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Section 13: Indirect Operations 203
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Run the program now and listen to the audible tone of the HP-41C as it starts with a
low pitch, works up to a high pitch, then back down to the low pitch.
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When you run the program, it executes through the first loop until the loop control
number in R01 equals 9. The TONE
function uses the loop control number in R01 indirectly
as a specification of the TONE value. When the loop control
number equals 9, the second loop begins execution until the loop control number equals 0.
TONE uses the loop control number in
R02 indirectly as the TONE
specification. The second loop does not execute TONE 0.
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Indirect Control of Branches and Subroutines
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Like indirect addressing of storage registers, you can address routines, subroutines,
even entire programs using indirect addressing.
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To indirectly address a subroutine with an ALPHA or numeric label (e.g.,
LBL TRIGO, LBL 10), use the
GTO g nn (go to
indirect) instruction in the program. (The calculator displays the prompt
IND following the function name.)
When the running program encounters the
GTO IND nn instruction, the calculator
searches the current program for a numeric label and all of program memory for an
ALPHA label that is specified by the indirect address register. (If the label is not
found, or if the label is not a legal label—e.g., the numeric label is
grater than 99, the display shows
NONEXISTENT.) Local labels (A through J,
a through e) cannot be used indirectly with GTO .
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As an example, with the ALPHA label SOLVE stored in register
R15, when the
GTO IND 15 instruction is encountered,
execution is transferred to the last LBL SOLVE in memory.
If the label SOLVE is found. execution resumes there. A GTO
to a numeric label will not transfer execution out of a program file, but a
GTO to an ALPHA label will transfer execution out of a
program file (refer to
section 12 for a complete
discussion of label searching, branches and transferring execution).
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