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<h4 class="subsection">22.4.1 Overview about traditional Unix memory handling</h4>

<p><a name="index-address-space-2772"></a><a name="index-physical-memory-2773"></a><a name="index-physical-address-2774"></a>Unix systems normally provide processes virtual address spaces.  This
means that the addresses of the memory regions do not have to correspond
directly to the addresses of the actual physical memory which stores the
data.  An extra level of indirection is introduced which translates
virtual addresses into physical addresses.  This is normally done by the
hardware of the processor.

   <p><a name="index-shared-memory-2775"></a>Using a virtual address space has several advantage.  The most important
is process isolation.  The different processes running on the system
cannot interfere directly with each other.  No process can write into
the address space of another process (except when shared memory is used
but then it is wanted and controlled).

   <p>Another advantage of virtual memory is that the address space the
processes see can actually be larger than the physical memory available. 
The physical memory can be extended by storage on an external media
where the content of currently unused memory regions is stored.  The
address translation can then intercept accesses to these memory regions
and make memory content available again by loading the data back into
memory.  This concept makes it necessary that programs which have to use
lots of memory know the difference between available virtual address
space and available physical memory.  If the working set of virtual
memory of all the processes is larger than the available physical memory
the system will slow down dramatically due to constant swapping of
memory content from the memory to the storage media and back.  This is
called &ldquo;thrashing&rdquo;. 
<a name="index-thrashing-2776"></a>
<a name="index-memory-page-2777"></a><a name="index-page_002c-memory-2778"></a>A final aspect of virtual memory which is important and follows from
what is said in the last paragraph is the granularity of the virtual
address space handling.  When we said that the virtual address handling
stores memory content externally it cannot do this on a byte-by-byte
basis.  The administrative overhead does not allow this (leaving alone
the processor hardware).  Instead several thousand bytes are handled
together and form a <dfn>page</dfn>.  The size of each page is always a power
of two byte.  The smallest page size in use today is 4096, with 8192,
16384, and 65536 being other popular sizes.

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