Shared graphics memory: Difference between revisions

Revision as of 09:29, 24 May 2018

Revision as of 22:11, 3 November 2018

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In [[computer architecture]], '''shared graphics memory''' 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  [[Mebibyte|MiB]] RAM set up with 64MiB64 MiB graphics RAM will appear to the [[operating system]] and user to only have 448 MiB 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.

A similar approach that gave similar results is the boost up of graphics used in some [[Silicon Graphics|SGi]] computers, most notably the [[SGI O2|O2/O2+]]. The memory in these machines is simply one fast pool (2.1  GB per second in 1996) shared between system and graphics. Sharing is performed on demand, including pointer redirection communication between main system and graphics subsystem. This is called Unified Memory Architecture (UMA).

== History ==

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

An early hybrid system was the Commodore Amiga which could run as a shared memory system, but would load executable code preferentially into non-shared "[[Amiga Chip RAM|fast ramRAM]]" if it was available.

== See also ==