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Mach's [[virtual_memory]] subsystem uses *memory objects* for supplying the
content of regions of virtual memory in an [[virtual_address_space]].

All of these objects are managed by *memory manager*s, that are also called
*pager*s.  These can be implemented as user-space processes.

Both the memory objects, and their managers are kernel objects, and are
accessed by [[port]]s.

A system's physical memory is conceived as a *memory cache* that contains
*memory cache objects*.  So when a [[thread]] accesses a page in its task's
address space, the memory object that includes this page is *cached* in the
memory cache.  Memory objects are [[paged out and paged
in|external_pager_mechanism]] by the aforementioned memory managers.  The
decision when they should be paged in or paged out is left to [[Mach]].  Each
memory object has an ordered list of memory managers that provide paging.  The
last one tried is the *default memory manager* that resides in the microkernel,
in contrast to most of the others.  The default memory manager is needed
because the microkernel can't wait infinitely for someone else to free the
memory cache: it just calls the next memory manager hoping it to succeed.

Read about [[GNU Mach's memory management|gnumach/memory_management]].