The memory that holds the video image is also referred to as the frame buffer and is usually implemented on the graphics card itself. Early systems implemented video memory in standard DRAM. However, this requires continual refreshing of the data to prevent it from being lost and cannot be modified during this refresh process. The consequence, particularly at the very fast clock speeds demanded by modern graphics cards, is that performance is badly degraded.

An advantage of implementing video memory on the graphics board itself is that it can be customised for its specific task and, indeed, this has resulted in a proliferation of new memory technologies:

  • Video RAM (VRAM): a special type of dual-ported DRAM, which can be written to and read from at the same time. It also requires far less frequent refreshing than ordinary DRAM and consequently performs much better
  • Windows RAM (WRAM): as used by the hugely successful Matrox Millennium card, is also dual-ported and can run slightly faster than conventional VRAM
  • EDO DRAM: which provides a higher bandwidth than DRAM, can be clocked higher than normal DRAM and manages the read/write cycles more efficiently
  • SDRAM: Similar to EDO RAM except the memory and graphics chips run on a common clock used to latch data, allowing SDRAM to run faster than regular EDO RAM
  • SGRAM: Same as SDRAM but also supports block writes and write-per-bit, which yield better performance on graphics chips that support these enhanced features
  • DRDRAM: Direct RDRAM is a totally new, general-purpose memory architecture which promises a 20-fold performance improvement over conventional DRAM.

Some designs integrate the graphics circuitry into the motherboard itself and use a portion of the system’s RAM for the frame buffer. This is called unified memory architecture and is used for reasons of cost reduction only. Since such implementations cannot take advantage of specialised video memory technologies they will always result in inferior graphics performance.

The information in the video memory frame buffer is an image of what appears on the screen, stored as a digital bitmap. But while the video memory contains digital information its output medium, the monitor, uses analogue signals. The analogue signal requires more than just an on or off signal, as it’s used to determine where, when and with what intensity the electron guns should be fired as they scan across and down the front of the monitor. This is where the RAMDAC comes in.

The table below summarises the characteristics of six popular types of memory used in graphics subsystems:

EDO VRAM WRAM SDRAM SGRAM RDRAM
Max.

throughput

(MBps)

400 400 960 800 800 600
Dual- or

single-ported

single dual dual single single single
Typical Data Width 64 64 64 64 64 8
Speed (typical) 50-60ns 50-60ns 50-60ns 10-15ns 8-10ns 330MHz clock speed

1998 saw dramatic changes in the graphics memory market and a pronounced market shift toward SDRAMs caused by the price collapse of SDRAMs and resulting price gap with SGRAMs. However, delays in the introduction of RDRAM, coupled with its significant cost premium, saw SGRAM – and in particular DDR SGRAM, which performs I/O transactions on both rising and falling edges of the clock cycle – recover its position of graphics memory of choice during the following year.

The greater number of colours, or the higher the resolution or, the more video memory will be required. However, since it is a shared resource reducing one will allow an increase in the other. The table below shows the possible combinations for typical amounts of video memory:

Video memory Resolution Colour depth No. colours
1Mb 1024×768

800×600

8-bit

16-bit

256

65,536

2Mb 1024×768

1280×1024

800×600

8-bit

16-bit

24-bit

256

65,536

16.7 million

4Mb 1024×768 24-bit 16.7 million
6Mb 1280×1024 24-bit 16.7 million
8Mb 1600×1200 32-bit 16.7 million

Even though the total amount of video memory installed may not be needed for a particular resolution, the extra memory is often used for caching information for the graphics processor. For example, the caching of commonly used graphical items – such as text fonts and icons – avoids the need for the graphics subsystem to load these each time a new letter is written or an icon is moved and thereby improves performance.

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