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Video RAM (VRAM)
The traditional, standard DRAM used for video cards typically does not have enough bandwidth to handle the demands of running a card at high resolution and color depths, with acceptable refresh rates. The main reason why is the two competing access factors for the video memory: the processor writing new information to the memory, and the RAMDAC reading it many times per second in order to send video signals to the monitor.
To address this fundamental limitation, a new type of memory was created called video RAM or VRAM. As the name implies, this memory is specifically tailored for use in video systems. The fundamental difference between VRAM and standard DRAM is that VRAM is dual-ported. This means that it has two access paths, and can be written to and read from simultaneously. The advantages of this are of course enormous given what the video card does: many times per second a new screen image is calculated and written to the memory, and many times per second this memory is read and sent to the monitor. Dual-porting allows these operations to occur without bumping into each other.
Note: Don't confuse VRAM with
the generic term "video RAM" or "video memory", which just refer to
the memory in the video subsystem in general.
VRAM provides substantially more bandwidth than either standard DRAM or EDO DRAM; double in many cases. It is more suited for use in systems requiring high resolution and color depth displays. The only reason that it hasn't replaced standard DRAM entirely is of course: cost. VRAM is more complex and requires more silicon per bit than standard DRAM, which makes it cost more.
Next: Window RAM (WRAM)