[Is an explanation of BASIC needed?]

TWENEX's BASIC interpreter is LOTS BASIC, developed at Stanford University.

The following tutorial is adapted from chapter 17. “Using BASIC” in LOTS DECSYSTEM-20 Overview Manual¹⁾.

For those already familiar with BASIC and who would like a summary of LOTS BASIC commands, skip the following example and see BASIC Command Summary.

Here is an example of how you might start BASIC, enter a program and run it, then leave BASIC. In the example, what the user types in is underlined. We start in the EXEC, which prompts with the @. The BASIC command starts the BASIC program.

@basic
Welcome to LOTS-BASIC
For help, type HELP
To exit, type MONITOR

BASIC types a greeting message, spaces down a line and stops. We may no begin typing in our program, including the line numbers. At the end of each line we press the RETURN key; BASIC will then be ready to accept another line or a command.

10 print “Type in a number”
20 input a
30 if a=0 go to 70
40 let s2=a*a
50 print “The square of”,a,“is”,s2
60 go to 10
70 stop

We have finished typing in the program, so we ask BASIC to run it by typing the RUN command. BASIC knows this command is not part of the program because it does not begin with a line number. BASIC types out the name of the program and the date and time. The heading “NONAME” stems for the fact that we haven't named the program yet.

run
NONAME 17:00 10-Aug-84
? NO END INSTRUCTION
TIME: 0.04 SECS.

READY

BASIC tells us that the program needs an END statement, then it tells us how long the run took. When BASIC types READY, it means that it is waiting for another command or a new program line. We need to add an END statement, which is always the last line in a program. We can give it any number greater than 70. The line numbers aren't required to be incremented by tens.

100 end

Now our program should run, so we try it again. The “?” tells us that we should type something in. We type in a 15 and BASIC tells us the square of it.

run

NONAME 17:01 10-Aug-84

Type in a number
? 15
The square of 15 is 225
Type in a number
? 666
The square of 666 is 443556
Type in a number
? 0
TIME: 0.24 SECS.

READY

By the way, you probably understood the program as it was written, without any commentary on our part; this illustrates the advantage of BASIC: it is easy to understand. Now let's add one more line.

55 print “And that's the truth!”

It makes perfect sense to insert this line between lines 50 and 60, and this is what BASIC does. Let's see what the program looks like now. The “NH” we append to the “LIST” stands for “no header”.

listnh

10 PRINT “Type in a number”
20 INPUT A
30 IF A=0 GO TO 70
40 LET S2=A*A
50 PRINT “The square of”,A,“is”,S2
55 PRINT “And that's the truth!”
60 GO TO 10
70 STOP
100 END

READY

We rerun the program to see the effect of the new line.

runnh

Type in a number
? 4
The square of 4 is 16
And that's the truth!
Type in a number
? 0
TIME: 0.19 SECS.

READY

Having put so much effort into our program, we would like to keep it around for later use. In saving it, we decide to give it the name TEST.

save test

READY

In the future, when we are in BASIC and want to retrieve this program, we would use the command “OLD TEST”. We have reached the end of our example, se we return to the EXEC.

monitor
@

We will find our saved program on our disk area under the name TEST.BAS.

This is a summary of commands in version 17D of LOTS BASIC.

You should read a tutorial on using one of the available text editors before reading this tutorial. This tutorial will assume that you're comfortable running programs from the EXEC and creating text files.

For a full guide to the BASIC language available, see the original manual.

This section will explain the fundamentals of creating a BASIC program, compiling and running it.

BASIC programs require line numbers. You should increment each line by 10 to allow for adding lines in between later.

Start by creating a file with the file extension of .B20 with the contents:

10 PRINT “Hello, World!”

This is the infamous Hello, World! program for the TOPS-20 flavor of BASIC.

To run the program, start the BASIC program:

@BASIC

You will now need to load your program. At the READY prompt, type:

OLD

You'll then be prompted for the filename of the source file you created. For instance, if you called the file HELLO.B20 then that's what you should enter.

At the next READY prompt, type RUN to run the program. You'll see “Hello, World!” printed on the output.

In order to build the program for use without the BASIC program, type BUILD. Then return to the EXEC by typing MON. Assuming the program was named HELLO.B20, you should now see a HELLO.EXE in the same directory. When you run this, you'll see the “Hello, World!” output as before.

Variable assignment in BASIC is pretty similar to every other programming language. It takes the form of:

VAR = VALUE or (optionally) LET VAR = VALUE

Variables do not need to be declared before use. Standard arithmetic operations like +, -, / and * are available for use with either number literals or variable names.

BASIC variable types are determined by an optional suffix to the variable name. A suffix of $ indicates that the variable holds a string. A suffix of % indicates that the variable holds an integer. No suffix indicates that a variable holds a number (possibly with fractional part).

It is important to remember that X, X$ and X% are all different variables, not just different interpretations of the same variable.

Reading input from the terminal is rather simple. The function used is called INPUT.

The value read can be a number, integer or string depending upon the variable type given as a parameter.

The “Hello, World!” program can be extended to be personalised like so:

10 PRINT "Enter your name:"
20 INPUT X$
30 PRINT "Hello, " + $X + "!"

This is also an example of string concatenation.

Given that every line has a line number, it can become tempting to use the GOTO function. This function gives an unconditional jump to the specified line number. This is a bad idea.

“The unbridled use of the go to statement has as an immediate consequence that it becomes terribly hard to find a meaningful set of coordinates in which to describe the process progress. … The go to statement as it stands is just too primitive, it is too much an invitation to make a mess of one's program.” – Edsger Dijkstra (March 1968). “Go To Statement Considered Harmful”. Communications of the ACM 11 (3): 147–148.

There are three structured programming loop structures available for use in BASIC: for, while and do.

Users should feel free to add their programs to this section.

• Using the BASIC Interpreter
• Advanced BASIC Concepts

• TOPS-20 BASIC-PLUS-2 Language Manual