HP HP-67 - Page 250

246 Using the I-Register for Indirect Control
When (@ has been performed in a running program, execution
then continues until the next or instruction is encountered,
whereupon the running program stops. Thus, if you pressed with
the instructions shown above loaded into the calculator, the
instructions in steps 201 through 207 would be executed in order.
Then the program would jump backward and execute step 195 next,
continuing with 196, 197, etc., until the instruction was
encountered in step 200. The running program would then stop.
With a negative number stored in the I-register, [ (@ also
transfers execution backward the number of steps specified by the
number in I. However, subsequent instructions are then executed as
a subroutine, so when the next instruction is encountered,
execution transfers back to the instruction following the @
instruction (just like a normal subroutine would be executed.)
The section of program memory below shows how {@ operates.
If you press f8, —12 will be stored in the I-register. When
{f is then executed a running program jumps back 12 steps from
step 207 and resumes execution with step 195. When the (return)
instruction in step 200 is encountered, execution returns and
continues with step 208.
193
194 (3)
| 195
| 196 @
| 197 (5]
| | 198
With
—12
stored
|
199
in
I,
execution
!
200
T
transferred
:
201
QG
|
backwards
|
202
'
Then
the
12
steps
by
l
203
(4
:
instruction
causes
@.
||
204
(2
I'a
return,
and
I
205
|
execution
:
206
Iresumes
with
—
|
207
@
I
step
208.
208 -+
209 (% +