HP HP-67 - Page 249

Using the 1-Register for Indirect Control 245
As you know, when a or instruction is executed, the
calculator does not execute further instructions until it has searched
downward through program memory and located the next label
addressed by or G58). When @ or 23 (@ is executed
in a running program, with O or a positive 1 through 19 stored in
the I-register, the running program searches downward through
program memory until it locates the next addressed by the
number in I. Then execution resumes.
With anegative number stored in the I-register, however, execution is
actually transferred backward in program memory when @ or
1] {@ is executed. The calculator does not search for a label,
but instead transfers execution backward the number of steps
specified by the negative number in the I-register. (This is advanta-
geous because the search is often much faster than searching for a
label, and because you can thus transfer execution even though all
labels in the calculator have been used for other purposes.)
For example, in the section of program memory shown below, —12
is stored in the I-register. Then, when step 207, (@ is executed,
the running program jumps backward 12 steps through program
memory to step 195 (that is, step 207 — 12 = 195), and execution
resumes again with step 195 of program memory.
193
194 (3
|-
195
@
|
196
(3
|
197
(3
:
198
i
199
YVlth
-12
sf[ored
{
200
in I, execution
transferred
backwards
:
201
[LBC
12
steps
by
|
202
@
!
203
(@
!
204
(2
|
205
|
206
([sTI
I-—
[
207
208 liant