The LC-3

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The LC-3 Chapter 5

• COMP 2620

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Methods of Loop Control

• Recall from last class, we designed a loop to
  implement the addition of 12 integers
• That is, a loop is a sequence of instructions
  that executed a number of times in repetition
• The code the executes in the loop is called the
  body of the loop
• Each time the loop executes, this is called one
  iteration

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Methods of Loop Control

• There are two common methods for controlling
  loops that are used in programming
• You probably have seen both of these in your
  C/C/Java courses in some form

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Methods of Loop Control

• The first method is the one we used last class
• It requires a counter variables and the loop
  terminates when the counter variable reaches a
  certain value
• That is, we execute the loop n times by setting a
  counter to n and decrementing each time

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Methods of Loop Control

• Then, we of course check to see when the loop
  counter reaches 0.
• If it is not zero, we set the PC to the start of
  the loop and continue with another iteration
• Alternatively, we could start the loop counter at
  0 or 1 and increment up to n

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Methods of Loop Control

• The second method of loop control is called a
  sentinel
• This is usually used when we do not know ahead of
  time how many iterations we need to run
• We choose a value that can never occur in
  processing to indicate termination of the loop
• This value is called the sentinel

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Methods of Loop Control

• For a sequence of positive numbers this could be
  a negative value
• For a list of pointer values, it could be NULL
  value for the current pointer
• When we find this value, the loop terminates

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Sentinel Example

• We consider a similar example to adding the 12
  integers, but this time we assume the numbers are
  all positive
• In this case, we could choose any negative value
  for the sentinel
• As convention many times, we choose the number -1

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Sentinel Example

• Pseudocode
• addr 0x3100
• sum0
• Load value from memoryaddr
• Is value the sentinel? if yes, end loop
• Sumsumvalue
• addraddr1
• Load value from memoryaddr

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Sentinel Example
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Sentinel Example
Address Instruction Comments
x3000 1 1 1 0 0 0 1 0 1 1 1 1 1 1 1 1 R1 ? x3100 (PC0xFF)
x3001 0 1 0 1 0 1 1 0 1 1 1 0 0 0 0 0 R3 ? 0
x3002 0 1 1 0 1 0 0 0 0 1 0 0 0 0 0 0 R4 ? MR1
x3003 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 BRn 0x3008
x3004 0 0 0 1 0 1 1 0 1 1 0 0 0 1 0 0 R3 ? R3 R4
x3005 0 0 0 1 0 0 1 0 0 1 1 0 0 0 0 1 R1 ? R1 1
x3006 0 1 1 0 1 0 0 0 0 1 0 0 0 0 0 0 R4 ? MR1
x3007 0 0 0 0 1 1 1 1 1 1 1 1 1 0 1 1 BRnzp 0x3003