The LC-3

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The LC-3 Chapter 6

• COMP 2620

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Problem Solving

• Recall from Chapter 1 that for electrons to solve
  problems, we need to get through several levels
  of transformation
• We go from natural language description of the
  problem down to electrons
• Recall the second step was to write the problem
  as an algorithm

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Problem Solving

• That is, we write a step-by-step process to solve
  our problem
• Recall the properties of an algorithm
• Finiteness terminates properly
• Definiteness each step is precisely stated
• Effective Computability it can be carried out
  by a computer or Turing machine

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Problem Solving

• For this course, we will use the concept of
  structured programming
• This originated in the 1960s as a way describing
  a complex system and decomposing it into smaller,
  more manageable tasks
• Sometimes we call this systematic decomposition

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Problem Solving

• We will use systematic decomposition as a way to
  design computer programs that are more complex
• This technique is useful at any level

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Three Constructs

• The main idea behind systematic decomposition is
  taking a task and breaking it into smaller units
• These units, when put together, work the same as
  the larger task
• We can repeat the process over and over
• Works well with extremely complex tasks

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Three Constructs

• This process is sometimes referred to as stepwise
  refinement
• That is, we take each larger step and refine it
  into smaller, more detailed tasks
• The smaller tasks are generally simplier that the
  larger task

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Three Constructs

• There are three constructs for doing this
• Sequential
• Conditional
• Iterative

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Three Constructs
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Three Constructs

• The sequential construct is the one to use if the
  task can be broken into two subtasks, one
  following the other
• The first task executes, then the second one
  happens
• Many times, the first task computes something
  that is needed by the second task
• There is no repetition here!

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Three Constructs
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Three Constructs

• The conditional construct tests a condition and
  performs one of two subtasks depending on the
  result
• If condition is true, we perform one task
• Otherwise, there is another task that we perform

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Three Constructs

• This correlates with an if-else statement
• Either task may be vacuous
• There is no repetition of the check or the tasks

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Three Constructs
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Three Constructs

• The iterative construct is used when there needs
  to be a task run multiple times
• First, a condition is tested to determine where
  the iteration should continue
• If so, the subtask runs
• Otherwise the iteration is done

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Three Constructs

• After the subtask is finished, the condition is
  rechecked
• Then, the process repeats until the condition is
  false
• This is similar to a while() loop

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Three Constructs
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Using the LC-3 with Constructs

• We will now illustrate how the control
  instructions on the LC-3 can implement this three
  constructs
• How do we use LC-3 instructions to encodethe
  three basic constructs?
• Sequential
• Instructions naturally flow from one to the
  next,so no special instruction needed to gofrom
  one sequential subtask to the next.

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Using the LC-3 with Constructs

• Conditional and Iterative
• Create code that converts condition into N, Z, or
  P. Example Condition Is R0 R1? Code
  Subtract R1 from R0 if equal, Z bit will be
  set.
• Then use BR instruction to transfer control to
  the proper subtask.

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Using the LC-3 with Constructs
PC offset toaddress C
Exact bits depend on condition being tested
Unconditional branchto Next Subtask
PC offset toaddress D
Assuming all addresses are close enough that
PC-relative branch can be used.
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Using the LC-3 with Constructs
PC offset to address C
Exact bits depend on condition being tested
Unconditional branchto retest condition
PC offset toaddress A
Assuming all addresses are on the same page.