VIRTUAL MEMORY TERMINAL

UK9-80-005 Virtual Memory Terminal A microprocessor controlled terminal (1) with keyboard input (4) and CRT display output (6) employs virtual memory and store management techniques to enable an operator to run any of a wide variety of different applications (e.g. text, image, graphics, data entry, reservation, payroll etc.) in the terminal. Application procedure records and data records relating to a selected application are copied as required from a local backing store (14) and/or over a communications link (9) from a main store (16) of central processing unit (12) into a dynamically managed region (20) of random access memory (RAM) (3) under control of primitive microprocessor control instructions permanently held in read-only store (10). In view of the virtual memory techniques employed, the backing store (14) and main store (16) appear to be logical extensions of RAM (3). The records copied to region (201 are contiguously stored as variable length segments in successive free storage locations and are chained together for subsequent access in a plurality of double-threaded chains. Previously marked procedure (and possibly data) are copied from region (20) into directly addressable fixed location (19) of RAM (3) and serve to configure terminal (1) to interact with data and procedure in region (20), in accordance with the requirements of the selected application. Contiguity of segments in region (20) is restored following disruption caused during running of the application. Storage management techniques provide for store-through of segments in RAM (3) to the backing store (14) and main store (16) and identifies segments within RAM (3) available for deletion from RAM on a least-recently-used basis to provide additonal space for new segments.