Identifying Instructions In Program Lines - HP -10C Owner's Handbook Manual

Section 5: Programming Basics 59
When a program is run, each instruction in program memory is executed—that
is, the keystroke in that program line is performed, just as if you were pressing
the key manually—beginning with the current line in program memory and
proceeding sequentially with the higher numbered program lines.
Whenever the calculator is in Program mode, the display shows information
about the program line to which the calculator is currently set. At the left of the
display is the number of the program line within program memory. The
remaining digits in the display comprise a code that indicates what instruction
has been stored in that program line. No code is shown for program line 00,
since no instruction is stored there.

Identifying Instructions in Program Lines

Each key on the HP-10C keyboard—except for the digit keys 0 through 9—is
identified by a two-digit "keycode" that corresponds to the key's position on the
keyboard. The first digit in the keycode is the number of the key row, counting
from row 1 at the top; the second digit is the number of the key in that row,
counting from left to right: 1 for the first key through 9 for the ninth key and 0
for the tenth key in the row. The keycode for each digit key is simply the digit
on the key. Thus, when you keyed the instruction . into program memory,
the calculator displayed:
This indicates that the key for the instruction in program line 03 is in the fourth
row on the keyboard and is the eighth key in that row: the . key. When you
keyed the instruction + into program memory, the calculator displayed: