Virtual Memory - Revision history

Dmiyusov: /* Overlays */

2007-11-20T23:22:56Z

Overlays

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	==Overlays==		==Overlays==
	This technique allows programs to be larger than the CPU's main memory. In this method a program is divided into self-contained object code blocks called overlays where the size of the overlay is limited according to memory constraints. The place where overlays are loaded are refered to as 'regions' (r1/r2 in the diagram). These regions can be of a different size. An overlay manager, such as an operating system loads the required overlay from external memory into this region. [3]		This technique allows programs to be larger than the CPU's main memory. In this method a program is divided into self-contained object code blocks called overlays where the size of the overlay is limited according to memory constraints. The place where overlays are loaded are refered to as 'regions' (R1/R2 in the diagram). These regions can be of a different size. An overlay manager, such as an operating system loads the required overlay from external memory into this region. [3]

	[[Image:diagram2.jpg]]		[[Image:diagram2.jpg]]

Dmiyusov: /* Dynamic Paging algorithms */

2007-11-20T23:22:15Z

Dynamic Paging algorithms

← Older revision		Revision as of 23:22, 20 November 2007
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	[1] http://www.ptdd.com/datarecovery/swap.htm		[1] http://www.ptdd.com/datarecovery/swap.htm

	[2] Nutt, Gary. Operating systems ~~2004~~. Boulder Colorado.		[2] Nutt, Gary (2004). Operating systems. Boulder Colorado.

	[3] http://wikipedia.org/		[3] http://wikipedia.org/

Dmiyusov: /* Paging */

2007-11-20T23:19:51Z

Paging

← Older revision		Revision as of 23:19, 20 November 2007
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	5. Loads the requested data into the available page.		5. Loads the requested data into the available page.

	6.Returns control to the program, transparently retrying the instruction that caused page fault.		6. Returns control to the program, transparently retrying the instruction that caused page fault.
	[3]		[3]

Dmiyusov: /* Paging */

2007-11-20T23:19:30Z

Paging

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	1. Handles the page fault, in a manner invisible to the causing program, and takes control.		1. Handles the page fault, in a manner invisible to the causing program, and takes control.

	2. Determines the location of the data in auxiliary storage.		2. Determines the location of the data in auxiliary storage.

	3. Determines the page frame in RAM to use as a container for the data.		3. Determines the page frame in RAM to use as a container for the data.

	4. If a page currently residing in chosen frame has been modified since loading (if it is dirty), writes the page to auxiliary storage.		4. If a page currently residing in chosen frame has been modified since loading (if it is dirty), writes the page to auxiliary storage.

	5. Loads the requested data into the available page.		5. Loads the requested data into the available page.

	6.Returns control to the program, transparently retrying the instruction that caused page fault.		6.Returns control to the program, transparently retrying the instruction that caused page fault.
	[3]		[3]

Dmiyusov at 23:19, 20 November 2007

2007-11-20T23:19:03Z

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	The main functions of paging are:		The main functions of paging are:

			1. Handles the page fault, in a manner invisible to the causing program, and takes control.
			2. Determines the location of the data in auxiliary storage.
			3. Determines the page frame in RAM to use as a container for the data.
			4. If a page currently residing in chosen frame has been modified since loading (if it is dirty), writes the page to auxiliary storage.
			5. Loads the requested data into the available page.
			6.Returns control to the program, transparently retrying the instruction that caused page fault.
			[3]

Dmiyusov: /* Paging */

2007-11-20T23:16:21Z

Paging

← Older revision		Revision as of 23:16, 20 November 2007
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	==Paging==		==Paging==
	~~Thisis an~~ allocation strategy ~~in a paging system to reduce external fragmentation by transferring a~~ fixed-~~size unit of virtual address space(page) whenever a virtual address in it is needed~~ to ~~execute the program~~. Every process’s virtual address space is logically divided into pages, ~~with each~~ page ~~having the same number~~ of ~~location.~~		A memory allocation strategy using fixed-length pages as opposed to variable-length segments. Every process’s virtual address space is logically divided into pages.
			Main function of paging are performed when a program tries to access pages that do not currently reside in RAM, this is a situation which causes page fault.
			The main functions of paging are:

	The program translation image is constructed to fit in a set of G continguous memory locations, with virtual addresses ranging between 0 and G-1. When the process executes it is allocated enough primary memory to store the contents of H memory locations where H<G (that is the process has fewer physical memory locations than virtual addresses)[2]

	In a binary computer the paging system maps the virtual addressees in the absolute module (0-G) into a set of n = 2^g pages, each of size c = 2^h. If G is not an integer multiple of the page size then part of the last page will be lost to internal fragmentation; the paging system will adjust G upward to hold the entire virtual address space. [2]

	Physical address space is the portion of the primary memory allocated to the process. The units of allocation are page frames and these are blocks of primary memory which are the same size as the page. The physical address space can be though of as a set of m = 2^j page frames each of size c = 2^h, so the amount of primary memory allocated to the process is H = 2^(h+j) [2]


	[[Image:The_address_space_and_pages.JPG]]		[[Image:The_address_space_and_pages.JPG]]


	==Paging Algorithms==		==Paging Algorithms==

Dmiyusov: /* Dynamic Paging algorithms */

2007-11-20T22:56:02Z

Dynamic Paging algorithms

← Older revision		Revision as of 22:56, 20 November 2007
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	[1] http://www.ptdd.com/datarecovery/swap.htm		[1] http://www.ptdd.com/datarecovery/swap.htm

	[2] Nutt, Gary. Operating systems 2004. Boulder Colorado.		[2] Nutt, Gary. Operating systems 2004. Boulder Colorado.

	[3] http://wikipedia.org/		[3] http://wikipedia.org/

Dmiyusov at 22:55, 20 November 2007

2007-11-20T22:55:47Z

← Older revision		Revision as of 22:55, 20 November 2007
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	==Virtual Memory==		==Virtual Memory==
	Virtual mermory is system memory which is simulated by the hard drive. In the case that all of the RAM is used up, the computer can simulate extra memory by swapping data to the hard drive and back, giving an impression that there is more memory. Extending memory can be done by other ways including overlays and swapping.		Virtual mermory is system memory which is simulated by the hard drive. In the case that all of the RAM is used up, the computer can simulate extra memory by swapping data to the hard drive and back, giving an impression that there is more memory. Extending memory can be done by other ways including overlays and swapping.[3]

	==Swapping==		==Swapping==
			A technique that enables computer to execute programs and manipulate data files larger than main memory. An operating system copies as much data as possible into main memory leaving the rest on disk. When it needs data from disk , thsi data exchanges a portion of data (page or segment) in main memory with a portion of data on disk [1]
	A memory management technique where a process may periodically have its primary memory space deallocated. This forces the swapped-out process to compete for memory before it can once again compete for the processor		A memory management technique where a process may periodically have its primary memory space deallocated. This forces the swapped-out process to compete for memory before it can once again compete for the processor

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	==Overlays==		==Overlays==
	This technique allows programs to be larger than the CPU's main memory. In this method a program is divided into self-contained object code blocks called overlays where the size of the overlay is limited according to memory constraints. The place where overlays are loaded are refered to as 'regions' (r1/r2 in the diagram). These regions can be of a different size. An overlay manager, such as an operating system loads the required overlay from external memory into this region.		This technique allows programs to be larger than the CPU's main memory. In this method a program is divided into self-contained object code blocks called overlays where the size of the overlay is limited according to memory constraints. The place where overlays are loaded are refered to as 'regions' (r1/r2 in the diagram). These regions can be of a different size. An overlay manager, such as an operating system loads the required overlay from external memory into this region. [3]

	[[Image:diagram2.jpg]]		[[Image:diagram2.jpg]]
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	;'''Data fragmentation'''		;'''Data fragmentation'''
	Reduces the amount of fragmentation in file systems. Occurs when a piece of data in memory is broken up into many pieces. Two types of fragmentation which exist are internal fragmentation and external fragmentation		Reduces the amount of fragmentation in file systems. Occurs when a piece of data in memory is broken up into many pieces. Two types of fragmentation which exist are internal fragmentation and external fragmentation [2]

	;'''Internal Fragmentation'''		;'''Internal Fragmentation'''
	Space is wasted in return for increased efficiency or simplicity. An example is that many file systems files always start at the beginning of a sector, which simplifies organization and makes it easier to grow files. Any space that is left over between the last byte of the file and the first byte of the next sector is internal fragmentation. Internal fragmentation occurs when the index pages are not being used to their maximum volume		Space is wasted in return for increased efficiency or simplicity. An example is that many file systems files always start at the beginning of a sector, which simplifies organization and makes it easier to grow files. Any space that is left over between the last byte of the file and the first byte of the next sector is internal fragmentation. Internal fragmentation occurs when the index pages are not being used to their maximum volume [2]

	;'''External Fragmentation'''		;'''External Fragmentation'''
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	Thisis an allocation strategy in a paging system to reduce external fragmentation by transferring a fixed-size unit of virtual address space(page) whenever a virtual address in it is needed to execute the program. Every process’s virtual address space is logically divided into pages, with each page having the same number of location.		Thisis an allocation strategy in a paging system to reduce external fragmentation by transferring a fixed-size unit of virtual address space(page) whenever a virtual address in it is needed to execute the program. Every process’s virtual address space is logically divided into pages, with each page having the same number of location.

	The program translation image is constructed to fit in a set of G continguous memory locations, with virtual addresses ranging between 0 and G-1. When the process executes it is allocated enough primary memory to store the contents of H memory locations where H<G (that is the process has fewer physical memory locations than virtual addresses)		The program translation image is constructed to fit in a set of G continguous memory locations, with virtual addresses ranging between 0 and G-1. When the process executes it is allocated enough primary memory to store the contents of H memory locations where H<G (that is the process has fewer physical memory locations than virtual addresses)[2]

	In a binary computer the paging system maps the virtual addressees in the absolute module (0-G) into a set of n = 2^g pages, each of size c = 2^h. If G is not an integer multiple of the page size then part of the last page will be lost to internal fragmentation; the paging system will adjust G upward to hold the entire virtual address space.		In a binary computer the paging system maps the virtual addressees in the absolute module (0-G) into a set of n = 2^g pages, each of size c = 2^h. If G is not an integer multiple of the page size then part of the last page will be lost to internal fragmentation; the paging system will adjust G upward to hold the entire virtual address space. [2]

			Physical address space is the portion of the primary memory allocated to the process. The units of allocation are page frames and these are blocks of primary memory which are the same size as the page. The physical address space can be though of as a set of m = 2^j page frames each of size c = 2^h, so the amount of primary memory allocated to the process is H = 2^(h+j) [2]

	Physical address space is the portion of the primary memory allocated to the process. The units of allocation are page frames and these are blocks of primary memory which are the same size as the page. The physical address space can be though of as a set of m = 2^j page frames each of size c = 2^h, so the amount of primary memory allocated to the process is H = 2^(h+j)

	[[Image:The_address_space_and_pages.JPG]]		[[Image:The_address_space_and_pages.JPG]]
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	''Edited by Dmitrii Miyusov ''		''Edited by Dmitrii Miyusov ''

			Rerefeces:

			[1] http://www.ptdd.com/datarecovery/swap.htm
			[2] Nutt, Gary. Operating systems 2004. Boulder Colorado.
			[3] http://wikipedia.org/

Dmiyusov at 22:45, 20 November 2007

2007-11-20T22:45:41Z

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	Classic problem of memory management:		Classic problem of memory management:
	''Fragmentation'' - difference between size of overlay area and size of the overlays		'''Fragmentation''' - difference between size of overlay area and size of the overlays

	Instead of having one overlay area what if we had many?		Instead of having one overlay area what if we had many?

Dmiyusov at 22:43, 20 November 2007

2007-11-20T22:43:14Z

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	'''~~Memory Management &~~ Virtual Memory Lecture 10'''		'''Virtual Memory Lecture 10'''

	==Virtual Memory==		==Virtual Memory==