Campbell 21X Operator's Manual page 43

lf
Then/Else may be nested to form
logical AND pr OR
branching. Figure 3.8-2
illustrates
anlAND construction. lf conditions A
and
B
are trde, the instructions included between
lF B and
thelfirst End Instruction will
be
executed.
lf either of the
conditions
is
false,
execution will jump to the
corresponding
End
Instruction, qkipping
the instructions between.
rF
A
(BB-92
WITH
COMMAND
J0)
SECTION
3.
INSTRUCTION SET BASICS
encountered,
execution branches to the END
Instruction
95
which closes the case
test (see
lnstruction
93).
3.8.2
END, INSTRUCTION 95
END, Instruction
95, is required to mark the end
of:
1.
A Subroutine (starts
with lnstruction
87)
2.
A Loop (starts with Instruction
85)
3.
An
|F... THEN DO sequence (starts with
one
of lnstructions 89-93 with the THEN
DO
command 30).
4.
A case statement (starts with Instruction
93)
The lF instructions 89-93 require Instruction 95
only when the
THEN DO command 30 is
used.
lf one
of
the above instructions is used without
the corresponding END, the 21X will display
error 22 when compiling the
program.
Error
21
is displayed if END is used
without being
preceded by one of these instructions (Section
3.10).
An END instruction is always paired with the
most recent instruction that requires an END and
does not already have
one.
A way of visualizing
this
is
to draw lines between each instruction
requiring
an END and the END paired with it (as
in Figure 3.8-2). The
lines must not
cross.
To
debug logic
or
find a missing or extra END error,
list
the program and draw the lines.
Subroutines can
be
called
from other
subroutines; they cannot be embedded within
other
subroutines. A subroutine must end before
another subroutine begins (Enor 20).
Any loops
or lF...THEN DO sequences started within
a
subroutine must end before the subroutine.
3.8.3 NESTING
A branching or loop instruction which occurs
before a previous branch or loop has been
closed with the END instruction is
nested.
The
maximum nesting level is 9
deep.
Error
30
is
displayed when attempting to compile
a program
which is nested too deep.
The Loop Instruction, 87, counts as
1
level.
lnstructions 86, 88, 89, 91, and 92 each count as
one
levelwhen
used with the THEN
DO
command
30.
Use
of Else, lnstruction 94, also
counts as one nesting leveleach time it is used.
For example, the AND construction above
is
--
rF
B
(88-e2
!V|TH cor/MAND
Jo)
I
I
,
INSTRUCTIONS
TO
.i:
EXECUTE
IF
A
AND
B
ARE
BOTH
TRUE
END
B
(e5)
END
A
(s5)
FIGURE
3f8-2. Logical
AND
Construction
Figure 3.8-3]illustrates the instruction sequence
that
will resuft in subroutine X being executed
if
either A or B
is
true.
lF
A
(88-92 with command 30)
Calllsubroutine X (86, command=X)
ELSE
(e4)
lF
B
(88-92 with command
3O)
Callsubroutine X (86, command=X)
ENq B (e5)
ENq A (es)
FIGURE
$.8-3. Logical
OR
Construction
A logical
Ofl
can also be constructed by setting
a
flag if
a
colnparison
is
true.
(The flag
is
cleared before making the comparisons.) After
all
comparis0ns have been made, execute the
desired instructions if the
flag is set.
The Begin Case Instruction 93 and lf Case
Instruction 88 allow
a
series
of
tests on the value
in
an input ldcation. The case test
is
started with
Instruction 90 which specifies
the location to test.
A series
of Instructions 83 are then used to
compare
thQ
value in the location with fixed
values.
Wh0n the
value
in
the input location
is
less than th4 fixed
value specified in Instruction
83 the comrhand
in
that Instruction 83
is
executed; when
the next lnstruction 83
is
3-5