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Shared graphics memory

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In computer architecture, Shared Memory Architecture (SMA) refers to a design where the graphics chip does not have its own dedicated memory, and instead shares the main system RAM with the 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 chips are required on the board. There is usually some mechanism (via the BIOS or a 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 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 MB RAM set up with 64MB graphics RAM will appear to the operating system and user to only have 448 MB 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.

Other approach that gave similar final state is boost up of graphics, that is used in some SGi computers, most notably O2/O2+. The memory in these machines is simply one fast pool (2,1 GB/s per second in 1996) shared between system and graphics is done on demand including pointer redirection communication between main system and graphics subsystem. This is called Unified Memory Architecture.

History

The first PC to use the SMA was the IBM PCjr, released in 1984. Video memory was shared with the first 128K 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 Amiga in 1985 where the earlier models featured graphics RAM only (256-2048 KB chip RAM) which the CPU also used as system memory with the ability to add CPU-only RAM (fast RAM) through extension boards.

See also

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