Patent GB-1354849-A (GB)
Title: Monolithic memories
IPC: G11C29/00
Filed: 1972-04-05
Abstract: 1354849 Digital data storage INTERNATIONAL BUSINESS MACHINES CORP 5 April 1972 [25 June 1971] 15560/72 Heading G4C  A monolithic memory comprises a plurality of units each containing a plurality of storage cells, at least some of the units being defective in that they include one or more suspect cells, and the arrangement is such that, in each word addressed, the number of bits derived from suspect cells is limited in a predetermined manner, e.g. to one bit per word. In a first embodiment, the memory comprises a plurality, e.g. 16, of cards each providing one bit of a word and comprising a plurality of modules made up of four chips as shown in Fig. 2. Seven bits of a 15-bit address are decoded to energize lines X0-X7 and Y0-Y15 to select a chip on each card at corresponding circuits 34. The remaining eight bits of the address are decoded at 30, 32 on the selected chip to select one bit cell. Each card is regarded as being divided into 16 logical sectors defined by address bits E-H, and each chip is likewise divided into 16 logical sectors defined by address bits A-D. In manufacture, the chips are sorted into perfect and defective groups and the defective group is further sorted into subgroups in each of which the chips have defective cells in a common chip sector. Each card is then constructed by using chips which contain defective cells in chip sector 0 only in card sector 0, chips with defective cells in chip sector 1 only in card sector 1 and so on. Perfect chips can be used in any card sector. The chip sector address wiring is then skewed by inverting a different selection of one or more of address bits A-D in all cards except one, e.g. card 0, whereby, for each chip sector address, each card supplies a bit from a different chip sector and the only suspect bit in the word is derived from the card in which the chip sector actually addressed corresponds to the addressed card sector. The suspect bit may be identified by the result of exclusive OR-ing address bits A-D with address bits E-H, and if associated error detection circuitry detects an error in the accessed word, the identified suspect bit is inverted, Fig. 6 (not shown). Preferably however, single-error correcting-circuitry normally independently corrects any single errors which occur, and only if a double error is detected is the identified suspect bit inverted so that, if a single error then remains, this error may be dealt with by the correction circuitry. In a second embodiment, an additional card comprised entirely of perfect chips is provided. During a write operation, data addressed to a cell in a defective chip sector is also written into a cell on the perfect card, a suspect cell being identified as in the first embodiment by exclusive OR-ing address bits A-D with E-H. On read-out, a bit from the perfect card is substituted for the bit from a suspect cell. In other embodiments described, the number of chip sectors is not equal to the number of bits in a word. For example, with eight chip sectors and a word comprising 16 data bits and one parity bit, the parity bits may be stored on a perfect card and a respective additional perfect card may be provided for each of 2 groups of 8 defective cards, the arrangement being similar to the second embodiment for each group of cards. When the number of chip sectors exceeds the number of bits in a word, the redundant card need not be perfect and cards may contain more than one defective chip sector in each card sector.