HP 15c Collector's Edition

102 Section 9: Subroutines
Subroutine Limits
A subroutine can call up another subroutine, and that subroutine can call
up yet another subroutine. This “subroutine nesting”—the execution of a
subroutine within a subroutine—is limited to stack of subroutines seven
levels deep (this does not count the main program level). The operation
of nested subroutines is as shown below:
Main Program
b A b 1 b 2 b 3 b 4
G 1 G 3
G 2 G 4
n n n n n
End
Examples
Example: Write a program to calculate the
slope of the secant line joining points
(x
1
, y
1
) and (x
2
, y
2
) on the graph shown,
where y = x
2
− sin x (given x in radians).
The secant slope is:
y
2
−y
1
x
2
−x
1
 or
x
2
2
− sin x
2
−x
1
2
−sin x
1

x
2
−x
1
The solution requires that the equation for y be evaluated twice—once
for y
1
and once for y
2
, given the data input for x
1
and x
2
. Since the same
calculation must be made for different values, it will save program space
to call a subroutine to calculate y.
The following program assumes that x
1
has been entered into the Y-
register and x
2
into the X-register.