A FOD (First-One-Detector) circuit for detecting the number of leading zeros counted from a most significant bit to a first one in a binary number includes a plurality of sub-FODs respectively having a plurality of unit blocks connected in cascade, when a fraction input is less than 16 bits, depending on the number of bits and respectively provided with a plurality of transmission transistors. When the fraction input is more than 16 bits, the plurality of sub-FODs respectively output the number of leading zeros with regard to predetermined bits of fraction inputs, and a determinative signal for determining whether the fraction inputs are all zeros. The sub-FODs respectively further include an encoding circuit for encoding the number of leading zeros outputted from the plurality of sub-FODs and the determinative signal and outputting the resultant number of leading zeros. The FOD circuit employs a fewer number of transistors and realizes a faster normalization by a quick detection of leading zeros.
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