23 What types of video RAM are available (or coming soon)?
Description
This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
23 What types of video RAM are available (or coming soon)?
Video cards use their on-board RAM in different ways, but for this example we will only consider it as a framebuffer. This means that it is used to store a digital 'snapshot' of what appears on the computer monitor. This framebuffer is used in two different ways. The video processor writes data to the framebuffer, and the RAMDAC reads data from the framebuffer, converts it to an analog signal, then sends that signal to the monitor. These two operations, reading and writing, must share the available bandwidth of the video RAM. At high pixel addressabilities and refresh rates, the RAMDAC can be quite demanding. At the same time, if more pixels have to be updated by the video processor (because of the higher pixel addressabilities) then there may not be enough video memory bandwidth available. As today's users move to higher and higher pixel addressabilities (1024x768 and up) and want to display more colours simultaneously (16.7 million and up) traditional type of RAM are becoming inadequate.
All video cards currently use some form of DRAM (Dynamic Random Access Memory) because of its high price/performance ratio. This section discusses some of the current and upcoming types of DRAM. There are two basic classifications used here; single-ported and dual-ported. See the section "What is the difference between VRAM and DRAM?" for a detailed explanation of single verses dual-ported RAM. The quick explanation is that single-ported RAM has only one data path that has to be shared between read and write operations. So, if the RAM has a bandwidth of, say, 80 Mbyte/s, then the read bandwith plus the write bandwidth must be less than 80 Mbytes/s. The read and write operations can share this bandwidth in any ratio, 50-50 or otherwise. Dual-ported RAM has separate read and write data paths, meaning that if it operates at the same speed as the RAM in the single-ported example, 80 Mbytes/s bandwidth is available for _each_ of the reading and writing operations. This effectively doubles total bandwidth of the RAM.
If that's the case, why don't all video cards use VRAM? Dual-ported RAM requires more discrete silicon components to store each bit of digital information than traditional DRAM, so the manufacturing cost goes up. Recent efforts have been focussed on trying to produced alternatives to VRAM which are as fast or faster, but at lower cost. Some of the new contenders are very fast, and not much more expensive than DRAM.
Standard DRAM (also called FPM DRAM for Fast-Page Mode) is the least expensive memory used in video cards. It is still the most popular type of memory used in video cards. DRAM typically runs at an i/o bus frequency in the 25-33 MHz range and provides a net bandwidth of ~90 Mbyte/s. Note that for DOS/VGA games, DRAM provides loads of bandwidth, and that faster video memory will _not_ give better performance.
VRAM (Video RAM) is one of the most expensive types of RAM, and is the most common type used in high-end graphics cards. It is dual- ported, providing double the effective bandwidth of DRAM running at the same speed.
WRAM (Window RAM) is dual-ported, and can be clocked at up to 50 MHz providing up to 50% more bandwidth than conventional VRAM. Due to its design, WRAM requires fewer silicon components than VRAM and as a result is ~20% cheaper. It has also been optimized to provide fast text and colour fills and aligned BitBLT's. For more info, refer to: http://www.matrox.com/mgaweb/wram.htm Benefits of WRAM Memory
EDO DRAM (Extended Data Out DRAM) is being used for both video cards and main system RAM due to it's improved performance and only marginally higher cost over FPM DRAM. It can be clocked at a higher i/o bus frequency (40-50 MHz) and provides higher bandwidth (~105 MHz).
Other types of RAM are under development, or have been implemented to a lesser extent. Most of these are single-ported designs which try to reduce memory latency through tricks like higher bus frequency, interleaving of multiple banks (MDRAM) and wider memory buses.
Summary:
Type Means I/O Bus Total Net
Frequency Latency Bandwidth
(MHz) (ns) (Mbyte/s)
FPM DRAM Fast Page-Mode RAM 25-33 80 80
VRAM Video RAM
WRAM Window RAM 120
EDO DRAM Extended Data Out DRAM 40-50 100 105
SDRAM Synchronous DRAM 66-100 102-75 166-253
RDRAM Rambus DRAM 250 108 206
MDRAM Multibank DRAM 125-166 22-19 405-490
SGDRAM Synchronous Graphics DRAM
EDRAM Enhanced DRAM
Note that these numbers come from data published in Electronic Engineering Times and Tseng Labs.
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