Patent GB-1024360-A (GB)
Title: Data mass memory system
IPC: G11C15/04
Filed: 1964-09-30
Abstract: 1,024,360. Magnetic disc stores. GENERAL PRECISION Inc. Sept. 30, 1964 [Dec. 10, 1963], No. 39870/64. Heading G4C. Each band of a content addressable magnetic disc store consists of a number of tracks for storing blocks of data signals and other tracks for the storage of blocks of index signals identifying the locations of corresponding data signals. As described the index signals consist of 18 characters, each of six bits and occupying two adjacent positions on three tracks, together with a three-bit space, as shown in Fig. 1, thus occupying 37 positions on three tracks. Each data block consists of 110 six bit characters together with a three-bit check character and a three bit space, thus occupying 222 positions on three tracks. In each band of the disc there are one index set of three tracks and six data sets of three tracks each, the layout being as shown in Fig. 3. In writing to the disc a block of information of 128 characters, a spare index block is found and the first 18 characters are written there, the remaining 110 characters being written into the data block which starts immediately after the index block, the required set of tracks being selected by a modulo. 6 counter actuated by a clock pulse track on the disc. To read a selected block of data the information in successive index blocks is compared with search criteria and when a match is found the succeeding data block is read out. A block diagram is given of the circuitry for selecting the required band, one of sixty in the embodiment described. Two counter/registers are used, each under the control of the associated data-processing equipment. One controls the read/write heads for the index tracks and the other controls the read/write heads for the data tracks. Thus it is possible to switch from one band to another without pause since any setting of the index head selection circuitry can take place while the previous data block is being written or read, and similarly setting of the data head selection circuitry takes place during writing or reading of the corresponding index block. To locate obsolete data blocks, a set of flag blocks are used. Each consists of 72 bits on two tracks and is the same length as the index blocks. Spacer bits are provided at each end of the flag block and so are " M " and " C " bits. These latter indicate respectively a data block which cannot be checked because an overlapping record is being read, and a block which meets the search criteria but has not been transferred to the data processor either because it is busy or because a " count only " instruction has been executed. Only one bit is used for each of these states as they are concerned only with one band at a time. The other sixty bits are used to indicate the corresponding index blocks in each of the sixty bands, being set to 1 if the block is obsolete. Four tracks are used for the flag blocks, the information being transferred from tracks 1 and 2 to tracks 3 and 4 at one point and transferred back again eight index blocks later. Thus flags are derived from logic circuitry 52 when a data block is in position for sensing and may be modified by circuitry 58 after the entire block has been sensed.