Operation Manual

101
Section 9
Branching and Looping
Although the instructions in a program normally are executed in order of their
program line numbers, in some situations it is desirable to have program
execution transfer or “branch” to a program line that is not the next line in
program memory. Branching also makes it possible to automatically execute
portions of a program more than once – a process called “looping.”
Simple Branching
The i (go to) instruction is used in a program to transfer execution to any
program line. The program line desired is specified by keying its three-digit line
number into the program line containing the i instruction. When the i
instruction is executed, program execution branches or “goes to” the program
line specified and then continues sequentially as usual.
You have already seen a common use of branching: the i000 instruction (that
is stored in program memory after the program you key in) transfers execution to
program line 000. A i instruction can be used to branch not only backward in
program memory – as in the case of i000 and as illustrated above – but also
forward in program memory. Backward branching is typically done to create
loops (as described next); forward branching is typically done in conjunction
with an o or m instruction for conditional branching (as described
afterward).
Looping
If a i instruction specifies a lower-numbered line in program memory, the
instructions in the program lines between the specified line and the i
instruction will be executed repeatedly. As can be seen in the illustration above
under Simple Branching, once the program begins executing the “loop” it will
execute it again and again.