COMPUTER SYSTEMS

摘要

1509193 Data processing NATIONAL RESEARCH DEVELOPMENT CORP 17 April 1975 [17 April 1974 14 June 1974] 16766/74 and 26497/74 Heading G4A A data processing system is adapted to process a set of nested program blocks and to preserve the states at the beginning of each block of those pre-existing items of information whose states change during the block whereby those items of information may be restored to their states prevailing when execution of a block was begun. As described the system is adapted to process error recovery programs consisting of nested "recovery" blocks each including a primary block, possibly one or more alternative blocks, and an acceptance test. Each primary or alternative block may contain one or more further recovery blocks to form the nested structure. On entering a recovery block e.g. recovery block II, the primary block, IIa, is executed, the initial states of any variables which are changed during the block being buffered in a cache memory as and when their states are changed. When the block is completed the acceptance test IIt is executed. If the test fails all the variables are restored to their states prevailing before the primary block IIa was started and the alternative block IIb is executed followed by the acceptance test IIt. If at any stage the test succeeds the recovery block II is terminated, the variables, apart from any which are purely local to the recovery block which are discarded, e.g. J, being kept in their current states. Processing then continues in the next higher level block Ia if such exists, at the point following the completed recovery block. Two basic embodiments are described, implemented by a micro-programmed or hardwired system, full details being given of the procedures involved in storing data in the cache memory to keep track of the states of variables down through the various levels of the nested structure. In a modification an arrangement is described which can selectively retain the states acquired by certain variables during a block even when processing is abandoned or an alternative block is attempted.