Abstract
The Eclipse S-200 is a Data General minicomputer available since 1975. It is a microprogrammed and microprogrammable computer: the user can buy a writable control store (WCS) to define his own specialized instructions. The Eclipse is also available with a memory allocation and protection unit (“MAP”) : so, the memory can be extended up to 128 K 16-bit words. However, the user's space is limited to 32 K words because of the address field structure in the instructions. For large programs, the user can define overlay areas or he can use chaining or swapping but these methods are sometimes not appropriate (e.g., for large recursive Algol programs). In this paper, we describe a realization of a virtual memory system that allows the user to have a working space larger than 32 K while relieving him from the management problems. Unfortunately, DG'S WCS feature is unsatisfactory: the last chapter describes some displeasing features. It should also be mentioned that Data General does not provide microutilities (microassembler or microloader), a fact which complicates the microprogramming task.
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