Patent GB-1118887-A (GB)
Title: Digital data storage system
IPC: G11B20/12
Filed: 1966-08-22
Abstract: 1,118,887. Electric digital data store. INTERNATIONAL BUSINESS MACHINES CORP. 22 Aug., 1966 [25 Aug., 1965], No. 37519/66. Heading G4C. An electric digital data storage system for the storage of variable length data records each comprising a plurality of data characters, includes a store having storage blocks sequentially identified each by a number and each of such storage capacity that, in general, a stored data record occupies more than one storage block, in which each stored data record is identified by a chain number which is stored in each block containing a portion of the stored data record, the chain number being the identifying number of a predetermined one of the blocks containing a portion of the stored record. The invention is described with respect to a magnetic drum or disc system. The memory area is divided into regions each region commencing with a region start character. This is read by a read head 101, detected by circuitry 142 and used to reset the circuitry required at the start of a read, write or erasing operation. Read operation.-In general each piece of data to be read occupies more than one storage block, each block containing a start symbol, a status symbol indicating whether the block is full or empty, a chain number and the data, with the final portion of the data containing an end of record symbol. The first portion of each piece of the data stored carries a chain number equal to the block number of the region in which it is stored (Fig. 3B). The remaining portions of that data are stored in different blocks but carry the same chain number. The data need not be stored in adjacent blocks (record 8 in Fig. 3B) but each portion carries a chain number equal to the lowest block number of the blocks in which it is stored. As each region is examined the detect region start signal prepares the appropriate circuitry, with switches 130-137 being in the R position. The first block start symbol advances a block counter 151. A full status symbol resets a flip-flop 156 and acts to enable a gate passing the chain number at the appropriate time. The number in the block counter is compared wth the chain number. Since for the first record these agree the comparison circuit 211 enables a read out gate 141 to pass the data to a central processing unit. The data is examined to see if it is the data required. If it is not required a reject signal is returned which sets the block counter flip-flop 156 so that the next block is counted. The remaining blocks carrying chain number 1 do not create a pulse from the comparison circuit so are not read out. When the record is reached that is required a reject signal is not returned, block counter flip-flop remains reset and the block counter is not incremented any further and thus carries the chain number of the data required. Coincidence at circuit 211 occurs for all records carrying this chain number and all blocks storing this data are read out until an end of record symbol is reached, at which point read out is stopped. Write operation.-In this operation switches 130-137 are in the W position. Data can only be written in blocks where the status symbol indicates that the block is empty so the signals are read as before with each block start advancing the block counter until a detect status empty signal 172 resets the block counter flipflop to maintain the block counter number fixed. Circuits are also enabled that allow this number to be recorded in the chain number situation at the appropriate time on the tape and change the status empty signal to status full. A block length counter is also enabled which causes reading-in of the data to stop when the block is filled. The data is then read-in to the empty blocks until an end of record signal is obtained. This is recorded and the writing process then ceases. Purge or erase operation.-If data is not required it can be effectively erased by changing a status full symbol to a status empty symbol. To do this switches 130-137 are set to R and switches 192, 251 to P. Data is read out as before until the required data is detected and a command purge signal is returned by the C.P.U. A purge flip-flop is reset and enables an AND gate 253 to pass a signal and cause a status empty symbol to be written in the appropriate position on the disc or drum. Data is recorded in series form, read out via the read head 101 and a read amplifier then converted into parallel form in a deserializer 110 and temporarily stored in a single character register 112. During the writing operation parallel data is rendered into serial form by a serializer 111. The start, status and chain number symbols may be recorded in each block in a different order to that described and may be accompanied by instructions indicating where they are situated. In particular the chain number may follow the data for the first block and be preceded by a character " chain number follows ". This would allow the C.P.U. to read all the data of the block and use the chain number time to decide if the record was the one required.