1. A virtual storage data processing system comprising: (a) a main memory for storing data and instructions; (b) an external memory for storing data and instructions to be stored in said main memory; (c) a job processor for accessing said main memory with virtual addresses to access data and instructions stored therein; (d) a file processor connected to external memory for accessing said main memory with virtual addresses to transfer data and instructions between said main memory and said external memory; (e) a memory controller connected to said main memory and including an address translation unit which translates virtual addresses from said job processor and said file processor into physical addresses and means for controlling accesses to said main memory with the translated physical addresses; (f) a common bus for connecting said job processor and file processor with said memory controller; and (g) the address translation unit of said memory controller having means providing status flags indicating that said file processor is transferring a partial data block between said main memory and said external memory, and means responsive to said status flags being set for causing said address translation unit to allow an access from said file processor to said data block, while inhibiting an access from said job processor to said data block and for outputting a page fault signal indicating that said data block does not exist in said main memory.