Abstract: A data compression system includes an input store (1) for receiving and storing a plurality of bytes of data from an outside source. Data processing means for processing successive bytes of data from the input store includes circuit means (21-25) operable to check whether a sequence of bytes is identical with a sequence of bytes already processed, output means (27) operable to apply to a transfer medium (12) each byte of data not forming part of such an identical sequence, and an encoder (26) responsive to the identification of such a sequence to apply to the transfer means (12) an identification signal which identifies both the location in the input store of the previous occurrence of the sequence of bytes and the number of bytes in the sequence.

Abstract: Digital data storage apparatus includes a data store DS having a plurality of multiple-bit word locations, and validity store VS having a lesser plurality of multiple-bit word locations arranged such that each separate bit corresponds to a separate word location in the data store. A first auxiliary store AS1 has a number of bit locations at least equal to the number of word locations in the validity store with each bit identifying the status of the validity bits in a separate word of the validity store. A number of further auxiliary stores are also provided, arranged in increasing order of significance and each having a lesser plurality of bit locations each of which corresponds to a separate word location in the next less significant auxiliary store. Circuit means CM are provided and connected in such a way that areas of the data store may be declared valid or invalid by setting appropriate bits in each of the auxiliary stores. Any area of the data store may be declared invalid in not more than two cycles.