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CS170 Computer Organization and Architecture I

CS170 Computer Organization and Architecture I. Ayman Abdel-Hamid Department of Computer Science Old Dominion University Lecture 24: 12/3/2002. Outline. Appendix B Example on Memory Elements More on Combinatorial Logic. Example on Memory Elements. Problem B.23 (See handout)

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CS170 Computer Organization and Architecture I

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  1. CS170 Computer Organization and Architecture I Ayman Abdel-Hamid Department of Computer Science Old Dominion University Lecture 24: 12/3/2002 CS170 Fall 2002

  2. Outline • Appendix B • Example on Memory Elements • More on Combinatorial Logic CS170 Fall 2002

  3. Example on Memory Elements • Problem B.23 (See handout) • Design a 3-bit counter using D latches and gates • As exercise, try to solve B.24 CS170 Fall 2002

  4. More on Combinatorial Logic1/4 • ROM read-only memory • A set of locations that can be read • The contents of these locations are fixed, usually at the time the ROM is created • PROM, EPROM • A set of input address lines (just as a PLA), say M of these.; there are 2M addresses and these point to 2M distinct words • A set of output lines (just as a PLA) say N, giving a word of N bits • Contrast a ROM and a PLA CS170 Fall 2002

  5. More on Combinatorial Logic2/4 • Don’t Cares • Situations where we do not care what the value of some output is, either because another output is TRUE or because a subset of input combinations determines the values of the outputs. • Two types of Don’t cares • Output: don’t care about the value of an output for some input combination (appear as X in the output portion of truth table) • Input: Output depends on only some of the inputs (appear as X in the input portion of the truth table) CS170 Fall 2002

  6. More on Combinatorial Logic3/4 If A or C is true, output D is true, whatever the value of B If A or B is true, output E is true, whatever value of C Output F is true if exactly one of the inputs is true. We don’t care about the value of F, whenever D and E are both true. Simplified truth table Input/Output don’t cares Truth table without don’t cares Truth table with output don’t cares CS170 Fall 2002

  7. More on Combinatorial Logic4/4 If A or C is true, output D is true, whatever the value of B If A or B is true, output E is true, whatever value of C Output F is true if exactly one of the inputs is true. We don’t care about the value of F, whenever D and E are both true. • How many product terms for original truth table? • How many product terms for simplified truth table? Simplified truth table Input/Output don’t cares Truth table without don’t cares Truth table with output don’t cares CS170 Fall 2002

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