HP 33s - Manual (Page 186)

13–2 Programming Techniques
Calling Subroutines (XEQ, RTN)
A subroutine is a routine that is called from (executed by) another routine and
returns to
that same routine when the subroutine is finished. The subroutine must
start with a LBL and end with a RTN. A subroutine is itself a routine, and it can call
other subroutines.
„ XEQ must branch to a label (LBL) for the subroutine. (It cannot branch to a line
number.)
„ At the very next RTN encountered, program execution returns to the line after
the originating XEQ.
For example, routine Q in the "Normal and Inverse–Normal Distributions"
program in chapter 16 is a subroutine (to calculate Q(x)) that is called from routine
D by line 
 % . Routine Q ends with a RTN instruction that sends
program execution back to routine D (to store and display the result) at line D0004.
See the flow diagrams below.
The flow diagrams in this chapter use this notation:

 ! 
€ M Program execution branches from this line to
the line marked
 M ("from 1").

  
 M Program execution branches from a line
marked
€ M ("to 1") to this line.

   Starts here.

  "! %

 %
€ M Calls subroutine Q.

 !
 N Returns here.

 #$

!  Starts D again.
 
 M Starts subroutine.
.
.
.
 !
€ N Returns to routine D.