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Multibank DRAM (MDRAM)

A new type of memory that attempts to address two problems with conventional video memory, Multibank DRAM or MDRAM was invented by MoSys specifically for use in graphics cards. MDRAM differs substantially in design from other types of video memory. Conventional memory designs use a single monolithic "block" of memory for the frame buffer. MDRAM breaks its memory up into multiple 32 KB banks that can be accessed independently. This provides the following advantages:

• Interleaving: Memory accesses can be interleaved between banks, allowing accesses to overlap to provide greater performance. This has the effect of increasing performance without the use of dual porting; the concept is similar to the use of interleaving for the system memory on high-end chipsets.
• Flexibility in Memory Sizing: With conventional memory, it is only practical to make video cards with whole megabytes of memory: you see cards with 1 MB, 2 MB, 4 MB etc. of video RAM. This can cause a great deal of memory waste. For example, to run 1024x768 resolution in true color (24 bits) requires 2.25 MB of video memory for the frame buffer, but a conventional video card would have to be outfitted with 4 MB of memory to support this mode. With MDRAM this restriction is removed and a card can be created with exactly 2.25 MB if desired.
• No Size-Related Performance Penalties: In some conventional video card designs the access speed to the memory is related to the amount of memory used. This means that a 1 MB DRAM card will run slower than a 2 MB one. This limitation does not apply to MDRAM.

MDRAM is also cost-effective to manufacture compared to VRAM and is efficiently organized to reduce waste. MDRAM is suitable for use in high-end applications and is becoming popular due to its performance-enhancing and cost-reducing features. Its most popular implementation is probably on the Tseng Labs ET6000 chipset, currently being used by several card manufacturers.

Next: Comparison of Video Memory Technologies

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