Because the older calculators could only display numbers, each key was assigned a code so that the programs could be read. The code was simple - each number represented either:

- The row and column on the keyboard. For example, the ENTER↑ key on the HP-65 would be code 41. All codes that don't start with 0 obey this rule. On the HP-97, keys on the right keypad were given codes beginning with −.
- A number. All codes starting with 0 represent the corresponding number. For example 04 means the 4 key.

The HP-55 had an exception to the above rules. GTO was displayed as a minus (−) thus GTO 25 would display as −25.

The HP-9100A & B don't use this convention. They list their keycodes on the pull-out reference card in their bases. If your card is missing, the keys are: (* marks codes/keys found only on the HP 9100B. Each of these codes can represent two functions which is determined by context. i.e. a 77 after a GO TO is a Sub not a Return.)

Code | Key | Code | Key | Code | Key | Code | Key | |||
---|---|---|---|---|---|---|---|---|---|---|

00 | 0 | 20 | Clear | 40 | y→() | 60 | Acc+ | |||

01 | 1 | 21 | . | 41 | Stop | 61 | Rcl | |||

02 | 2 | 22 | Roll ↑ | 42 | Fmt | 62 | To Polar | |||

03 | 3 | 23 | x→() | 43 | If Flag | 63 | Acc- | |||

04 | 4 | 24 | y⇄() | 44 | Go To () () | 64 | Int x | |||

05 | 5 | 25 | ↓ | 45 | Print/Space | 65 | Ln x | |||

06 | 6 | 26 | Enter Exp | 46 | End | 66 | To Rect | |||

07 | 7 | 27 | ↑ | 47 | Continue | 67 | Hyper or x ←()* |
|||

10 | 8 | 30 | x⇄y | 50 | If x = y | 70 | Sin x | |||

11 | 9 | 31 | Roll ↓ | 51 | 71 | Tan x | ||||

12 | e | 32 | Chg Sign | 52 | If x<y | 72 | Arc | |||

13 | a | 33 | + | 53 | If x>y | 73 | Cos x | |||

14 | b | 34 | − | 54 | Set Flag | 74 | e^{x} |
|||

15 | f | 35 | ÷ | 55 | |y| | 75 | Log x | |||

16 | c | 36 | × | 56 | π | 76 | √x | |||

17 | d | 37 | Clear x | 57 | Pause | 77 | Sub* or Return* |

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