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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) - PowerPoint PPT Presentation
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Lecture 24: 12/3/2002 CS170 Fall 2002 1
CS170 Computer Organization and Architecture I
Ayman Abdel-Hamid
Department of Computer Science
Old Dominion University
Lecture 24: 12/3/2002
Lecture 24: 12/3/2002 CS170 Fall 2002 2
Outline
•Appendix B
•Example on Memory Elements
•More on Combinatorial Logic
Lecture 24: 12/3/2002 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
Lecture 24: 12/3/2002 CS170 Fall 2002 4
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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
Lecture 24: 12/3/2002 CS170 Fall 2002 5
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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)
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Inputs Outputs
A B C D E F
0 0 0 0 0 0
0 0 1 1 0 1
0 1 0 0 1 1
0 1 1 1 1 0
1 0 0 1 1 1
1 0 1 1 1 0
1 1 0 1 1 0
1 1 1 1 1 1
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.
Truth table without don’t cares
Inputs Outputs
A B C D E F
0 0 0 0 0 0
0 0 1 1 0 1
0 1 0 0 1 1
0 1 1 1 1 X
1 0 0 1 1 X
1 0 1 1 1 X
1 1 0 1 1 X
1 1 1 1 1 X
Truth table with output don’t cares
Inputs Outputs
A B C D E F
0 0 0 0 0 0
0 0 1 1 0 1
0 1 0 0 1 1
X 1 1 1 1 X
1 X X 1 1 X
Simplified truth table
Input/Output don’t cares
Lecture 24: 12/3/2002 CS170 Fall 2002 7
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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.
Truth table without don’t cares
Inputs Outputs
A B C D E F
0 0 0 0 0 0
0 0 1 1 0 1
0 1 0 0 1 1
0 1 1 1 1 X
1 0 0 1 1 X
1 0 1 1 1 X
1 1 0 1 1 X
1 1 1 1 1 X
Truth table with output don’t cares
Inputs Outputs
A B C D E F
0 0 0 0 0 0
0 0 1 1 0 1
0 1 0 0 1 1
X 1 1 1 1 X
1 X X 1 1 X
Simplified truth table
Input/Output don’t cares
•How many product terms for original truth table?
•How many product terms for simplified truth table?
