Described is an interstorage transfer mechanism 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 the request stacks. A tag storage serves as an index to the data concurrently resident in high-speed storage and a directory storage acts as an index to data currently in main storage. Requests for data in each port cause the tag storage to be interrogated to determine whether the desired data is in highspeed storage. If not, then the desired data is retrieved from main storage and placed into high-speed storage by the interstorage transfer mechanism. Priority means for accessing said high-speed storage are provided, said interstorage transfer mechanism being given first priority to access said high-speed and tag storages in case of conflicts in access between said interstorage transfer mechanism and at least one of said plurality of request ports. Means are provided for choosing a target address in High-speed storage wherein said desired data will be relocated. The tag indexing said target address is updated by said interstorage transfer mechanism to reflect the new data. Means are further provided for invalidating all requests currently in transit at the time said tag is changed to insure data integrity in case said requests refer to old data in said target line. The aforementioned tags contain a bit indicating that the corresponding address in high-speed storage has recently been accessed. Cold generator means are provided for periodically resetting this bit in each tag to mark the corresponding high-speed storage physical address as a candidate for replacement target.