HP HP-67 - Incrementing and Decrementing the I-Register

Controlling the I-Register 215
Incrementing and Decrementing
the I-Register
You have seen how a number can be stored in the I-register and then
changed, either by storing another number there, or by using the
0 operation.
You will find either of these methods useful, whether you are
utilizing them as instructions in a program or using them manually
from the keyboard.
Another way of altering the contents of the I-register, and one that
is most useful during a program, is by means of the B8 (7]
(increment I, skip if zero) and £ (decrement I, skip if zero)
instructions. These instructions either add the number 1 to (increment)
or subtract the number 1 from (decrement) the I-register each time
they are executed. In a running program, if the number in the
I-register has become zero, program execution skips the next step after
the (2] or instruction and continues execution (just like a false
conditional instruction).
The 3 (52) and 7 instructions always increment or decre-
ment first; then the test for zero is made. For test purposes, numbers
between but not including —1 and +1 are the same as zero.
Example: Here is a program that illustrates how {8 (5Z) works.
It contains a loop that pauses to display the current value in the
I-register, then uses the [} (5Z) instruction to increment that value.
The program will continue to run, continually adding one to and
displaying the contents of the I-register, until you press (or any
key) from the keyboard.
To key in the program:
Slide the W/PRGM-RUN switch WPrRGM [[[MIMRUN to W/PRGM.
Press Display
&
0@0n
RCI Recalls I-register
contents.
003 35 72| Pauses to display
contents.