Shared graphics memory: Difference between revisions

Revision as of 16:09, 29 June 2010

Revision as of 17:18, 1 August 2010

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In [[computer architecture]], '''Shared Memory Architecture''' (SMA) refers to a design where the [[Graphics Card|graphics chip]] does not have its own dedicated memory, and instead shares the main system [[Random access memory|RAM]] with the [[Central processing unit|CPU]] and other components.

This design is used with many integrated graphics solutions to reduce the cost and complexity of the [[motherboard]] design, as no additional memory [[silicon chip|chips]] are required on the board. There is usually some mechanism (via the [[BIOS]] or a [[jumper (computing)|jumper]] setting) to select the amount of system memory to use for graphics, which means that the graphics system can be tailored to only use as much RAM as is actually required, leaving the rest free for applications. A [[Unintended consequence|side effect]] of this is that when some RAM is allocated for graphics, it becomes effectively unavailable for anything else, so an example computer with 512 [[MegabyteMebibyte|MBMiB]] RAM set up with 64MB64MiB graphics RAM will appear to the [[operating system]] and user to only have 448 MBMiB RAM installed.

The disadvantage of this design is lower performance because system RAM usually runs slower than dedicated graphics RAM, and there is more contention as the [[memory bus]] has to be shared with the rest of the system. It may also cause performance issues with the rest of the system if it is not designed with the fact in mind that some RAM will be 'taken away' by graphics.

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== History ==

The first PC to use the SMA was the [[IBM PCjr]], released in 1984. [[Video memory]] was shared with the first 128K128KiB of RAM. The exact size of the video memory could be reconfigured by software to meet the needs of the current program.

Another early design was the [[Commodore International|Commodore]] [[Amiga]], introduced in 1985. It initially featured 256 [[Kibibyte|KiB]] of "[[chip RAM]]" (later up to 2048 KiB, depending on the model). This RAM was used by both the CPU (as main memory) and the Amiga's [[Original Amiga chipset|custom chipset]] (for sound/graphics/IO). By default, most Amiga computers were shipped with chip RAM only, but could be expanded with RAM that only the CPU could access (called "Fast RAM"), through expansion boards.

== See also ==