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→Memory Design
Effectively, memory is divided into "pages" of a set size (such as 4kb, 1MB, or 4MB). These pages are ''mapped'' using a memory mapping table or address translation table, which renumbers the addresses of the locations within that page. Pages which do not appear in the memory map are not accessible to the CPU. It is also possible to mark specific attributes for each page in the mapping table, such as "do not execute" and "read only".
For example, a computer may have three processes, "A", "B", an and "C". Only one process is active at a time (assuming a single-core model), and the operating system switches between the processes whenever they are eligible to run to create the appearance of concurrent execution. (A program is not eligible to run if it is blocked by something -- for example, when it is waiting for data from the disk, network, or keyboard, and that data hasn't arrived yet).
Process A and B are running two separate programs, so the memory map is set so that the pages used by the first program are visible when process A is running; pages used for the data used by that process are also made visible. The memory map is changed so that the pages of the second program and data space are visible when process B is running. Neither process can access the software or data of the other program.
