Page 134 - DCAP403_Operating System

Unit 7: Memory Management

In a virtual memory system, it is common to map between virtual addresses and physical Notes
addresses by means of a data structure called a page table. A page table is the data structure
used by a virtual memory system in a computer operating system to store the mapping between
virtual addresses and physical addresses. Virtual addresses are those unique to the accessing
process. Physical addresses are those unique to the CPU, i.e., RAM.

The page number of an address is usually found from the most significant bits of the address; the
remaining bits yield the offset of the location within the page. The page table is normally indexed
by page number and contains information on whether the page is currently in main memory, and
where it is in main memory or on disk.
Conventional page tables are sized to the virtual address space and store the entire virtual address
space description of each process. Because of the need to keep the virtual-to-physical translation

time low, a conventional page table is structured as a fixed, multi-level hierarchy, and can be very
inefficient at representing a sparse virtual address space, unless the allocated pages are carefully

aligned to the page table hierarchy.
7.6 Segmentation

It is very common for the size of program modules to change dynamically. For instance, the
programmer may have no knowledge of the size of a growing data structure. If a single address
space is used, as in the paging form of virtual memory, once the memory is allocated for modules
they cannot vary in size. This restriction results in either wastage or shortage of memory. To
avoid the above problem, some computer systems are provided with many independent address
spaces. Each of these address spaces is called a segment. The address of each segment begins with
0 and segments may be compiled separately. In addition, segments may be protected individually
or shared between processes. However, segmentation is not transparent to the programmer like
paging. The programmer is involved in establishing and maintaining the segments.
Segmentation is one of the most common ways to achieve memory protection like paging. An
instruction operand that refers to a memory location includes a value that identifi es a segment
and an offset within that segment. A segment has a set of permissions, and a length, associated
with it. If the currently running process is allowed by the permissions to make the type of
reference to memory that it is attempting to make, and the offset within the segment is within the
range specifi ed by the length of the segment, the reference is permitted; otherwise, a hardware
exception is delivered.
In addition to the set of permissions and length, a segment also has associated with it information
indicating where the segment is located in memory. It may also have a flag indicating whether

the segment is present in main memory or not; if the segment is not present in main memory,
an exception is delivered, and the operating system will read the segment into memory from
secondary storage. The information indicating where the segment is located in memory might
be the address of the first location in the segment, or might be the address of a page table for the

segment. In the first case, if a reference to a location within a segment is made, the offset within
the segment will be added to address of the fi rst location in the segment to give the address in
memory of the referred-to item; in the second case, the offset of the segment is translated to a
memory address using the page table.
A memory management unit (MMU) is responsible for translating a segment and offset within
that segment into a memory address, and for performing checks to make sure the translation can
be done and that the reference to that segment and offset is permitted.

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