Download Memory Organization

Module IV
Memory Organization
Virtual Memory
• In a virtual memory system, the OS manages
the long programs.
• User programs can be larger than the physical
main memory (2GB).
• OS stores the entire program on a hard disk
whose capacity is much larger than the
physical memory size.
Virtual Memory
• At a given time, only some portions of the
program are brought into the main memory from
the hard disk.
• As and when needed, a portion which is not in
main memory is taken from the hard disk, and at
the same time, a portion of the program which is
present is released out of the main memory and
stored on the hard disk. This process is known as
swapping.
• In a virtual memory system, when a program is
executed, swapping is performed as per the
requirement on a continuous basis.
Virtual Memory
Virtual Memory
• The CPU fetches instruction and data from the main
memory.
• Whenever the required instruction or data is not
present in main memory, the hardware raises an
interrupt known as virtual memory interrupt or page
fault.
• In response, the operating system loads a section of
the program (containing the required instruction or
data) from the hard disk drive to the main memory.
• The page fault occur within an instruction cycle.
• After the page fault interrupt is serviced, the CPU will
continue processing the partially executed instruction.
Virtual Memory
Advantages of Virtual Memory
• Program size is not limited by physical
memory size.
• User do not have to do memory allocation.
Main memory allocation is done automatically
according to the demands of the program.
• Program can be loaded in any area of physical
memory
• It allows processes to share files easily.
Virtual Memory
• It is implemented using two techniques:
– Segmentation
– Paging
Segmentation
• This memory management technique
supports user’s view of memory.
User’s view of a program
Segmentation
• Users prefer to view memory as a collection of
variable-sized segments :
– Code : to store program
– Data : variables defined by the code are stored
– Stack : Used by sub-routines and interrupt service
routines to hold temporary data and addresses.
Segmentation
• All applications need to be loaded into main
memory before they can be executed.
• In order to do so, a temporary 'segment' is
created by the memory manager
A typical segment
Segmentation
• There can be many segments in memory at
the same time.
• Each one is a separate process or application
and each may be a different size
Segmentation
• Segmentation allows the OS to do is to place
each one of those segments in different parts
of physical memory.
Segmentation
Segmentation
• In segmentation, a memory reference includes a
value that identifies a segment (segment
number) and an offset within that segment.
• A segment is associated with
– a flag indicating whether it is present in main memory
or not
– information indicating where the segment is located
in memory(the address of the first location in the
segment)(base address)
– has a set of permissions
– Length
Segmentation
• If the segment is not present in main memory,
an exception is raised and the OS will read the
segment into memory from secondary
storage.
Segmentation
• If a reference to a location within a segment is
made,
– Physical address = base address +offset
• A memory management unit (MMU) is
responsible for
– translating a segment and offset (logical address)
into a memory address,
– checking to make sure the translation can be done
and that the reference to that segment and offset
is permitted.
Segmentation
• The mapping is done with help of segment table.
• Logical address consists of two parts: a segment
number s, and offset d
• s is used as an index to the segment table.
• Each entry of segment table has base and limits.
– base contains starting physical address where resides in
memory
– limit specifies length of the segments
• d must be between 0 and the segment limit. If not, it
traps to addressing error
• If offset is valid, it is added to base to produce physical
address
Segmentation: Hardware
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