Magnetic bubble logic

Although the literature on the bubble logic devices is limited, the concepts and device configurations are diverse. In conductor-access devices, logic can be performed by bubble transfer operations. In field-access devices, logic can be performed by providing alternative paths which are selected by interaction between bubbles. Examples include the conjugate logic gates, the resident-bubble cellular logic, and the chevron 3-3 circuits. Logic can also be performed by counting bubbles, such as in the symmetric switching function implementation. The various mechanisms for implementing bubble logic are all described by truth tables. To assess their efficiency, they are compared in terms of space and delay when they are used to implement the same logic element - a full adder. They are all comparable except for the resident-bubble cellular logic which requires excessive space and delay. However, it is important to point out that only the symmetric switching function devices offer rewrite-ability to eliminate the part number problem, and accommodation for a large number of inputs to ease interconnection and delay equalization problems.