HP 15c - Manual (Page 67)

Section 6: Programming Basics 67
Location in Program Memory. Program memory – and therefore the
calculator's position in program memory – is demarcated by line numbers.
Line 000 marks the beginning of program memory and cannot be used to
store an instruction. The first line that contains an instruction is line 001.
Program lines other than 000 do not exist until instructions are written for
them.
You can start a program at any existent line (designated nnn), but it is
simplest and safest to start an independent program (as opposed to a
subroutine) at the beginning of program memory. As you write, any existing
program lines will be preserved and ―bumped‖ down in program memory.
Press t “ 000 (in Program or Run mode) to move to line 000
without recording the t statement. In Run mode, ´ CLEAR M
will also reset the calculator to line 000- without clearing program memory.
Alternatively, you can clear program memory, which will erase all
programs in memory and position you to line 000. To do so, press ´
CLEAR M in Program mode.
Program Begin. A label instruction – ´b followed by a letter
(A through E) or number (0 through 9 or .0 through .9) – is used to
define the beginning of a program or routine. The use of labels allows you
to quickly select and run one particular program or routine out of several.
Keystrokes Display
´ CLEAR
M 000- Clears program memory and
sets to line 000 (start of
program memory).
´ b A 001-42,21,11
Recording a Program. Any key pressed—operator or constant—will be
recorded in memory as a programmed instruction.
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Except the nonprogrammable functions, which are listed on page 80.