HP HP-20S - Subroutines

A subroutine is a routine that is called from (executed by) another
routine and returns control to that same routine when it finishes. The
subroutine must start with a LBL and end with a RTN. A subroutine
can call other subroutines.
If a subroutine is at the end of program memory and does not end
with (>][RTN], control is still transferred to the step after the originat-
ing when the routine completes. It is as if the subroutine had
ended with (] [RTN].
Calling Subroutines (XEQ)
Use label to call a specific subroutine. The subroutine must start
with the label A through F or 0 through 9. Searching begins at the
and proceeds down the program, wrapping around through line
00 until the label is found. Within a program, label transfers exe-
cution of a running program to the program line containing that label,
wherever it may be. The program continues running from the new
location. Then, at the next [>] statement, execution returns to
the line after the originating and continues.
For example, to write a program that calculates the average slope be-
tween x; and x, on the graph, where y = x> — sin x, you would use
the formula:
(x,” — sin x,) — (x,° — sin x;)
slope =
X, — X
6: Programming 77