Virtual vs Cache

7/25/2019 Virtual vs Cache

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Virtual Memory Cache Memory

Introduction and Difference

Introduction

Virtual Memory

Virtual (or logical) memory is a concept that, when implemented by a computer and its operating system,

allows programmers to use a very large range of memory or storage addresses for stored data. The

computing system maps the programmer's virtual addresses to real hardware storage addresses. Usually,

the programmer is freed from having to be concerned about the availability of data storage.

In addition to managing the mapping of virtual storage addresses to real storage addresses, a computer

implementing virtual memory or storage also manages storage swapping between active storage (RAM)

and hard disk or other high volume storage devices. Data is read in units called "pages" of sizes ranging

from a thousand bytes (actually 1,024 decimal bytes) up to several megabytes in size. This reduces the

amount of physical storage access that is required and speeds up overall system performance.

Cache Memory

A small amount of high-speed memory residing on or close to the CPU is called Cache Memory. Cache

memory supplies the processor with the most frequently requested data and instructions. Level 1 cache(primary cache) is the cache closest to the processor. Level 2 cache (secondary cache) is the cache second

closest to the processor and is usually on the motherboard.

Cache memory helps to alleviate the gap between the speed of a CPU's megahertz rating and the ability

of RAM to respond and deliver data. It reduces the frequency that the CPU must wait for data from the

main memory.


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Description

Virtual Memory

Virtual memory is acomputersystem technique which givesan application program the impression that it has

contiguous working memory (an address space), while in

fact it may be physically fragmented and may even overflow

on to disk storage.

Developed formultitaskingkernels,virtual memory provides

two primary functions:

1.

Each process has its own address space, thereby not

required to berelocatednor required to userelative

addressing mode.

2.

Each process sees one contiguous block of free

memory upon launch.Fragmentationis hidden.

All implementations (excluding emulators) require hardware

support. This is typically in the form of a Memory

Management Unitbuilt into the CPU.

Systems that use this technique make programming of large

applications easier and use real physical memory (e.g.RAM)

more efficiently than those without virtual memory. Virtual

memory differs significantly from memory virtualization in

that virtual memory allows resources to be virtualized as memory for a specific system, as opposed to a

large pool of memory being virtualized as smaller pools for many different systems.

Note that "virtual memory" is more than just "using disk space to extend physical memory size" - that is

merely the extension of thememory hierarchyto includehard disk drives.Extending memory to disk is a

normal consequence of using virtual memory techniques, but could be done by other means such as

overlays or swapping programs and their data completely out to disk while they are inactive. The

definition of "virtual memory" is based on redefining the address space with a contiguous virtual memory

addressesto "trick" programs into thinking they are using large blocks of contiguousaddresses.

Modern general-purpose computer operating systems generally use virtual memory techniques for

ordinary applications, such as word processors, spreadsheets, multimedia players, accounting, etc.,

except where the required hardware support (a memory management unit) is unavailable. Older

operating systems, such asDOSof the 1980s, or those for themainframesof the 1960s, generally had no

virtual memory functionality - notable exceptions being theAtlas,B5000andApple Computer'sLisa.

Embedded systems and other special-purpose computer systems which require very fast and/or very

consistent response times may opt not to use virtual memory due to decreased determinism. This is

based on the idea that unpredictable processor exceptions produce unwantedjitteron CPU operated I/O,

which the smaller embedded processors often performdirectlyto keep cost and power consumption low.

And the associated simple application has little use for multitasking features.
http://en.wikipedia.org/wiki/Computerhttp://en.wikipedia.org/wiki/Computerhttp://en.wikipedia.org/wiki/Computerhttp://en.wikipedia.org/wiki/Computer_multitaskinghttp://en.wikipedia.org/wiki/Computer_multitaskinghttp://en.wikipedia.org/wiki/Kernel_%28computing%29http://en.wikipedia.org/wiki/Kernel_%28computing%29http://en.wikipedia.org/wiki/Kernel_%28computing%29http://en.wikipedia.org/wiki/Relocation_%28computer_science%29http://en.wikipedia.org/wiki/Relocation_%28computer_science%29http://en.wikipedia.org/wiki/Relocation_%28computer_science%29http://en.wikipedia.org/wiki/Addressing_mode#PC-relativehttp://en.wikipedia.org/wiki/Addressing_mode#PC-relativehttp://en.wikipedia.org/wiki/Addressing_mode#PC-relativehttp://en.wikipedia.org/wiki/Addressing_mode#PC-relativehttp://en.wikipedia.org/wiki/Fragmentation_%28computer%29http://en.wikipedia.org/wiki/Fragmentation_%28computer%29http://en.wikipedia.org/wiki/Fragmentation_%28computer%29http://en.wikipedia.org/wiki/Memory_Management_Unithttp://en.wikipedia.org/wiki/Memory_Management_Unithttp://en.wikipedia.org/wiki/Memory_Management_Unithttp://en.wikipedia.org/wiki/RAMhttp://en.wikipedia.org/wiki/RAMhttp://en.wikipedia.org/wiki/RAMhttp://en.wikipedia.org/wiki/Memory_virtualizationhttp://en.wikipedia.org/wiki/Memory_virtualizationhttp://en.wikipedia.org/wiki/Memory_hierarchyhttp://en.wikipedia.org/wiki/Memory_hierarchyhttp://en.wikipedia.org/wiki/Memory_hierarchyhttp://en.wikipedia.org/wiki/Hard_disk_driveshttp://en.wikipedia.org/wiki/Hard_disk_driveshttp://en.wikipedia.org/wiki/Hard_disk_driveshttp://en.wikipedia.org/wiki/Overlay_%28programming%29http://en.wikipedia.org/wiki/Overlay_%28programming%29http://en.wikipedia.org/wiki/Operating_systemhttp://en.wikipedia.org/wiki/Operating_systemhttp://en.wikipedia.org/wiki/Memory_management_unithttp://en.wikipedia.org/wiki/Memory_management_unithttp://en.wikipedia.org/wiki/DOShttp://en.wikipedia.org/wiki/DOShttp://en.wikipedia.org/wiki/DOShttp://en.wikipedia.org/wiki/Mainframe_computerhttp://en.wikipedia.org/wiki/Mainframe_computerhttp://en.wikipedia.org/wiki/Mainframe_computerhttp://en.wikipedia.org/wiki/Atlas_Computer_%28Manchester%29http://en.wikipedia.org/wiki/Atlas_Computer_%28Manchester%29http://en.wikipedia.org/wiki/Atlas_Computer_%28Manchester%29http://en.wikipedia.org/wiki/B5000http://en.wikipedia.org/wiki/B5000http://en.wikipedia.org/wiki/B5000http://en.wikipedia.org/wiki/Apple_Inc.http://en.wikipedia.org/wiki/Apple_Inc.http://en.wikipedia.org/wiki/Apple_Inc.http://en.wikipedia.org/wiki/Apple_Lisahttp://en.wikipedia.org/wiki/Apple_Lisahttp://en.wikipedia.org/wiki/Apple_Lisahttp://en.wikipedia.org/wiki/Embedded_systemhttp://en.wikipedia.org/wiki/Embedded_systemhttp://en.wikipedia.org/wiki/Deterministic_algorithmhttp://en.wikipedia.org/wiki/Deterministic_algorithmhttp://en.wikipedia.org/wiki/Jitterhttp://en.wikipedia.org/wiki/Jitterhttp://en.wikipedia.org/wiki/Jitterhttp://en.wikipedia.org/wiki/Bit-banginghttp://en.wikipedia.org/wiki/Bit-banginghttp://en.wikipedia.org/wiki/Bit-banginghttp://en.wikipedia.org/wiki/Bit-banginghttp://en.wikipedia.org/wiki/Jitterhttp://en.wikipedia.org/wiki/Deterministic_algorithmhttp://en.wikipedia.org/wiki/Embedded_systemhttp://en.wikipedia.org/wiki/Apple_Lisahttp://en.wikipedia.org/wiki/Apple_Inc.http://en.wikipedia.org/wiki/B5000http://en.wikipedia.org/wiki/Atlas_Computer_%28Manchester%29http://en.wikipedia.org/wiki/Mainframe_computerhttp://en.wikipedia.org/wiki/DOShttp://en.wikipedia.org/wiki/Memory_management_unithttp://en.wikipedia.org/wiki/Operating_systemhttp://en.wikipedia.org/wiki/Overlay_%28programming%29http://en.wikipedia.org/wiki/Hard_disk_driveshttp://en.wikipedia.org/wiki/Memory_hierarchyhttp://en.wikipedia.org/wiki/Memory_virtualizationhttp://en.wikipedia.org/wiki/RAMhttp://en.wikipedia.org/wiki/Memory_Management_Unithttp://en.wikipedia.org/wiki/Memory_Management_Unithttp://en.wikipedia.org/wiki/Fragmentation_%28computer%29http://en.wikipedia.org/wiki/Addressing_mode#PC-relativehttp://en.wikipedia.org/wiki/Addressing_mode#PC-relativehttp://en.wikipedia.org/wiki/Relocation_%28computer_science%29http://en.wikipedia.org/wiki/Kernel_%28computing%29http://en.wikipedia.org/wiki/Computer_multitaskinghttp://en.wikipedia.org/wiki/Computer


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Cache Memory

Pronounced cash, a special high-speed

storage mechanism. It can be either a

reserved section ofmain memoryor an

independent high-speedstorage device.

Two types of caching are commonly

used inpersonal computers:memory cachingand disk caching.

A memory cache, sometimes called a cache storeorRAM cache, is a portion ofmemorymade of high-

speed static RAM (SRAM) instead of the slower and cheaper dynamic RAM (DRAM) used for main

memory. Memory caching is effective because mostprogramsaccessthe samedataorinstructionsover

and over. By keeping as much of this information as possible in SRAM, the computeravoids accessing the

slower DRAM.

Some memory caches are built into thearchitectureofmicroprocessors.The Intel 80486 microprocessor,

for example, contains an 8K memory cache, and the Pentium has a 16K cache. Such internal cachesare

often calledLevel 1 (L1) caches.Most modern PCs also come with external cache memory, calledLevel 2

(L2) caches.These caches sit between theCPUand the DRAM. Like L1 caches, L2 caches are composed of

SRAM but they are much larger.

Disk caching works under the same principle as memory caching, but instead of using high-speed SRAM, a

disk cacheuses conventional main memory. The most recently accessed data from the disk (as well as

adjacentsectors) isstoredin a memory buffer. When a program needs to access data from the disk, it

first checks the disk cache to see if the data is there. Disk caching can dramatically improve the

performance ofapplications,because accessing abyteof data inRAMcan be thousands of times faster

than accessing a byte on ahard disk.

When data is found in the cache, it is called a cache hit,and the effectiveness of a cache is judged by its

hit rate.Many cachesystemsuse a technique known as smart caching,in which the system can recognize

certain types of frequently used data. The strategies for determining which information should be kept in

the cache constitute some of the more interesting problems incomputer science.

Pronounced cash, a special high-speedstoragemechanism. It can be either a reserved section ofmain

memory or an independent high-speed storage device. Two types of caching are commonly used in

personal computers:memory cachingand disk caching.

A memory cache, sometimes called a cache storeorRAM cache, is a portion ofmemorymade of high-

speed static RAM (SRAM) instead of the slower and cheaper dynamic RAM (DRAM) used for mainmemory. Memory caching is effective because mostprogramsaccessthe samedataorinstructionsover

and over. By keeping as much of this information as possible in SRAM, the computeravoids accessing the

slower DRAM.

Some memory caches are built into thearchitectureofmicroprocessors.The Intel 80486 microprocessor,

for example, contains an 8K memory cache, and the Pentium has a 16K cache. Such internal cachesare

often calledLevel 1 (L1) caches.Most modern PCs also come with external cache memory, calledLevel 2

(L2) caches.These caches sit between theCPUand the DRAM. Like L1 caches, L2 caches are composed of

SRAM but they are much larger.
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Disk caching works under the same principle as memory caching, but instead of using high-speed SRAM, a

disk cacheuses conventional main memory. The most recently accessed data from the disk (as well as

adjacentsectors) isstoredin a memory buffer. When a program needs to access data from the disk, it

first checks the disk cache to see if the data is there. Disk caching can dramatically improve the

performance ofapplications,because accessing abyteof data inRAMcan be thousands of times faster

than accessing a byte on ahard disk.

When data is found in the cache, it is called a cache hit,and the effectiveness of a cache is judged by its

hit rate.Many cachesystemsuse a technique known as smart caching,in which the system can recognize

certain types of frequently used data. The strategies for determining which information should be kept in

the cache constitute some of the more interesting problems incomputer science.

References www.wikipedia.org

www.answers.com

www.webopedia.com

www.intel.com
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Virtual vs Cache