A memory map is a massive table, in effect a database, that comprises complete information about how the memory is structured in a computer system. A memory map works something like a gigantic office organizer. In the map, each computer file has a unique memory address reserved especially for it, so that no other data can inadvertently overwrite or corrupt it.
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In order for a computer to function properly, its OS (operating system) must always be able to access the right parts of its memory at the right times. When a computer first boots up (starts), the memory map tells the OS how much memory is available. As the computer runs, the memory map ensures that data is always written to, and read from, the proper places. The memory map also ensures that the computer's debuggers can resolve memory addresses to actual stored data.
If there were no memory map, or if an existing memory map got corrupted, the OS might (and probably would) write data to, and read data from, the wrong places. As a result, when data was read, it would not always pertain to the appropriate files or application programs. The problem would likely start out small and unnoticeable, worsen with time, and become apparent only after considerable damage had been done to stored data and programs. In the end, some or all of the applications would fail to run, and many critical data files would be ruined.
Continue reading about memory maps:
WiseGEEK describes how memory maps work.
Joe Forster portrays a complete memory map for the Commodore 64 computer.