Vertual Memory Overlays Исполнитель

Vertual Memory

Overlays

Many programs are too long to fit into the space in main memory that can be allocated to them at run time. In a uniprogramming system, this will be true  when the amount of space required by the program is greater than the total  memory available to problem programs. In a multiprogramming system it may be true because the amount of space  that is needed is more than the operating system is willing to allocate to this program up into sections, segments, or overlays so that the entire program need not be in main memory at the same time. The  term folding has sometimes been used for this process.

In many old systems, the programmer had the responsibility for breaking the program  into overlays and for providing the loading instructions that bring necessary overlays into main memory as they are needed. Many software systems provided aids to overlay planning. The user  could name the overlays so that all symbolic addresses in an overlay would automatically be tagged with a special identifier that indicated which overlay they belonged to. A loader or linker created an object program organized as a set of overlays and a root segment containing information about  the overlay structure. The root segment would be loaded into main memory along with the segments needed to get the program started. Any reference to a symbolic address in a segment not in main memory would cause a call on the supervisor to load the required segment, overlaying other segments if necessary. 

Vocabulary

loader - погрузчик

Willing- желающий

Necessary- необходимый

segments - доли