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SECTION
12.
PROGRAM
CONTROL INSTRUCTIONS
A delay of
0 means that there is
no
delay
between passes through
the
loop.
Each
time the
table
is
executed
all iterations of
the loop will be completed and execution
will pass on to the
following instructions.
lf
the delay
is
5, every fifth time that
the
execution interval comes up, one pass
through
the loop is made; only those
instructions
in
the loop will
be
executed and
other portions of the table
are not executed
in
the
interim.
When
the loop is executed,
execution starts
at
the loop, skipping over
any previous instructions
in
the table.
When
a
fixed number of iterations are executed,
the time spent
in
the loop is equal to the product
of
the
execution interval, delay, and the number
of
iterations.
For example,
a loop with a delay
of
1
and
a
count of
5
will take
5
seconds
if
the
execution interval
is
1
second.
When
the loop
is
first entered,
one pass through the loop is made,
then the 21X
delays
untilthe
next execution
interval and makes the
second pass through the
loop. After
making the
fifth pass through the
loop,
there
is
the
fifth delay, after which
execution passes to
the instruction following the
END instruction which goes with
the
loop.
While in a loop with delay, the table
will not
be
initiated at each execution
interval.
(However,
the overrun decimals
will not be displayed.)
Some consequences of this
are:
The Output
Flag
will not be automatically cleared between
passes through
the
loop.
Because
Table
2
cannot interrupt
Table
1,
Table
2
will not
be
executed while
Table
1
is
in
a loop with delay.
Table
1
will not interrupt Table 2
in
the middle of
an output array.
Thus,
if
the
Output Flag
is
set
in
Table 2 prior
to entering the loop
or
within the
loop, the
flag must be specifically cleared before
the end
of
the pass or
Table
1
will not be able to
interrupt.
Input locations
for Processing lnstructions within
a loop can be entered as Indexed locations. An
Indexed location causes
the input location to
be
incremented
by
1
with
each pass through the
loop.
(The lndex
counter
is
added to
the
location number
in
the program
table.)
lnput
locations which
are not indexed will remain
constant.
To specify
an Indexed location, depress the
C
key
at
some point
while keying
in
the digits
for
the input location and before entering the
location with the
A
key.
Two dashes, --, appear
in
the
two right most characters
of
the display,
indicating the entry is lndexed.
When
the same output processing is requiied
values in sequential input locations, it must
be
accomplished by using
the repetitions
of
the
Output Instruction, not by indexing the
input location within
a loop.
An
Output Instruction within a loop
is
allotted
same number
of Intermediate Storage locations
as
it
would receive if
it
were not
in
the
loop.
For
example, the
Average instruction with
a
single
repetition
is
allotted only
two Intermediate
locations: one for the number of samples and
one for
the running
total.
Each time through
the
loop the sample
counter is incremented and the
value
in
the referenced input location
is
added
the
total.
lf
the input location is indexed, the
values
from all input locations are added to the
same
total.
lf the Average instruction
with
1
repetition and location
1
indexed is placed
a loop
of
10
and the
Output Flag set high prior
entering
the loop,
10
values will be output.
These
will not
be
the averages
for locations
1-
10.
The first
will
be
the average of all the
readings in locations 1-10 since
the previous
output.
Because
the Intermediate locations are
zeroed each time an output occurs,
the next
nine values
will
be
the current
values (samples
at
the time of output) of Locations 2-10.
Loops
can be
nested.
Indexed locations within
nested loops
are indexed to the inner most loop
that they are
within. The
maximum nesting
in
the 21X is
9
deep.
This applies
to
lf
Then/Else comparisons and Loops or any
combination
thereof. An
lf Then/Else
comparison
which uses the Else lnstruction,
94
counts
as being nested 2 deep.
PAR.
DATA
NO.
TYPE
DESCRIPTION
2
Delay
4
lteration count
The following example involves
the use
of
the
Loop Instruction, without a delay,
to perform
a
block data transformation.
01:
02:
12-2
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