Video RAM

Video RAM (VRAM) is a term generally used in computers to describe RAM dedicated to the purpose of displaying bitmap graphics in raster graphics hardware. Video RAM is usually physically separate from the main RAM residing in a computer system. This is not always the case as in some systems the VRAM is shared with the system RAM.

The term VRAM is used more specifically to describe a type of dual-ported RAM used in high performance raster graphics hardware. In a raster graphics system, the VRAM is called upon to constantly refresh the display, and at the same time be responsive to modifications from the host system. Without using VRAM a computer's processor (or the video card's own drawing engine) has contended access to the video RAM while the video hardware is reading the image onto the screen. Each access is held up by one or more cycles. This pause (called a wait state) becomes relatively expensive as the speed of the processor increases. Using VRAM the processor can effectively run at full speed all the time.

VRAM was typically used only for a short time when the pause was expensive enough to be an issue, but the computer bus and video DRAM was still fairly slow. It was used extensively in the Apple Macintosh models of the early 1990s for instance, at least those models with built-in video hardware. As the performance of common DRAM in video cards and computer busses increased the requirement for dedicated, expensive dual port RAM dropped away.