Incompletely specified functions

• Functions of n inputs have 2n possible input values

  • Some input combinations may be unused
    • Called “don’t cares”
  • Exploit don’t cares during logic minimization
    • Warning: don’t care ? no output

• Example: A binary coded decimal (BCD) function

  • BCD encodes the decimal digits 0–9 as 00002–10012
  • BCD doesn’t use 10102–11112
  • Consider a BCD “increment-by-one” function
    • Computes the next number in sequence
    • If the input is 00102, the output is 00112

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