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The LC-3 – Chapter 6 COMP 2620 Dr. James Money COMP 2620 1

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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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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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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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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 encode the three basic constructs? Sequential – Instructions naturally flow from one to the next, so no special instruction needed to go from 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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Exact bits depend on condition being tested PC offset to address C PC offset to address D Unconditional branch to Next Subtask Assuming all addresses are close enough that PC-relative branch can be used. Using the LC-3 with Constructs

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Exact bits depend on condition being tested PC offset to address C PC offset to address A Unconditional branch to retest condition Assuming all addresses are on the same page.

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