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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