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252 Part 2 ½ Regions of Computer Space
Section 2 ½ Memory Hierarchies and Multiple Processes

to show that the benefits and principles of the virtual machine are not compromised by the secondary modifications which are introduced to facilitate the efficient multiprogramming of several processes in separate virtual machines.

2. The Segmented Virtual Store

The relatively high cost of main store when compared with other storage devices, such as drums and discs, forces the computer architect to consider how this essential system component can be utilised with greatest efficiency. Amongst the more pressing problems in this area are:

a The process-image, and possibly even the user program alone, may exceed the size of available main store.

b Competition for main store by a number of programs may exist (e.g. in a time sharing system).

c Efficient use of main store for variable length tables, etc.

The most promising technique for solving such problems is the virtual storage concept, first used on the Atlas machines. In order to ensure that the user's needs are satisfied we shall look at this solution in the light of program structures.

The output of a compiler consists mainly of a series of logical regions comprising an object program. Most third generation architectures treat the object program as a single logical unit (e.g. for protection purposes), but certain advantages accrue if the logically separate regions, such as code sections and data areas, which we shall for the moment call program segments, are recognised as separate entities. For example, the separation of code segments from data segments considerably simplifies the production of "pure" reentrant code; this in principle allows separate virtual machines to use a single real copy of common code (e.g. operating system procedures) whilst allowing us to retain the concept of a process image containing all the code necessary to solve the user's problem. We shall see other advantages of the architectural recognition of segmentation in due course.

A characteristic feature of segments within a process-image is their need to cross-reference each other, the obvious technique for implementing this being to form an address consisting of segment number plus displacement within segment. If we now form for each virtual machine a "segment table" consisting of a list of entries (one per segment in the process-image), which map the segments onto main store addresses, and make this available to the hardware, then the hardware can calculate the exact main store location of any item cross-referenced by a "segment number plus displacement" address. If a segment table entry also contains a marker indicating whether the segment is present in main store, or is temporarily held on a secondary storage device (e.g. a drum), and a record of the length of each segment (see Fig. 1), then we have the rudiments of a segmented virtual store. This concept allows part or all of a process-image to reside temporarily outside main store on some secondary storage device, and thus in principle solves our problems of (a) a process-image which exceeds the size of main store, and (b) competition for main store usage in a time-sharing environment. Our remaining problem (c) of variable length segments can in principle be solved by allowing the recorded segment length to be changed.

The hardware procedure for translating a "segment number plus displacement" address (i.e. a virtual address) into a main store address is as follows. If Pi indicates that segment i is not in main store, the hardware causes an interrupt to allow the software to read the segment into main store; otherwise the virtual address i (segment number), j (displacement) is calculated as Ri + j. A further advantage of this scheme is that the test j >Li reveals erroneous attempts to jump to non-existent code or to access non-existent data beyond the upper bound of any segment.

Although this segmentation scheme is conceptually complete, the practicalities of multiprogramming require the introduction of certain modifications for the sake of efficiency. The existence of a separate entry in each virtual machine's segment table for those segments required in all virtual machines (e.g. operating systems

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