HP -25 Owner's Handbook Manual page 80

As you can see, the program memory for the HP-25 is separate
from the four stack registers, the LAST X register, and the
eight storage registers.
With[ 00 ]displayed in PRGM mode, you are ready to
key in your program. Surface area of a sphere is calculated using
the formula A = 7d?. The short list of keys for the area of a
sphere program is shown below:
Keys Comments
} These keys square the diameter.
} These keys place 7 in the X-register.
This key multiplies d* by .
Keycodes
Press the first key of the program and the display will change to:
The two numbers on the right of the display designate the key
stored in that step. Each key on the keyboard has a two-digit
keycode. For convenience, the digit keys are coded 00 through
09. All other keys are coded by their position on the keyboard.
The first digit denotes the row of the key and the second digit the
number ofthe key in that row. So 15 tells you that the key is in
the first row on the calculator and that it is the fifth key in that
row, the Elkey.
1st Row ==
ST0 RCL
5th Key
This handy matrix system allows you to easily determine the
code for each instruction without using a reference table.