Derive the circuit for a 9's complementer when decimal digits are represented in the excess-3 code (Table 3-6). A mode control input determines whether the digit is complemented or not. What is the advantage of using this code over BCD?

Table 3-6

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TABLE 3-6 Four Different Binary Codes for the Decimal Digit Decimal BCD digit 8421 0 1 2 3 4 5 6 789 9 Unused bit combi- nations 2421 0000 0000 0001 0001 0010 0010 0011 0011 0100 0100 0101 1011 0110 1100 0111 1101 1000 1110 1001 1111 1010 0101 1011 0110 1100 0111 1101 1000 1110 1001 1111 1010 Excess-3 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 0000 0001 0010 1101 1110 1111 Excess-3 gray 0010 0110 0111 0101 0100 1100 1101 1111 1110 1010 0000 0001 0011 1000 1001 1011