Overlays In CP/M

                        This concept of “overlays” is going to be simple with CP/M. But in more advanced operating systems, this is going to be a little trickier. The maximum of memory in CP/m was constrained, by the amount of the memory that processor could address. Initially this was 64 KB (with Intel 8080, 8085, Zilog Z-80), but some processors which were introduced later, allowed more memory to be addressed. But what if a program would not fit in the available memory space? This problem has been an issue, almost since the birth of electro-mechanical computing.
                        A human can solve this kind of problem very easily: If the available space is small, then bring only what is needed at a time. But computers only do the things that they are told to do. So after tell it, a program that manipulates large amount of data, brings only the required data on memory, remaining data is kept on the disk till it is needed. Similar stuff is done with programs, which have large binary codes: only the required code is brought in the memory. These parts “overlay” each-other at the same location in memory, and are called as overlays.
                        Programs with large amount of binary code, have to be divided into different parts. The main part of the program is always memory resident. The programmer (while conceiving the program) has to identify the parts of the program, that should be grouped together in an overlay. While designing this, it is important to avoid one overlay calling a different overlay that would take the place of the first one in memory. Actually, loading of the overlays is done by the programming language’s runtime library, which uses CP/M OS system calls to load an overlay. The programmer has to indicate the compiler that which parts of a program, which functions, and procedures should be in each overlay. Then the compiler produces the loadable overlay code. In following diagram, the program has one main part and three overlays. Only one of the overlay will be in memory at any particular time.