Incompletely specified functions

• Functions of n inputs have 2n possible input configurations

  • Some input combinations may be unused
  • Unused input combinations are called “don’t cares”
  • We exploit don’t cares during logic minimization
  • Don’t care ? no output

• Example: A binary coded decimal (BCD) function

  • BCD encodes the decimal digits 0–9 as 00002–10012
  • We don’t care about the binary numbers 10102–11112
  • Consider a function that takes a BCD input and computes the next number in sequence
    • If the input is 00102, the output is 00112

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