HP HP-67 - Page 251

Using the I-Register for Indirect Control 247
Rapid reverse branching using (i and (2 (i are extremely
useful instructions as part of your programs. Rapid reverse branching
permits you to transfer execution to any step number of program
memory. With a negative number stored in the I-register, the resulting
step number can always be found by combining the negative number
in I with the step number of the (@ or (7 (@ instruction.
Execution can even be transferred backward past step 000. To find
the resulting step number of program memory, find the sum of the
negative number in the I-register and the step number containing the
@ or {@ instruction, then add 224. Thus, if the I-register
contained —12 and a (@ instruction were encountered in step
007, execution would be transferred to step 219 of program memory
(7 — 12 + 224 = 219).
Example: Named after a 13™-century
mathematician, the Fibonacci series is
a series of numbers that expresses many
relationships found in mathematics,
architecture, and nature. (For example,
in many plants, the proliferation of
branches follows a series of Fibonacci
numbers.) The series is of the form
0,1,1,2,3,5,8, 13 ..., where each
element is the sum of the two preceding
elements.
The program on page 248 contains an infinite loop that generates and
displays
the
Fibonacci
series.
Although
you
normally
would
probably
not
set
up
a
single
routine
that
began
in
step
211
and
continued
through
step
008,
the
routine
illustrates
how
the
[
instruction
coupled with a negative number in the I-register can transfer program
execution
back
in
program
memory,
even
past
step
000.