#1 (permalink) Fri May 06, 2011 9:22 am Can you please change this text? |
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I want to change a paragraph using appropraite sentence so if u can help me out please check it and sent the same i would be thank ful to you. Please change the paragrah it should not be as it is however the meaning should be the same Transforming Circuits to CNF
In a logic circuit, the values assigned to internal circuit lines are constrained by the gates
attached to those lines. The output of an AND gate will be 1 iff all of its inputs are 1; the
output of an OR gate will be 0 iff all of its inputs are 0, and so on. Given any logic circuit,
a CNF formula can be constructed which expresses the same constraints. This is done by
generating sub-formulas expressing the constraints of each gate in the circuit, and then taking
the conjunction of all such sub-formulas.
A CNF formula representing the constraints for a gate can be generated from its charac-
teristic function [35]. This is a function χ(·) which evaluates to 1 if the values assigned to the
inputs and output of the gate are consistent, and 0 otherwise. The characteristic function can
be converted to a minimal CNF formula by means of a Karnaugh map [26: p. 452] or any other
standard logic minimization technique. For example, the characteristic function for an AND gate
with inputs x1 and x2 and output y is shown in Figure 2.1.
This process can be repeated for any gate type, including multi-output gates. The construc-
tion will be polynomial in the number of inputs and outputs as long as “counting”-type gates
are assumed to have a fixed maximum number of inputs [9]. Table 2.1 gives the CNF formulas
for all gate types which are used in the algorithms presented in this thesis. Note that inverted
gate types like NAND and NOR can be made from their non-inverting counterparts by negating
the output literal wherever it appears in the formula. Compare, for example, the CNF formula
for the NAND gate to that for the AND gate.
The last three gates in the table are the multiplexer (mux), half-adder, and full-adder gates.
The mux is used heavily in Section 4.2 and Chapter 5. The adders are used in Section 4.3.
These are not always considered to be “elementary” gate types. They are usually built up
from several of the other gate types listed. However, it is often beneficial not to decompose
them into simpler gate types, since doing so will usually result in a larger CNF formula. For
example, the MUX gate can be decomposed into three NAND gates and a NOT gate, but doing so
would result in a CNF formula with eleven clauses instead of four. Taking the conjunction of
the sub-formulas for each gate in a circuit results in a CNF formula that represents the entire
circuit. Any variable assignment that satisfies this formula directly yields a valid assignment of
logic values to circuit lines |
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Meenaharani New Member
Joined: 06 May 2011 Posts: 1
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