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4-7-7 Branching Instruction Lines
When an instruction line branches into two or more lines, it is sometimes neces-
sary to use either interlocks or TR bits to maintain the execution condition that
existed at a branching point. This is because instruction lines are executed
across to a right-hand instruction before returning to the branching point to
execute instructions on a branch line. If a condition exists on any of the instruc-
tion lines after the branching point, the execution condition could change during
this time making proper execution impossible. The following diagrams illustrate
this. In both diagrams, instruction 1 is executed before returning to the branching
point and moving on to the branch line leading to instruction 2.
Instruction 1
00002
00000
Instruction 2
Branching
point
Instruction 1
00002
00000
Instruction 2
Branching
point
Diagram B: Incorrect Operation
Diagram A: Correct Operation
00001
Address Instruction Operands
00000 LD 00000
00001 Instruction 1
00002 AND 00002
00003 Instruction 2
Address Instruction Operands
00000 LD 00000
00001 AND 00001
00002 Instruction 1
00003 AND 00002
00004 Instruction 2
If, as shown in diagram A, the execution condition that existed at the branching
point cannot be changed before returning to the branch line (instructions at the
far right do not change the execution condition), then the branch line will be
executed correctly and no special programming measure is required.
If, as shown in diagram B, a condition exists between the branching point and the
last instruction on the top instruction line, the execution condition at the branch-
ing point and the execution condition after completing the top instruction line will
sometimes be different, making it impossible to ensure correct execution of the
branch line.
There are two means of programming branching programs to preserve the
execution condition. One is to use TR bits; the other, to use interlocks
(IL(02)/IL(03)).
TR Bits The TR area provides eight bits, TR 0 through TR 7, that can be used to tempo-
rarily preserve execution conditions. If a TR bit is placed at a branching point, the
current execution condition will be stored at the designated TR bit. When return-
ing to the branching point, the TR bit restores the execution status that was
saved when the branching point was first reached in program execution.
Inputting, Modifying, and Checking the Program Section 4-7