HP 9100A/B Programming

HP 9100A/B Programming The Museum of HP Calculators Warning: On these calculators the ENTER button

HP 9100A/B Programming

The Museum of HP Calculators

Warning: On these calculators the ENTER button reads programs from magnetic
cards. It is called “the ENTER button” in this text. The key that enters
numbers on the stack is labeled with an upward pointed arrow but is shown
in this text as “ENTER^”. The downward pointed arrow is shown as “drop” here.



  • Memory space shared between program and registers with no sizing command
    (allows self-modifying code.)
  • Up to 16 registers on the HP 9100A. Up to 32 on the HP 9100B.
  • Up to 196 program steps on the HP 9100A. Up to 392 on the HP 9100B. (Unmerged
    – each keystroke takes a step.)
  • An external memory expansion option adds up to 248 registers or 3472
    program steps.
  • Core memory requires no power to maintain state.
  • Step number addressing.
  • Unconditional and conditional branching based on register values.
  • Subroutines on the HP 9100B only. (Possible by self-modifying code on the

Basic Programming

Entering And Running A Sample Program

A simple program is essentially just the keystrokes you would press to solve
the program manually. The calculator remembers a sequence of keys and executes
them in order at the touch of a key. In the simplest programs, there is only
the addition of END keystroke.

To enter a program, make sure the calculator’s PROGRAM/RUN switch is set
to RUN and press END. This sets the program counter to 00.  Now switch
to PROGRAM mode.

With the PROGRAM/RUN switch set to PROGRAM the number you see on the left
side indicates the step number. The HP 9100A shows each program step in the
X register. The HP 9100B shows the current step in the Z register and the
previous step in the X register. (Don’t count on the previous step being
meaningful on line 00.) As keystrokes are entered, their octal key
codes will be displayed on the right.
The first key you press will go into step 00.

Enter the program below to compute the area a circle:

ENTER^     This key is labeled with an up arrow
x          Square the radius
x          Multiply by PI

Now set the PROGRAM/RUN switch to RUN and reset the program counter  to
00 by pressing END. To run the program key in a radius and press CONTINUE
(CONT on the HP-9100B) and the area will be displayed in the Y register almost
instantly. The program can be run as many times as you like by entering new
values and pressing CONTINUE. The display will blank during programs, but
the calculator is so fast that the simple examples shown here will cause
a barely noticeable blink. For many simple programs, that’s all you need
to know!

Stopping, Interrupting and Entering Data

Many programs only require data to be entered at the beginning as the one
above did. Remember that you can use the stack and store values in registers
before execution. (Use x->() or y->() to store numbers. a-f are recalled
by just pressing the keys, 0-9 are recalled with y<>() or x<-()
on the HP 9100B.) 

You can also enter STOP instructions into the program to allow additional
data to be entered. The user can enter the data and then press CONTINUE.
If you create a program that runs for too long (possibly due to an infinite
loop) you can stop it by pressing STOP and if necessary, resume it by pressing
CONTINUE. If you press STOP in the middle of a program, you can see where
the program is executing by switching to PROGRAM mode.


The HP 9100A has 16 registers of core memory. 14 of these registers (0-d)
can be used to hold 14 program steps each which allows up to a maximum of
196 steps with two storage registers e and f. The memory layout is shown

There is no command to determine how much space is dedicated to storage registers
versus program code. Use is determined “on the fly” and you can overwrite
code (intentionally or accidentally) by storing to a register via y->(),
y<>() etc. Generally, you should start programs at step 00 and use
registers starting from the top down in order to avoid a collision. Besides,
the upper registers a-f can be recalled with one key whereas the only way
to retrieve a value from registers 0-9 on the HP 9100A is to swap it via
the y<>() instruction. The x<-() key makes this easier on the HP
9100B but it still requires one more keystroke than the alpha registers.

The HP 9100B has two pages of core labeled + and -. GO TOs are assumed to
go to the current page unless a + or – precedes the address like GO TO +
n n. Storage and recall operations assume the + page unless the minus page
is specified like y->() – a to store in register a on the – page. (Precede
the address with the minus key – don’t use the CHG SIGN key.)

As an optimization on either calculator, consider placing program initialization
code at the highest addresses of the program where it can be overwritten
by register stores when it is no longer needed.

Stepping And Editing programs

STEP PGRM steps one instruction ahead. In RUN mode the next instruction is
executed and the X register is then displayed making this instruction useful
for debugging. It can also be used for moving forward without execution in
PROGRAM mode. This command is not recordable.

GO TO nn can be used in RUN mode to position the calculator to step nn. The
calculator can then be switched to PROGRAM mode for editing at that step.
This function is recordable so the position of the PROGRAM-RUN switch must
be properly set.

Entering an instruction causes the new instruction to overwrite the one currently
displayed. (Most HPs add or overwrite new instructions after the one
currently displayed.) To replace a single instruction, display the instruction
and key in the correct instruction which will replace the error. If you find
that you need to add instructions to a program, you can do one of
the following:

  1. Rewrite it from the point of the error on down.
  2. Insert a GO TO nn at the point where the new instructions would be needed,
    perform the new steps elsewhere and use a GO TO to go back to the step after
    the first GO TO. Remember to include the instructions that were overwritten
    by the first GO TO in your new block of code. Or
  3. Use the card reader to patch the program. (See Saving and
    Loading Programs
    below for an example of this method).

Here’s an example of inserting a GO TO (method 2):

Original code;

10 -
11 y->() 
12 4             Oops! Need to square x and take the log before storing!
13 5
14 *

Patched code:

10 -
11 GOTO ()()
12 4
13 0              Overwrite store 4 & PI with a jump to some unused memory
14 *              Return here
15 ...
40 ENTER^         The new steps
41 x              square x
42 log x
43 y->()          The steps overwritten by GO TO 40
44 4
45 5             
46 GO TO ()()     Could have used GO TO SUB / RETURN on the HP 9100B
47 1
48 4              Return to the normal program flow

Saving and Loading Programs

To save a program to magnetic card, set the PROGRAM-RUN is switch to RUN
(this is different from most HPs). Hold the card with the label facing the
you and with the the arrow of the track you want to record facing down and
insert the card all the way into the read/writer slot.

Press GO TO n n to select a position to begin writing. (END is a handy shortcut
if you want to start at 00.) Now press the RECORD button and the card will
rise back out of the slot. The X register is destroyed when cards are written.

The first 14 registers (0-d inclusive) are written to the card (up to the
END statement.) On the HP 9100A, the calculator’s memory is stored on just
one track of a card and the other side is available for a second program.
The HP 9100B requires both tracks to save its entire core memory so after
recording side A, reverse the card (still label toward you) to record side
B.  (The calculator does you the favor of doing a GO TO – 0 0 to get
ready to write the second page after recording track A. Press GO TO – n n
if you want to start from a different address on the second page.)

Note that while it’s possible to write functioning programs without END
statements, programs recorded without END statements may or may not
read back properly. If programs steps are recorded in both pages of the HP
9100B, each page should have exactly one END statement.

You may cut off the corner of a magnetic card to prevent it from being
overwritten. Don’t clip more than shown below or you could loose data. Cards
can be marked with a permanent felt tip pen or lightly with a soft pencil.

To load a program from a magnetic card, make sure the PROGRAM-RUN is switch
set to RUN and place a card (label side toward the keyboard) all the way
into the card reader/writer.

Press GO TO n n to select a position to begin reading. (END is a handy shortcut
if you want to start at 00.) Now press the ENTER button next to the reader
and the card will come back out of the slot. Remember to GO TO – n n (usually
0 0) and read side B if you’ve recorded both pages on an HP 9100B. The stack
is undisturbed when cards are read.

Here’s an example of using the card reader to insert three program steps
(method 3 from stepping and editing above). Suppose you need to insert two
steps at line 42. Make sure the PROGRAM/RUN switch is set to RUN and press
GO TO 42 to position the calculator. Now record the memory from steps 42
and up onto the card by inserting the card and pressing the RECORD button.
Now insert the two instructions at line 42 by pressing GO TO 42, switching
to PROGRAM mode and keying the instructions.

Now (leaving the calculator positioned after the instructions just entered)
switch the calculator back to RUN, insert the same program card and press
the ENTER button next to the card reader. The calculator will read the rest
of the program back into memory after the insertion. Note, however, that
you’ll need to adjust any GO TOs that reference addresses in the moved

Saving and Loading Data

It’s possible, though a bit tricky to store data on a card along with the
program. The tricky part is that you need to have your program end after
the the data you want to save. For example if you wanted to preserve the
data in registers up to c, you could reserve register d for the end of your
program. At the end of your program, add GO TO d 0.  Then switch to
RUN mode and press GO TO d0 and switch back to PROGRAM mode and press END.

Then you must fill all program memory from the end of your program (right
after the GO TO d 0) up to the first data register you want to save with
CONTINUE operations which serve as No-ops.  For example, if your program
ended with GO TO d 0 at step 65 and your data data is in registers 8-c, you
would place CONTINUE operations in steps 66-7d. (The reason for this is to
insure that there isn’t another END lurking in this unused space that would
end the writing earlier.)

Then you would switch to RUN mode, and save the program as usual. You can’t
save e and f since they are out of program space.

An alternative method is to leave your program unchanged and simply start
the recording at the lowest data register to save. For example, to save a-d,
press GO TO a 0 and then insert your card and press the RECORD button. The
calculator starts writing and when it gets to the end of memory, it continues
writing the card starting at step 00 because it still hasn’t found an END.
This is tempting because it is much simpler for writing but remember
that you must always press GO TO a 0 before reading this card back in.

You should make a note of this requirement on the card.

Keep in mind that if you’re using one of these methods to store constants
that are used just once or twice in the program, it may be more space efficient
to simply write them into the code. For example 3.717 requires 5 characters
as code but an entire 14 character register when stored as data.

Programming Techniques


The HP-9100A & B use step addressing allowing you to jump to any line.
Addresses are two digit numbers in base 14 of the form 01, 02, … 09, 0a,
0b, 0c, 0d, 20, 21, … 2c, 2d, 30, … dd.

The HP 9100B has two pages of core labeled + and -. GO TOs are assumed to
go to the current page unless a + or – precedes the address. (Storage operations
are assumed to refer to + unless overridden by -.)


Both calculators have:

  • GO TO nn where -dd <= nn <= (+)dd jumps to that step number. nn is
    a base 14 number (see the memory layout above) so each
    digit should be less than d.
  • END sets the program counter to 0 0 and stops execution.

The HP 9100B adds:

  • GO TO SUB nn saves the return address and branches to address nn. The function
    at nn must end with a RETURN. The HP 9100 can nest subroutines 5 levels deep.
    To refer to the other page, add a page specifier like GO TO SUB + 5 d.
  • RETURN returns from a subroutine that was called by GO TO SUB.

Don’t worry that SUB and RETURN are the same key. The calculator knows which
you mean because SUB always immediately follows GO TO and RETURN never does.

The program below adds 1 to the X register, displays it for a second and
then repeats. Key it in after pressing END and switching to PROGRAM mode.

1             top of the loop
PAUSE         display for 1/8 second
GO TO ()() 
0             loop

Now switch to RUN mode, press END, key a number, press ENTER^ and press CONTINUE
(CONT on the 9100B). When you get tired of it, press STOP.  (You might
have to press STOP a few times, because of the PAUSE.)

The following program calculates the volume of a cylinder by first calling
a subroutine to calculate the area of its base and then multiplying by the

GO TO ()()
8             Call the subroutine at line 08 to calculate the area
x             Multiply by the length in y
STOP          Display it
GO TO ()()    This shouldn't be an END!
0             Ready for the next case
ENTER^        This key is labeled with an up arrow
x             Square the radius
PI            Multiply by PI 
x             (Now have X=PI, Y=area, Z=length)
roll dn       Move area to X, length to Y

Now switch to RUN mode and press END. Key in a cylinder length, press ENTER^,
key in a radius and press CONTINUE. The volume will be displayed in the Y

You may be wondering why the program used GO TO 0 0 in the middle rather
than an end. The calculator requires any program that is written to a magnetic
card to have exactly one END instruction at the bottom of the program. Even
if you’re not recording your program, it’s good to get in the habit of using
END only at the end of a program.

Here’s an example of a factorial program for integers <= 69.  Press
END, switch to PROGRAM mode and enter:

ENTER^   loop here
IF x>y   If we're below 2, we're done
1        Go to the exit routine
1        at step 11 (remember addresses are base 14)
drop     (down arrow on the keyboard)
1        compute n-1
drop     (down arrow on the keyboard)
*        n*n-1 (leaving n-1 in the x register - very handy)
drop     exit routine (step 11): Leave result in y

Now switch to RUN mode, press END, press 5 and press CONTINUE. The result
of 120 will be displayed in the Y register. Try some larger numbers. Even
the maximum computable input of 69 takes under a second.

Conditional Tests and Flags

The HP 9100 has three instructions for comparing X to Y.

  • IF x < y
  • IF x = y
  • IF x > y

If the condition is false, the next two steps are skipped. If the condition
is true, execution continues with the next step. When the condition is true,
the calculator examines the next two steps and if they contain an address,
then the calculator acts as if the Conditional test was a GO TO and branches
to that address. If they are not an address, then the calculator simply executes
them with no implied GO TO.

Two program fragments follow which show the conditional used both as an implied
GO TO and as a conditional skip.

IF x < y      If x is equal to y
c             GO TO (implied) step C4
cos x         else compute the cosine of x

IF x = y      If x is equal to y
PI            Add PI to y
x<>y          Execute these steps regardless of conditional
cos x         compute cos of x or x+PI

The calculators have a single flag which is set by the SET FLAG instruction
and tested by IF FLAG. IF FLAG acts like a conditional test skipping two
steps if the FLAG is not set. The flag is cleared by testing it or whenever
a CLEAR is executed. (Note that CLEAR also clears the stack and registers
e and f.)


STOP can be used as an instruction or pressed from the keyboard. If a program
is stopped, pressing CONTINUE (CONT on the HP 9100B) starts it. If the program
is running, pressing STOP stops it. A STOP can also be inserted into a program
to allow the user to input or record data.

For example, this program accepts the length and radius of a cylinder. It
computes the area of the base and stops to allow the answer to be recorded.
When the user resumes, it computes the volume.

x       Radius squared
x       Area of the base
STOP    Allow user to record it
drop    (Down arrow key)
x       Multiply by length still on the stack

Set the PROGRAM-RUN switch to RUN, press END, key in a length and press ENTER^.
Then key a radius and press CONTINUE. After the area is displayed, press
CONTINUE again to display the volume. (Both results are displayed in Y.)

For brief displays, one or more PAUSE instructions may be used to show the
display for 1/8 of a second. If the PAUSE key is held down during program
execution, a STOP will occur at the next PAUSE in the program. (Press Continue
to resume the program in this case.) Using the PAUSE in this way allows you
to watch the progress of a program and optionally make adjustments to the

The PRINT/SPACE key is used with the optional printer. If no printer is attached,
it behaves like STOP so the user can record the display.

The illegal operation lamp is to the left of the display and will light on
operations such as divide by zero, sqrt(n) where n<0 etc. It will not
stop a running program but will remain lit until a key is pressed.

back to the HP 9100A/B

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