripts. A single letter or a letter followed b y a digit form s the name

of the variable. Th e subscript is forme d by o ne or tw o integers enclosed in parentheses and separated by com ma s.

Up

to 31 arrays are possible in an y program, subject only t o the amo unt of m emory available for d ata storage. For

examp le, an array might be described as A(J) w here

J

goes from

1

to 5 , as follows:

This allows reference t o be m ade t o each

of

the five elements

in

the array A.

A

two-dimensional array A(J,K) can be

defined in a similar man ner, bu t th e sub scripted variable

A

must always have the same number of subsc ripts (that is,

A(J) an d A(3 ,K) can not be used in th e same program). It is possible, howev er, to use the same variable nam e

as

b o t h

a subsc ripted and an unsubscripted variable. Both A and A(J) are valid variable names and can be used in the same

program. F )r more info rmation on arrays and the use of sub scripts, see the description of the DIM statem ent

(Section

6.7.10.1).

6.6.5 Arithmetic Operations

6.6.5.1

Operators BASIC performs add ition, sub tractio n, mu ltiplication, division and exp on entia tion, as well as

mo re complicated op eratio ns (explained in detail later

in

th e manual). The five ope rato rs used in writing mos t

formulas are :

Symbol

Op erator Meaning Example

t Addit ion A+B

Sub tractio n A- B

Multiplication A*B

-

*

I

Divis ion A D

-(or**) Ex pon entia tion A^B o r (A**B)

(Raise A t o th e B Power)

6 .6 .5 .2 Priority

In any given mathematical formu la, BASIC performs th e arithm etic operations in th e follow-

ing order:

1 . Parentheses receive to p prio rity. Any expression within parentheses is evaluated before an unparen-

thesized expression.

2. In the absence of parentheses, the order or priority is:

a. Exponentiat ion

b. Multiplication and Division (of equa l priority)

c. Addition and Subtraction (of equal priority)

3 .

If either sequence 1 or

2

above does not clearly designate the order of priority, then the evaluation o f

expressions proceeds from left

to

right.

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BASIC

The expression A+B-C is evaluated from le ft t o right as follows:

1

A+B = step

1

2 . (result of step 1)-C = answer

The expression A/B*C is also evaluated from left to right since multiplication and division are of equal priority:

1

A/B

=

step

1

2 . (result of step l)*C

=

answer

6.6.5.3

Parentheses Parentheses may be used to change the orde r

or

priority because expressions within paren -

thesis are always evaluated first. Th us, by enclosing expressions appro priate ly, the order

of

evaluation can be

controlled. Parentheses may be ne sted, that is, enclosed by o ne

or

mo re sets of parentheses. In this case, the ex-

pression within the innermo st parentheses is evaluated first, and then t he n ext in nerm ost, and so

on,

un til all have

been evaluate d. Consider th e following examp le:

The o rder of p riority is:

1 BA2 =

step

1

2 . (result of step 1)+4 = step 2

3 . (result of step 2)/X

=

step 3

4 . (result of step 3)*7 = answer

Parentheses also prevent an y confusion or d ou bt as to how th e expression is evaluated .

For

example,

A*BA2/7tB/C+DA2

((A*BA2>/7)+((B/C)+D12)

Both of these formulas will be executed in the same way. However, the second example may b e easier t o und er-

sta nd . Spaces may also be used to increase readab ility. Since th e BASIC compiler ignores spaces, th e tw o

statements:

10 LET B=DA2+1

10LETB=DT+l

are identical, bu t spaces in th e first statem ent provide ease in reading.

6.6.5.4 Rules

for

Exp one ntiatio n Th e following rules apply in evaluating the expression AAB.

1 If B=O, then A-B=l 3^0=1

3 .

4 .

If

B is an integer

X,

hen A^B=A, *A2 *A 3. . .*A,, where n=B

5 . If

B is an integer

O

then A B=l/A1 *A2*A ,.

.

.*A,), where n=B

6 .

If B is a decimal (non integ er) and A>o hen A^B=EXP(B*LOG(A))

7 .

If

B is a positive or negative decimal (nonin tege r) and A X , he

2 . If A=O and B X , hen AAB=O 0 -2=0

If A=O and B


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BASIC

6.6.5.5 Relational Op erato rs A program may require tha t two values be compared at some poin t to discover

their relation to on e anoth er.

To

accomplish th is, BASIC makes use of the following relational opera tors:

equal

=

>

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