Abstract: A data processing system with virtual memory and the ability to vary the program window size for each program to be processed. The system includes at least one processor, a main memory, a secondary storage and a channel to handle data transfer between the secondary storage and main memory. Each processor is provided with means to measure the processing time of the processor and the data transfer time of the channel for transferring either variable segments or fixed pages from the secondary storage to main memory in response to page faults. A program table is provided in the processor to contain names of pages which reside in main memory for the current program, which table implements a page replacement mechanism such that as new pages are transferred to main memory for the current program, the least recently used pages are removed therefrom.