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Section 12: Subroutines 188
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First
Program
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Six returns can be pending.
Subroutines
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LBL MAIN
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LBL 01
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LBL 02
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LBL 03
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LBL 04
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LBL 05
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LBL 06
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XEQ 01
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XEQ 03
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XEQ 05
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XEQ 02
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XEQ 04
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XEQ 06
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RTN
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RTN
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RTN
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RTN
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RTN
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RTN
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END
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The calculator can return back to the first program from subroutines that are six
deep, as shown. However, if you call up subroutines that are more than six deep,
the calculator will return only six subroutines deep. For example, if you call up
seven subroutines deep, when the seventh subroutine is completed, execution will
transfer back only six subroutines, back to the second subroutine executed.
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Naturally, the calculator can execute an END or
RTN instruction as a stop any number of times. Also,
if you execute any of the subroutines manually from the keyboard (or you
press g
RTN ) all pending END
and RTN instructions are forgotten by the calculator.
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Single-Line Execution of Subroutines
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If you are executing a program one step at a time with SST
key in normal mode, and encounter an XEQ instruction, the
calculator will then transfer execution to the specified subroutine. You can
then execute the subroutine one line at a time with SST .
When you encounter the END or RTN
in the subroutine, execution transfers back just like a running program. You can
execute programs this way, with SST , and the HP-41C will
remember up to six pending returns, as in a running program.
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Local Labels
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Earlier in section 7
you learned how to label or name a program with a string of ALPHA characters.
There are 15 ALPHA labels on the HP-41C that have special functions that are
called ‘‘local labels.’’ These labels are
LBL A through LBL J and
LBL a through LBL e (shifted A
through E). Any time you label a portion of a program or a subroutine with one of
these labels, it is a local label.
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When the HP-41C is in USER mode and you press one of the keys in the top two rows
(or g and a top row key),
the calculator immediately begins searching for the corresponding (A through J, a
through e) local label within the current program. If the local label is not
found, the calculator executes the function printed on the face of, or above, the
key.
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For example, when you press Σ+ in USER mode, the
calculator first searches for a LBL A instruction in the
current program. The calculator searches downward from the current position in
program memory to the end of the program. Then it begins searching at the beginning
of the program and back to where the search began.
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