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Controlling the I-Register
217
In practice, you will find that you will usually use (7] and
with
numbers that are integers, since these instructions are most useful
as counters—that is, to control the number ofiterations of a loop—
and to select storage registers, subroutines, or display settings.
(More about using the I-register as a selection register later.)
The
(decrement I, skip if zero) instruction operates in the same
manner as the increment instruction, except that it subtracts, rather
than adds, one each time it is used. When a running program
executes an
instruction, for example, it subtracts 1 from
the contents of the I-register, then tests to see if the I-registeris 0.
(A number between +1 and —1 tests as zero.) If the numberin the
I-registeris greater than zero, execution continues with the next step
of program memory. If the number in the I-register is zero, the
calculator skips one step of program memory before resuming
execution.
Example: The island of Manhattan
was sold in the year 1624 for $24.00. {%
The program on the next page shows &=
how the amount would have grown
each year if the original amount had j §
been placed in a bank account drawing
5% interest compounded annually. The !
number of years for which you want to
see the
amount is
stored in the
I-register, then the
instruction
is used to keep track of the number of
iterations through the loop.
Were you to prepare a magnetic card to store this program, it might
look like this:
{1 mAnN /7'79'77;'7/(/ VALLVE
)
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