SEQUENCE INTERLOCKING AND PRIORITY APPARATUS

Described is a sequence interlock generator and priority apparatus combination suitable for use in a storage control system for a two-level storage, wherein the storage system includes a high-speed storage against which requests for data are processed and a slower, larger-capacity main storage. Requests can be received and serviced concurrently at a plurality of request ports in the system where they are buffered in request stacks. A tag storage serves as an index to the data currently resident in high-speed storage and a directory storage acts as an index to data currently in main storage. The sequence interlock generator interlocks requests in the plurality of request stacks to insure that requests to the same storage area are performed in proper sequence to insure data integrity. A sequence interlock vector is computed when the request first enters its request port and is appended to the request until the request is clear to request service. A request is received in terms of a logical address. The logical address is transformed into a plurality of physical addresses in high-speed storage. The physical addresses are used to address corresponding tags from the tag storage to determine the contents of the desired addresses in high-speed storage. The priority apparatus essentially splits the request into two priority determinations, one for priority to access the tag storage and the second for priority to access the data storage. The ideal situation is for tags and storage to be accessed concurrently. However, the invention makes provision for action to begin upon the tags if the tag storage can be accessed due to conflicts for accessing the addressable entity in high-speed storage. As a consequence of the invention, initiation of service of a given request is begun at the earliest possible time.