Yeda Research and Develpment & Co. Ltd

Avshalom Houri

Ehud Y Shapiro

A flat concurrent Prolog (Fcp) computer comprises a memory in which all the data is stored, three sets of registers, several queues or lists and a computer program for controlling the computer. The memory is a single data area called the 'heap' which also includes two small data areas called the 'trail' and the suspension table as well as the queues. The queues include the resolvent which is also called the active queue or the process queue, the activation queue, the process free list and the suspension free list. The registers include a set of general registers, a set of procedure try registers and a set of clause try registers. The general registers include a heap backtrack register, a queue front register, a queue back register, a process free list register, and a suspension free list register. The procedure try registers include a current process register, a time slice register that identifies the number of process iterations that can be done, a program counter, a failure label register that contains the address of the first instruction in the next clause to be tried, and a suspension table pointer. The clause try registers include a heap pointer register, an activation pointer register, a trail pointer register, an argument pointer register, a structure pointer register that points at an argument of a compound structure, a mode register that can specify either read or write mode, and a plurality of temporary registers for holding intermediate values during a clause try.

Method and apparatus for implementing a concurrent logic program

A suspension list is maintained for each variable on which one or more processes has been suspended because the variable is uninstantiated. The suspension list for each variable is a circular list of suspension records, each record in the list being associated indirectly with one process that is suspended because that variable is uninstantiated. Each record comprises a first pointer to a memory address or register (called a 'hanger') at which is stored the address of the process record for the suspended process and a second pointer that points to the memory address of the next record in the suspension list. Since. the same process can be suspended on different variables, it can happen that records in more than one suspension list point to the same hanger at which is stored the address of the process record for that process. If a variable is subsequently instantiated, each record in the suspension list associated with that variable is used to identify one of the processes that has been suspended on that variable. Accordingly each such process can then be enqueued in the process queue. At the same time, as each process is identified by the record in the suspension list, the address of the process record that is stored in the hanger is reset, illustratively, to zero. As a result, if at a later time another variable is instantiated and the suspension list for that variable also contains a record that points to the same hanger, the hanger will be found to point to its reset value rather than the address of the process record. Consequently, the process will not be identified a second time and will not be enqueued again in the process queue.

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Netovot, Israel

Avshalom Houri