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BASIC on the Altair

A program that makes a decision

Let a program choose what to do with an answer. Compare the number of tickets with zero, follow a different path when there are none, and catch a negative answer before calculating a price.

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Continue your session, or load BASIC

If the terminal below already shows your earlier BASIC session, press the browser's RUN button to continue it. You do not need to load the tape again. This tab keeps the machine, program, terminal, and pending input as you move between BASIC lessons, visit All lessons, or refresh. Returning always pauses execution first. Other tabs work independently; a newly opened or duplicated tab may begin with a copy of this tab's saved session.

Begin a lesson's commands at OK. If your previous program is still asking a question at ?, finish that conversation first; the ticket loop finishes when you answer 0. The lesson's setup explains which stored lines to keep or replace. Typing NEW at OK clears the stored program and variables without reloading BASIC.

If this is a fresh machine, follow the loading steps below.

This is the original 4K BASIC 3.2 interpreter running on our Altair. An interpreter is a program that reads your commands and carries them out. Before it can understand PRINT, the Altair must load that interpreter into memory.

  1. Get the selected paper-tape file from the machine guide (opens in a new tab). Return here and choose it in Paper tape below. The file stays on your computer.
  2. Raise only switches A11 and A10. Leave PC at 0000: do not press EXAMINE. You may then close the front panel to bring the terminal closer.
  3. Press the browser's RUN button. Click Terminal keyboard and answer each question when it appears:
When you see Enter
MEMORY SIZE? Just Enter, to let BASIC find the available memory.
TERMINAL WIDTH? Just Enter, to keep the default.
WANT SIN? Y then Enter. We will not need this math function today.

When 727 BYTES FREE and OK appear, BASIC is ready. Leave the processor running for the lesson. Switching tabs pauses it; press the browser's RUN button when you return.

If you need to start loading again, use Start fresh, raise A11 and A10 again, then RUN. This discards the current BASIC program and uses the tape you already selected. Reset CPU + serial does not reload BASIC.

The saved session stays in this browser tab's temporary storage. It is not sent to the site. Copy your numbered lines somewhere to keep them beyond the browser session. If the browser cannot save or restore a session, a message beside the machine explains that; you can still use the current page.

A computer waiting for your instructions

From paper tape to BASIC.

Resume a saved session with RUN. To load BASIC for the first time, choose the 4K BASIC 3.2 tape (guide opens in a new tab), raise the loading switches, then RUN. Loading instructions

This tab can keep your BASIC session between lessons. Checking for a saved session…

The file stays on your computer. Selecting a verified tape prepares a fresh, stopped machine.

No tape selected.

Front panel · address, data, and sense switches

To load BASIC, raise only A11 and A10 and leave PC at 0000. Do not press EXAMINE after raising the loading switches. You can then close this panel to bring the terminal closer.

ALTAIR 8800

Memory and sense switches

Address lights PC · next instruction

0 decimal · 0000 hex

Data lights The byte at that location

0 decimal · 00 hex

A lit lamp is 1; a dark lamp is 0. The digits help you read the lights.

Data / address switches Up = 1 · Down = 0

15–8 · Address + sense

7–0 · Address or data

All switches as an address
0 decimal · 0000 hex
Lower eight as a byte
0 decimal · 00 hex

Setting the switches prepares a number. It does not read or write memory. The CPU can read the upper eight switches at port FF, even while running.

Select the address in the switches and read its byte.

Move to the next address and read its byte.

Write the lower eight switches to the selected address.

Move to the next address, then write the lower eight switches.

EXAMINE would select address 0. DEPOSIT would write 0 to the selected address, 0.

Last memory operation

Enable JavaScript to operate the panel.

EXAMINE and NEXT change PC. Only 0000–0FFF contain RAM; other addresses read FF and discard deposits. Lights show instruction boundaries, not individual bus cycles.

Stopped · 0 instructions

RUN lets the processor execute; STOP pauses it. At BASIC’s prompt, type RUN to run your BASIC program. Break sends Control-C to BASIC.


  
  

    
    
  

  
0 keyboard bytes queued

  
Use uppercase commands. Enter sends a line; Backspace sends BASIC’s erase character (_).
    Paste one line at a time. Only the machine’s echoes appear above.

  

  
Inspect the running machine
    

    
Latest twelve instructions on this page · PC before → after (hexadecimal)

    

  

  
Reset keeps RAM and the switches but resets the CPU and serial device. Start fresh discards the program, restores the bootstrap,
    rewinds the selected tape, and clears the terminal; raise the switches again before RUN.




  

    
Start with the question


Load or resume BASIC using the instructions above, and begin at OK. If you
continued directly from the previous lesson,
keep its four-line ticket program and skip the setup block below. Each ticket
costs 2 pounds; N holds the number of tickets.


If you are starting here or returning from a different lesson, type NEW to
clear the stored program and variables, then enter these four lines.


Type one numbered line at a time, pressing Enter and waiting for the complete
echo before sending the next:


Type, then Enter
NEW
BASIC replies
OK
Type one line at a time, then Enter
10 PRINT "HOW MANY TICKETS"
20 INPUT N
30 PRINT N*2
40 END


So far, every answer takes the same route: ask, receive, calculate, finish.
We would like zero tickets to produce the message NO TICKETS instead of a price.


Ask a yes-or-no question


Add these three lines:


Type one line at a time, then Enter
25 IF N=0 THEN 60
60 PRINT "NO TICKETS"
70 END


Read line 25 as “if N equals zero, go to line 60.” The comparison N=0 is a
condition: it can be true or false. When it is true, BASIC continues at
line 60. When it is false, BASIC continues at the next stored line, 30.


Here = tests a value. It does not change N. In an assignment such as
LET N=0, it gives N a new value; IF N=0 asks whether the value already
there is zero.


The 60 after THEN is a line number to go to, not a value to print. We inserted
line 25 between 20 and 30 without having to renumber either of them. LIST shows
the result in line-number order:


Type, then Enter
LIST
BASIC replies
10 PRINT "HOW MANY TICKETS"
20 INPUT N
25 IF N=0 THEN 60
30 PRINT N*2
40 END
60 PRINT "NO TICKETS"
70 END
OK


Add the decision ↑


Try both paths


RUN the program and answer 0 at the question mark:


Type, then Enter
RUN
BASIC replies
HOW MANY TICKETS
?
Type, then Enter
0
BASIC replies
NO TICKETS

OK


The condition is true. BASIC goes straight from line 25 to line 60, prints the
message, and finishes at line 70. Lines 30 and 40 are passed over.


Now RUN again and answer 3:


Type, then Enter
RUN
BASIC replies
HOW MANY TICKETS
?
Type, then Enter
3
BASIC replies
 6

OK


The condition is false. BASIC continues from line 25 to line 30, calculates
6 pounds, and finishes at line 40. It never reaches the NO TICKETS message.






Answer
Is N equal to zero?
Lines followed after INPUT






0
Yes
25 → 60 → 70




3
No
25 → 30 → 40






Why keep END on line 40? It finishes the price path before the message on
line 60. A line number such as 60 gives us somewhere to go; it does not make
that line run only when chosen by IF. Execution can also reach a line by
continuing from the one before it.


Both paths belong to the same stored program. Each RUN asks for a new answer,
and that answer decides which path is taken. Leave the browser's processor
RUN control running while BASIC waits for input or returns to OK.


Try the two answers ↑


Catch an impossible answer


Zero tickets makes sense. Minus one ticket does not. Our program would still
multiply a negative answer by 2, so add a check before either existing path:


Type one line at a time, then Enter
22 IF N<0 THEN 80
80 PRINT "PLEASE USE ZERO OR MORE"
90 END


The sign < means less than. Line 22 asks whether N is below zero. If so,
it goes to line 80, prints our explanation, and finishes at line 90. Otherwise
it continues to line 25 and the zero check we already wrote.


Notice that 22 belongs between INPUT on line 20 and the earlier decision on
line 25. LIST now shows the complete program:


Type, then Enter
LIST
BASIC replies
10 PRINT "HOW MANY TICKETS"
20 INPUT N
22 IF N<0 THEN 80
25 IF N=0 THEN 60
30 PRINT N*2
40 END
60 PRINT "NO TICKETS"
70 END
80 PRINT "PLEASE USE ZERO OR MORE"
90 END
OK


RUN and answer -1:


Type, then Enter
RUN
BASIC replies
HOW MANY TICKETS
?
Type, then Enter
-1
BASIC replies
PLEASE USE ZERO OR MORE

OK


This is a message we wrote, not a BASIC error. The program finishes without
printing a price. It does not ask again automatically; type RUN for another try.


Check the boundary again


After changing a program, check the paths that already worked as well as the
new one. Predict the replies for 0 and 1. Neither is less than zero, but
only one is equal to zero.


Type, then Enter
RUN
BASIC replies
HOW MANY TICKETS
?
Type, then Enter
0
BASIC replies
NO TICKETS

OK
Type, then Enter
RUN
BASIC replies
HOW MANY TICKETS
?
Type, then Enter
1
BASIC replies
 2

OK


We have tried a number just below zero, zero itself, and a number just above
zero. These answers exercise all three paths:






Answer
Lines followed after INPUT
Result






-1
22 → 80 → 90
Explain that the number must be zero or more.




0
22 → 25 → 60 → 70
Say there are no tickets.




1
22 → 25 → 30 → 40
Print the price, 2 pounds.






We still expect whole numbers of tickets. This check rejects negative numbers;
it does not reject a fraction such as 1.5. The program can only check the rules
we have given it.


In the next lesson, we will make the program
ask again after an unsuitable answer, then let it calculate several prices
in one run.


Check all three paths ↑


The original Altair BASIC manual
describes comparisons on printed page 28 and IF…THEN on printed page 33.

    
You can keep experimenting in the terminal. This tab carries the BASIC machine and program between these lessons,
      including a visit to All lessons, and restores them paused after a refresh. Press the browser's RUN button to continue.
      Start fresh discards the program and reloads the retained tape. Copy your numbered lines somewhere for a lasting copy;
      browser session storage is temporary.

    
The machine guide (opens in a new tab)
      explains the hardware, loading process, and recovery controls. These lessons use that same machine and original interpreter.