Patent GB-922584-A (GB)
Title: Bit comparator
IPC: G11C15/02
Filed: 1961-09-22
Abstract: 922,584. Electric digital-data-storage apparatus. SPERRY RAND CORPORATION. Sept. 22, 1961 [Oct. 13, 1960], No. 34061/61. Class 106 (1). In data-storage apparatus, a stored word is compared bit-by-bit with an externally stored word to provide a given output when and only when the two bits compared are in a given relationship, such as equality, circuits being provided for indicating when the internally stored word has at least a predetermined number M of its bits in said given relationship with the bits of the externally stored word. The invention is applied to a magnetic thin film memory. A word in the store is stored in magnetic elements such as B 0 -B 4  each of which comprises two uniaxial ferromagnetic thin films with their easy axes mutually perpendicular, one film in each pair storing the digit and the other film being employed for non-destructive read-out. The remanent magnetization of the storage film biases the magnetization vector of the read-out film in one direction or the other according to the state of the storage film. An external comparison word is set up in a register 10, the direction of a transverse biasing field applied perpendicular to the easy axis of each read-out film depending on the value of the bit stored in the corresponding stage of the register 10, the transverse field rotating the magnetization vector of the read-out film further towards or away from its easy axis. Drivers D 0 -D 4  are arranged to provide successive drive fields parallel to the easy axes of the respective read-out films causing an output on a line 12 of one polarity if an external " 1 " is associated with an internal " 0," an output of the other polarity if an external " 0 " is associated with an internal " 1," and no output if the associated bits are equal. The output line 12 is connect ed through a sense amplifier 14 to an inverter 16 so that a pulse is applied to an accumulator 18 which may be a condenser, or a digital counter, each time an equality of bits is detected to provide a count therein of the number of equalities detected. The total count is added to N, the number of digits compared (N = 5 as shown), unity being added at 26 and the matching constant M subtracted at 28 to give an indication of N+1-M at 34. From this total, N is subtracted, an indicator 38 showing the total or indicating that the total is greater than zero, e.g. that the word stored internally has at least M of its bits equal to the external word. Each register in the store may be provided with the described calculating and indicating equipment. If the indicator 34 is not required, the units 20, 36 may be dispensed with. A similar arrangement may be employed to indicate inequalities instead of equalities. In a modification, a plurality of bits stored in a data store may be read out in parallel bit-by-bit, the successive bits of a word being applied to a gate which also receives the bits of the comparison word. In a modification (Fig. 3, not shown), the bits of the stored word may be applied successively to a gate which is also supplied with the corresponding bits of the comparison word.