Virtual storage implementation on a microcomputer

Abstract

Virtual storage is a hardware memory management technique which is gaining popularity amongst modern mainframe computers and top of the range minicomputers. It offers huge memory resources at lower cost in comparison to solid state memories of compatible size. The associated benefits are reduced component counts and lower power requirement. The trade off for the above mentioned advantages is an increase in the memory average access time. However this increase in the average access time does not appear to be a major handicap for many computer applications. With the widespread acceptance of microcomputer systems these days, more sophisticated and increasingly more complex programmes are being run on microcomputers. Demands are placed on the microcomputers for bigger memory recourses. Also the new generation microprocessors can now cater for an addressing space of many mega bytes. It is obvious that virtual storage technique is ideally suited for these new generation microprocessors since a large real memory implementation is impracticable and is out of question economically. This project investigated the feasibility of adapting an existing IMP-16C microcomputer system into a virtual storage system, assuming that a microcomputer system is single user orientated. Some of the virtual storage techniques were reviewed, in particular those that have been studied with a computer simulation of a virtual storage system. A working virtual storage computer system was implemented on the National Semiconductor IMP-16C microcomputer. the results of the simulation study. The design was based on A virtual memory of 256 K words of 16 bits was achieved. The cost to equip a system with an equivalent size real memory is about ten times the cost to manufacture the virtual storage controller. The average access time of the virtual storage computer system as implemented is an order higher than the conventional real memory system. Supplementary techniques and faster auxiliary storage can be employed to improve the average access time. This project demonstrated that a virtual storage controller can be coupled to an existing microprocessor to provide virtual memory storage at about one tenth the cost to provide the equivalent real memory storage. The virtual storage controller can be divided quite readily into functional blocks. Each block is suitable for chip level integration with LSI or VLSI technology. Implementing a virtual storage system around an existing microprocessor would be a much simpler task when these functional chips are available.