A routine typically starts with a label (LBL) and ends with an instruction that alters or stops program execution, such as RTN, GTO, or STOP, or perhaps another label.

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 XBQ.

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 D04. See the flow diagrams below.

The flow diagrams in this chapter use this notation:

 !  € 1

   1

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

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

13–2 Programming Techniques

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