1 / 11

The LC-3 – Chapter 5

The LC-3 – Chapter 5. COMP 2620. 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

callie
Télécharger la présentation

The LC-3 – Chapter 5

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. The LC-3 – Chapter 5 COMP 2620 Dr. James Money COMP 2620

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. Sentinel Example • Pseudocode: • addr= 0x3100 • sum=0 • Load value from memory[addr] • Is value the sentinel? – if yes, end loop • Sum=sum+value • addr=addr+1 • Load value from memory[addr]

  10. Sentinel Example

  11. Sentinel Example

More Related