Two-level combinational logic using a ROM

A B C F0 F1 F2 F3

0 0 0 0 0 1 00 0 1 1 1 1 00 1 0 0 1 0 00 1 1 0 0 0 1

1 0 0 1 0 1 1

1 0 1 1 0 0 0

1 1 0 0 0 0 1

1 1 1 0 1 0 0

ROM8 words x 4 bits/word

address

outputs

A B C F0 F1 F2 F3

• ROMs directly store and decode truth table

• Use two-level canonical form (no need to minimize logic)

  • F0 = A'B'C + AB'C' + AB'C
  • F1 = A'B'C + A'BC' + ABC
  • F2 = A'B'C' + A'B'C + AB'C'
  • F3 = A'BC + AB'C' + ABC'

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