Program Versus Keyboard Use Of - HP -16C Owner's Handbook Manual

Section 9: Program Branching and Controls 95
Nesting. Subroutine '"nesting"'—the execution of a subroutine
within a subroutine—is limited to a nest of subroutine returns four
levels deep (not counting the main program level).
If you attempt to call a subroutine nested more than four levels
deep, the calculator will halt and display Error5 when it
encounters the at the fifth level. Note that there is no
limitation (other than memory size) to the number of nonnested
subroutines or sets of nested subroutines that you may use.
Program Versus Keyboard Use of [GSB].
The key is used for two different functions: 1) to execute
programs from the keyboard, and 2) to call subroutines within a
running program. Note that when is used from the keyboard
to execute a program, it does not establish pending subroutine
returns. When a is subsequently encountered, program
execution halts and returns to line 000—unless there were
intervening programmed instructions (subroutine calls).