Logic circuits and carry-lookahead circuits capable of performing high speed operations with simplified designs are described. The logic circuit is provided for searching a binary bit string from the most significant bit to the least significant bit for a first "0" or "1" bit and comprises a NOT gate circuit receiving the most significant bit of said binary bit string and composed of a dynamic logic circuit; NOR gate circuits provided in a one-to-one correspondence to the respective bits of said binary bit string, each NOR gate circuit receiving the bit of said binary bit string corresponding to the bit position of said each NOR gate circuit and, if any, the bit(s) of said binary bit string which is more significant than the bit corresponding to the bit position of said each NOR gate circuit except for the most significant bit; and two-input NOR gate circuits each of which receives two logic signals as output from adjacent ones of said NOT and NOR gate circuits.

A priority encoder-includes an encoder for coding an input consisting of a plurality of bits, selectors, respectively provided for bit input terminals of the encoder, for respectively receiving corresponding ones of a plurality of bits of an operand input, each of the selectors including a switch circuit to be controlled by an operand input bit, a carry line connected in series with the switch circuit and connected in series with all of the selectors, a first precharge circuit, connected to a carry line portion on one end side of the switch circuit, for precharging the carry line at a predetermined timing, a first detector which is controlled by an enable signal for designating upper bit priority and detects whether a potential of a carry line portion on an upper bit side of the switch circuit is at a discharge level, a second detector which is controlled by an enable signal for designating lower bit priority and detects whether a carry line portion on a lower bit side of the switch circuit is at a discharge level, and a third detector for detecting whether one of outputs from the first and second detectors and the operand input bit are both in an active state.

An adder comprises: a comparator circuit 2 for comparing values of n input signals, each of which comprises 1-bit data, with first to n-th predetermined values which are different from each other; a non-volatile memory 6 having first to n+1-th word lines, m (2.sup.m.gtoreq.n+1) bit lines which are provided so as to intersect the word lines, and memory cells, each of which is provided at an intersection of each of the word lines and each of the bit lines and each of which has stored 1-bit data; and a selector circuit 4 for selecting one of the n+1 word lines on the basis of n comparison results of the comparator circuit to activate the selected word line. Thus, a plurality of bits are added at high speed.

A priority encoder for receiving input request signals at a number of input request terminals and for providing an N-bit output binary code word indicating the binary identification number of the highest-priority, currently-active input request terminal. Each output bit of the output binary code word is provided from a respective logic circuit. Whenever any one of a first group of input request terminals, which are identified as having a logical TRUE value in a particular bit position of the N-bit output binary code word, is active, all of a second group of input request terminals, having a logical FALSE value for the particular bit position, are disabled. Sequential operation of the logic circuits for each output bit is obtained by successively delaying enablement of a logic circuit until high-order logic circuits have completed operation.

A digital encoder (34) is partitioned into a plurality of rank ordered encoder circuits (36-39) which concurrently encodes least significant bits of an input signal from a first digital format to an output signal in a second digital format. Simultaneously, at least one bit of the input signal is used by a most significant bit encoder (42) to provide at least one most significant bit of the input signal in the second digital format. The at least one most significant bit is also used to select an output encoding of one of the plurality of rank ordered encoder circuits (36-39)as a remainder of the bits of the output signal in the second digital format.