Dedicated input/output processor method and apparatus for access and storage of compressed data

System control of compression and decompression of data based upon system aging parameters, such that compressed data becomes a system managed resource with a distinct place in the system storage hierarchy. Processor registers are backed by cache, which is backed by main storage, which is backed by decompressed disk storage, which is backed by compressed disk storage then tape, and so forth. Data is moved from decompressed to compressed form and migrated through the storage hierarchy under system control according to a data life cycle based on system aging parameters or, optionally, on demand: data is initially created and stored; the data is compressed at a later time under system control; when the data is accessed, it is decompressed on demand by segment; at some later time, the data is again compressed under system control until next reference. Large data objects are segmented and compression is applied to more infrequently used data. A dedicated compression input/output processor (IOP) is controlled by host system defined data structures which include a bus transport mechanism (BTM) which is DMA loaded to the compression IOP storage. The BTM includes a request response control block (RRCB) comprising a compress or decompress operation command, a data out descriptor (DOD) for providing the address of data in host storage that needs to be compressed or decompressed, and data in descriptor (DID) for providing the address in host storage where the resulting decompressed or compressed data is to be stored.