PC Hardware

Video Memory

As mentioned earlier, the amount of memory on a display adapter is a major factor in determining the screen resolution and color depth that the card can manage. Just as with system RAM, the video memory must be able to operate at a speed that can keep up with the processor, and the demands of the system clock. If the display adapter is too slow at updating the image on the monitor, the user is left waiting or is presented with jerky mouse movements and keystrokes that appear in delayed bursts rather than as typed.

Fast Page DRAM

Early video cards used fast page-mode (FPM) DRAM, a series of chips that were basically the same as the RAM used on the early PC's motherboard. This memory form was fine for MDA and CGA cards, and even the 8514/A, but with the higher resolution, increasing pixel depth, and faster refresh rates of VGA displays and beyond, vendors sought improved memory models to get the most performance out of their video coprocessors.


Enter dual-ported memory in the form of VRAM (Video RAM). It can read and write to both of its I/O ports at the same time. It allows the processor to talk to the system bus and the monitor simultaneously: fast, but very expensive. VRAM showed up in the best cards, but vendors wanted a low-cost option as well. Some vendors just used FPM (fast page-mode) DRAM, leaving the user to discover that, at high resolution, the display was too slow for efficient operation. These cards did sell well in the low-end market, as they allowed the budget-minded user to operate in low-color modes for most tasks, switching only to higher color depth for projects that required high-color or true-color mode. Users who regularly worked in high-color or true-color mode often would quickly consider an upgrade.


An alternate is EDO (Extended Data Out) DRAM, which can begin reading a new set of instructions before the preceding set of instructions has been completed. This is a common form of system DRAM that boosts performance to about 15 percent above conventional DRAM.


WRAM (Window Random Access Memory, unrelated to the Microsoft operating system) is a high-speed variant of VRAM that costs less to produce and boosts performance by about 20 percent beyond regular VRAM. VRAM and WRAM have become the standard memory types for high-end display adapters.


The mid-range display market makes use of SGRAM (Synchronous Graphics RAM). As the name implies, it is tuned to the graphics-card market, offering faster transfers than DRAM, but not as fast as VRAM and WRAM.


Multibank DRAM (MDRAM) is the final stop on our tour of memory acronyms. It uses interleaving (the dividing of video memory into 32KB parts that can be accessed concurrently) to allow faster memory input/output to the system without expensive dual-porting. It is also a more efficient type of chip that is practical to produce in sizes smaller than a full megabyte. A vendor can save money by just buying the amount needed to actually draw the screen. This saves about 1.75 MB per card for a resolution of 1024 x 768.

The table below lists the standard memory requirements for the most common resolutions and pixel depths used today. As pointed out in the previous paragraphs, keep in mind that the minimum amount of memory for MDRAM is usually less than for other types of RAM. Some graphics cards offer additional memory, and even incorporate different types of RAM on the same card. In such cases, some of the memory might be used for features other than merely imaging the picture to be sent to the CRT in pixels.

by BrainBellupdated