A Guide To Video Memory

When shopping for a video card, there is no doubt that a prime feature bragged about the card is how much memory it has and what kind. But, what does that mean? And how does the type and amount of memory effect the performance of the video on your system?

Simply put, a video card relies on its memory to draw a picture. Different cards offer different amounts of this precious memory. Extreme low-end cards often have only 256 KB, while very high-end cards have as much as 8 MB of memory or more. The standard for the run-of-the-mill SVGA card is 1 or 2 MB.

The common misconception is that the more, the better. While this is somewhat true, it is different than expected. More video memory DOES NOT speed up the video system on the card. It only affects how many colors it can show at higher resolutions.

There is a mathematical equation to help you figure out just how much video memory you need. This depends on your resolution. The higher the resolution, the more memory you need, because each and every pixel on your screen must have a space in the memory for its data. Here is the computation:

Lets say you would like to display 256 colors on a screen resolution of 640×480. At this resolution, there is 307,200 dots, or pixels. 256 colors requires 8 bits or data for each pixel. You can figure this because with an eight digit binary, there are 256 possible combinations. For two colors, you need only 1 bit, either on or off. For 16 colors, you need 4 bits, 2 to the 4th power. 256 colors requires 8 bits, and it goes up from there. Anyway, multiply the number of dots by the number of bits per pixel to get the number of bits for the entire screen.

307,000 x 8 = 2,457,600 bits.
There are eight bits per byte and 1,024 bytes per kilobyte. So…

2,457,600 / 8 = 307,200 bytes = 300K

Therefore it requires exactly 300K of memory to display 256 colors at 640×480 resolution. But, after calculating this, you must consider the available amounts. You cannot buy a video card with 300K of memory. They are available at either 256K or 512K. So, to get this resolution and color scheme, you must buy a card with 512K of memory on-board.
Obviously, though, most users do not deal with low memory amounts such as this any more. Most users have a card with at least 2MB of memory. With this, you can display most any resolution you would want. At the most common resolution, 800×600, a 2 MB card can display 16,777,216 colors. When you get up to 4MB cards, you can show this many colors at 1280×1024 resolution. With the high-end cards, one sometimes sees 8M or memory boasted. As you can see, this much memory is absolutely useless unless you would like to display 24-bit color at 1600×1200 resolution, something which is rarely done apart from on the largest of high-end monitors.

Another issue with regards to memory is the local bus between the video chipset and the video memory. Basically, the wider the access to memory from the chipset, the faster the card. This access is a local bus wired into the card. When shopping, you will often see 64-bit or even 128-bit cards advertised. This refers to this memory bus, and is often confused with the bus slot the card plugs into. In reality, a 64-bit card means that it has a 64-bit memory bus on the card, but is actually a 32-bit card plugging into a PCI bus slot.

 

Types Of Video Memory

Most Video Cards in the past used a type of video memory called Dynamic RAM, or DRAM, to store image information. This type of RAM was easy to make and as a result was very cheap. However, DRAM was far too slow, which stems from the fact that DRAM needs to be constantly refreshed in order to save the information. DRAM also cannot be read at the same time as it is being written to. This slowness can be demonstrated by looking at today’s cards. At a standard resolution of 1024×768 with a 72Hz refresh rate, the DAC (Digital to Analog Converter) needs to access the memory’s information 72 times per second. If using 24-bit color, this translates to about 170 MB/sec of transfer from the DRAM, the max transfer of DRAM. This does not include the writing of new information, which can’t be done during the read process.

In response to this, a number of new memory technologies have been developed.

 

  • EDO RAM was the first to be used. EDO RAM is often used in the main system memory as well. EDO RAM, when used for video, is only slightly faster than DRAM, but the cost was about the same. It works by offloading its contents to separate circuits, thereby allowing it to receive new data at the same time that the last data cycle is being completed. EDO RAM is architecturally the same. The only difference is with the wiring onto the board.
  • Video RAM, or VRAM has been utilized on Video Cards for a while. VRAM is dual-ported, meaning it allows all hardware to access the memory at the same time, including the main processor, the video processor and the DAC. It’s more expensive than EDO, but the performance increase is noticeable.
  • Windows RAM, or WRAM, is modified VRAM. It boasts slightly better performance than VRAM and costs less to make. For this reason, it is often used in place of VRAM on many Video Cards.
  • MultiBank DRAM (MDRAM) is newer technology. It is aimed at the cost-sensitive user who requires good performance. The downside is that it is rather rare. The prime difference is its organization. While standard memory was limited to sizes or 256 KB, 512 KB, 1 MB, 2 MB, 4 MB and so on, MDRAM could be added in increments of 32K. This allows the user to calculate just how much memory they really need, using the above equation, and upgrade their video memory to be very close. Performance wise, MDRAM is much faster than VRAM or WRAM.
  • Synchronous Graphics RAM, or SGRAM, is a new arrival for high-end graphics users. It runs at 66MHz and can operate at up to 80MHz. While very fast, you won’t really be able to take advantage of it until the PCI bus is upgraded to 66MHz for the mainstream motherboard.

 

Installing Video RAM

Some Considerations

Upgrading your Video RAM is relatively easy. The biggest thing to consider is whether it is worth it for you. On most systems, 1 MB of Video RAM is enough. This will allow you to display 65,000 colors at a 640×480 resolution or a 800×600 resolution. If you try to jack the resolution up to 1024×768, though, you can only show 256 colors. In a case where you want higher resolution AND more colors, you may want to add RAM to your video card. But, also, before deciding you want more resolution, consider your monitor. Some monitors can only support 800×600 or lower. Also, a higher resolution may not be good for the size screen you have. Usually, a 1024×768 resolution only looks good with a 17″ or larger monitor.

Also, adding more Video RAM will not necessarily make your video card run any faster. It will only allow it more colors at higher resolutions. If you’re out for speed, you need to upgrade your entire video card to a graphics accelerator. Many companies, such as Matrox and Diamond, make great SVGA graphics accelerators.

Installing VRAM

  1. Turn off the system, unplug it. Remove the case. Locate your video card. Ground yourself, and remove your video card. Lay the card on a flat surface.
  2. Get the RAM chips. Each of the RAM chips will have a small dot in one corner. Likewise, each RAM socket will have one beveled corner. Match the chips dot into the same corner as the beveled corner on the socket. Make sure the pins on the RAM chip line up with the grooves on the side of the socket.
  3. Making sure of step 2, press the chip into the socket firmly. Don’t damage the card. The RAM chip should not fall out of the video card if turned upside down.
  4. Install any other RAM chips the same way.
  5. Reinstall the video card and boot the machine. You should now see the new RAM amount on boot up. If not, try reinstalling the RAM. If this doesn’t fix it, call the manufacturer. There may be something wrong with the new Video RAM.
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