All types of VRAM are special arrangements of dynamic RAM (DRAM). VRAM is a buffer between the computer processor and the display, and is often called the frame buffer. When images are to be sent to the display, they are first read by the processor as data from some form of main (non-video) RAM and then written to VRAM.
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From VRAM (the frame buffer), the data is sent as a digital signal via a DVI or HDMI port to a modern, flat-screen LED display. If the display is an older cathode ray tube model -- or if the modern display is connected by an older VGA connector to the video card -- the video signal is first converted by a RAMDAC into analog signals that are sent to the display.
Previous high-performance forms of VRAM were dual-ported, which means that while the processor is writing a new image to VRAM, the display is reading from VRAM to refresh its current display content. The dual-port design was the main difference between system RAM and VRAM in the 1980s and into the 1990s.
Types of VRAM
Multibank Dynamic RAM (MDRAM) is a high-performance RAM, developed by MoSys, that divides memory into multiple 32 kilobyte parts, or banks, that can be accessed individually. Traditional VRAM is monolithic; the entire frame buffer is accessed at one time. Having individual memory banks allows accesses to be performed concurrently, increasing overall performance.
MDRAM is also cheaper, since, unlike other forms of VRAM, cards can be manufactured with just the right amount of RAM for a given resolution capability, instead of requiring it to be in multiples of megabytes.
Synchronous Graphics RAM (SGRAM) is a clock-synchronized DRAM that is relatively low-cost video memory. SGRAM is single-ported memory, but it can act like dual-ported memory by opening two memory pages at the same time, instead of just one.
Window RAM (WRAM) is unrelated to Microsoft Windows. It is a very high-performance VRAM that is dual-ported and has approximately 25% more bandwidth than VRAM, but costs less. It has features that make it more efficient to read data for use in block fills and text drawing. WRAM can be used for very high resolution (such as 1,600 x 1,200 pixels) using true color.
Modern graphics cards use a version of SGRAM called GDDR5. As the name implies, GDDR5 is double data rate RAM, like the DDR4 used as system RAM in modern computers.
The only significant difference these days between VRAM and system RAM is the speed (DDR5 vs. DDR4 or lower) and the ability to mimic dual-port functionality common in older forms of specialized VRAM.
For applications that rely on complex data processing functions, the amount of VRAM on a system's graphics card is not nearly as important as the amount of system RAM. A modern business workstation can easily get by without a graphics card, often using a chip on the motherboard that shares system RAM for video display.
For graphic processing applications, like Adobe Photoshop, or high-end video games, like the 2016 game "Rise of the Tomb Raider," the more VRAM the better. "Rise of the Tomb Raider," for example, requires video cards with at least 2 gigabytes (GB) of VRAM, and at least 6 GB of system RAM.
As of this writing, it is possible to buy video cards based on NVIDIA's Titan X technology with 12 GB of GDDR5 VRAM, which can run video games at 4K resolution, at frame rates up to 60 frames per second.