Retro Review: HP 65
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Quick Facts
Company: Hewlett Packard
Years: 1974 - 1977
Type: Scientific, RPN (Reverse Polish Notation)
Memory: 9 memory registers, 100 steps
Batteries: originally Ni-Cad, there are battery packs that can use AAA batteries (Ebay seller: waterhosko, https://www.ebay.com/usr/waterhosko)
DISCLAIMER: I am not page for referring the website on eBay, nor I do not guarantee that inventory is available.
The HP 65 is first programmable calculator. I recently purchased a HP 65 from Persnickity Antiquity in Pomona, California. I saw that HP 65 a year earlier. What got me to purchase was it was for two reasons, (1) I have a HP 67 and fell in love with the classical HP calculators and (2), it had the AAA battery pack (see waterhosko above).Originally the HP 65 is ran with rechargeable Ni-Cad batteries.
Format Settings
The HP 65 has two format settings:
Scientific Format: [ DSP ] # (0-9)
Fixed Format: [ DSP ] [ . ] # (0 – 9)
Modifier Keys
There are three modifier keys: two orange shifts [ f ] and [ f^-1 ] and one blue shift [ g ]. The label for the orange shift is above the key and the label for the blue shift is below the key.
Inverse Key Table
[ f ] |
[ f^-1 ] |
LN |
e^x |
LOG |
10^x |
√x |
x² |
SIN |
SIN^-1 |
COS |
COS^-1 |
TAN |
TAN^-1 |
R→P (to polar) |
P→R (to rectangular) |
D.MS+ |
D.MS- |
→D.MS (decimal, minute, seconds) |
D.MS→ (decimal) |
→OCT (to octal base) |
→DEC (to decimal base) |
INT (integer part) |
FRAC (fractional part) |
SF 1/SF 2 (set flag 1 or 2) |
CF 1/CF 2 (clear flag 1 or 2) |
TF 1/TF 2 (is flag 1 or 2 set?) |
TF^-1 1/TF^-1 2 (is flag 1 or 2 clear?) |
Programming
Program Steps
In program mode, the HP 65 displays only the key code. The key code is usually two digits, the first is the row (top-down), second is column (left-right). The exception is the digit keys where they would be labeled in the format 0#.
Editing is limited to SST (single step forward) and delete key ([ g ] [ Clx ] (DEL)).
Partially Merged Steps
The first programmable calculator holds up to 100 steps. Steps are partially merged. The program commands that are merged are:
STO # (1 – 8) * does not include storage arithmetic |
Swap X and Y: x<>y |
Comparisons: x≠y, x=y, x≤y, x>y |
RCL # (1 - 8) |
Roll Down: R↓ |
All but STO/RCL are followed by the [ g ] shift key. |
NOP (No operation) |
Roll Up: R↑ |
|
Memory Registers
The HP 65 has nine memory registers R1 through R9. There is no R0 (register zero), which would be added in later calculators.
Two registers are used for specific purposes:
R8: Register 8 is used the counter in the DSZ command (Decrement and Skip if Zero command).
R9: Register 9 is used as a temporary register from trigonometric function calculations, rectangular/polar conversions, and for comparison tests, R9 is used as a Last X register.
R8 and R9 can be used for general use, but would be subject to change.
Comparisons and Labels
The HP 65 has four comparisons (x = y, x ≠ y, x ≤ y, x > y) and the DSZ command operate somewhat like most RPN calculators: if the test is true, the next step is executed. However, if the result is false, the next two steps are skipped. That’s right, the next two steps.
Example:
x > y
[ if true, x > y, execute this step; if false, x ≤ y, skip this step ]
[ also skip this step if x ≤ y ]
[ third step ]
Why two steps? The goto (GTO) command takes two steps on the HP 65. The HP 65 has 15 labels, 0 – 9 and A – E. If both steps are not needed, one can be filled by the NOP (No Op (Operation)) command.
Subroutines and Instant Labels
The labels A through E (A, B, C, D, E) are user programs that can be accessed by keys. They are the only labels act as subroutines. There is no XEQ/GSB command, subroutines are automatically called by pressing the corresponding key. Only one subroutine can be called at a time.
One quirk for the HP 65, if there is a program without a label A, then pressing [ A ] in run mode, execution starts from the first step.
The HP 65 loads five short default programs every time the calculator is turned on, which function is printed above the key in white:
[ A ] 1/x
[ B ] √x
[ C ] y^x
[ D ] R↓
[ E ] x<>y
Non-Continuous Memory
The memory on the HP 65 is not continuous. When the calculator is turned off, all memory is lost. The only way to save the memory registers and the steps require the use of thin memory cards and the built-in card reader. Each card can hold 100 steps.
I love the classic RPN HP calculators from the early 1970s. I have three of them now, the HP 45 (1973), HP 65 (1974), and HP 67 (1976).
Sources
“HP 65” The Museum of HP Calculators (MoHPC). https://www.hpmuseum.org/hp65.htm Retrieved April 1, 2026.
“HP-65 Programming” The Museum of HP Calculators (MoHPC). https://www.hpmuseum.org/prog/hp65prog.htm Retrieved April 1, 2026.
Hewlett-Packard. HP-65 Owner’s Handbook. Cupertino, CA. July 1974.
Eddie
All original content copyright, © 2011-2026. Edward Shore. Unauthorized use and/or unauthorized distribution for commercial purposes without express and written permission from the author is strictly prohibited. This blog entry may be distributed for noncommercial purposes, provided that full credit is given to the author.
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