Computer systems

Claim 1

Data processing apparatus for use in data processes which utilize a nested set of program blocks, each program block being followed by an acceptance test before entry into a succeeding block or return to an outer enclosing block, said apparatus being operative to preserve the states that have been taken up by each of a plurality of items of information immediately prior to the beginning of each block where said items of information change state during such block so that the states of such items of information prevailing at the beginning of the block can be restored if required, said apparatus comprising: first memory means for storing the current states of items of information; means for storing indicators which distinguish successive program blocks and indicate entry into and exit from a program block; second memory means for storing previous states of items of information including storing of indicators of the program blocks in which such states occurred; means for generating a "pass" signal or a "fail" signal on completion of an acceptance test; means for restoring the states of the memory means to that pertaining immediately prior to entry into a current program block if a "fail" signal is generated, said restoring means including means for discarding from the first memory means the items of information stored during the current program block, and means for transferring from the second memory means to the first memory means the states stored in the second memory means for the immediately preceding program block of those items of information which are discarded from the first memory means; and means for progressing the states of the memory means if a "pass" signal is generated, said progressing means including means for discarding from the second memory means at least those items of information local to the immediately preceding program block and those items of information for which entries already exist in second memory means in the immediately preceding program block.