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→Memory Design
In addition to cache, briefly described above, most modern computer systems use some type of paged memory design, memory maps, and a memory management unit (MMU) to control memory.
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", and "C". Only one process is active at a time (assuming a single-core model), and the operating system switches between the processes frequently 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).
