HP 33s - Manual (Page 162)

12–4 Simple Programming
When a program finishes running, the last RTN instruction returns the program
pointer to  ! , the top of program memory.
Using RPN, ALG and Equations in Programs
You can calculate in programs the same ways you calculate on the keyboard:
„ Using RPN operations (which work with the stack, as explained in chapter 2).
„ Using ALG operations (as explained in appendix C).
„ Using equations (as explained in chapter 6).
The previous example used a series of RPN operations to calculate the area of the
circle. Instead, you could have used an equation in the program. (An example
follows later in this chapter.) Many programs are a combination of RPN and
equations, using the strengths of both.
Strengths of RPN Operations Strengths of Equations and
ALG Operations
Use less memory. Easier to write and read.
Execute a bit faster. Can automatically prompt.
When a program executes a line containing an equation, the equation is
evaluated in the same way that
X evaluates an equation in the equation list.
For program evaluation, "=" in an equation is essentially treated as "–". (There's
no programmable equivalent to
‘ for an assignment equation — other than
writing the equation as an expression, then using STO to store the value in a
variable.)
For both types of calculations, you can include RPN instructions to control input,
output, and program flow.
Data Input and Output
For programs that need more than one input or return more than one output, you
can decide how you want the program to enter and return information.
For input, you can prompt for a variable with the INPUT instruction, you can get an
equation to prompt for its variables, or you can take values entered in advance
onto the stack.