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Physical memory on the system is divided between the area occupied by the kernel and the area available to user processes. Whenever the system is rebooted the size of these areas, as well as the total amount of physical memory, is logged in the file /usr/adm/messages under the heading mem:, for example:
mem: total = 32384k, kernel = 4484k, user = 27900kThis shows a system with 32MB of physical memory; the kernel is using just over 4MB of this memory with the remainder being available for user processes.
Physical memory is divided into equal-sized (4KB) pieces known as pages. When a process starts to run, the first 4KB of the program's text (executable machine instructions) is copied into a page of memory. Each subsequent portion of memory that a process requires is assigned an additional page.
When a process terminates, its pages are returned to the free list of unused pages.
Physical memory is continually used in this way unless the number of running processes require more pages of memory than currently exist on the system. In this case the system must redistribute the available memory by either paging out or swapping.