Video Hardware, Part 3 - Video RAM
(Page 4 of 12 )
Most video adapters rely on their own onboard memory that they use to store video images while processing them; although the AGP specification supports the use of system memory for 3D textures, this feature is seldom supported now that video cards routinely ship with 32MB, 64MB, or more of onboard memory. Many low-cost systems with onboard video use the universal memory architecture (UMA) feature to share the main system memory. In any case, the memory on the video card or borrowed from the system performs the same tasks.
The amount of memory on the adapter or used by integrated video determines the maximum screen resolution and color depth the device can support. You often can select how much memory you want on a particular video adapter; for example, 32MB, 64MB, and 128MB are common choices today. Although adding more memory is not guaranteed to speed up your video adapter, it can increase the speed if it enables a wider bus (from 64 bits wide to 128 bits wide) or provides nondisplay memory as a cache for commonly displayed objects. It also enables the card to generate more colors and higher resolutions and, for AGP cards (see the following), allows 3D textures to be stored and processed on the card, rather than in slower main memory.
Many types of memory have been used with video adapters. These memory types are summarized in Table 15.11.
Table 15.11 Memory Types Used in Video Display Adapters Memory Type | Definition | Relative Speed | Usage |
FPM DRAM | Fast Page-Mode RAM | Slow | Low-end ISA cards; obsolete |
VRAM 1 | Video RAM | Fast | Expensive; obsolete |
WRAM 1 | Window RAM | Fast | Expensive; obsolete |
EDO DRAM | Extended Data Out DRAM | Moderate | Low-end PCI-bus |
SDRAM | Synchronous DRAM | Fast | Low-end PCI/AGP |
MDRAM | Multibank DRAM | Fast | Little used; obsolete |
SGRAM | Synchronous Graphics DRAM | Very fast | High-end PCI/AGP; replaced by DDR SDRAM |
DDR SDRAM | Double-Data Rate SDRAM | Very fast | High-end AGP |
DDR-II SDRAM | DDR SDRAM, 4-bit per- cycle memory fetch | Extremely fast | High-end AGP, PCI Express |
GDDR-3 SDRAM | Modified DDR SDRAM | Extremely fast | High-end AGP, PCI Express |
(i) 1. VRAM and WRAM are dual-ported memory types that can read from one port and write data through the other port. This improves performance by reducing wait times for accessing the video RAM compared to FPM DRAM and EDO DRAM.
Note -To learn more about memory types used on older video cards (FPD DRAM, VRAM, WRAM, EDO DRAM, and MDRAM), see Chapter 15 of Upgrading and Repairing PCs, 12th Edition, available in electronic form on the disc packaged with this book.
SGRAM, SDRAM, DDR, and DDR-II SDRAM—which are derived from popular motherboard memory technologies—have replaced VRAM, WRAM, and MDRAM as high-speed video RAM solutions. Their high speeds and low production costs have enabled even inexpensive video cards to have 16MB or more of high-speed RAM onboard.
SDRAM Synchronous DRAM (SDRAM) is the same type of RAM used on many current systems based on processors such as the Pentium III, Pentium 4, Athlon, and Duron. The SDRAMs found on video cards are usually surface-mounted individual chips; on a few early models, a small module containing SDRAMs might be plugged into a proprietary connector. This memory is designed to work with bus speeds up to 200MHz and provides performance just slightly slower than SGRAM. SDRAM is used primarily in current low-end video cards and chipsets such as NVIDIA's GeForce2 MX and ATI's RADEON VE.
SGRAM Synchronous Graphics RAM (SGRAM) was designed to be a high-end solution for very fast video adapter designs. SGRAM is similar to SDRAM in its capability to be synchronized to high-speed buses up to 200MHz, but it differs from SDRAM by including circuitry to perform block writes to increase the speed of graphics fill or 3D Z-buffer operations. Although SGRAM is faster than SDRAM, most video card makers have dropped SGRAM in favor of even faster DDR SDRAM in their newest products.
DDR SDRAM Double Data Rate SDRAM (also called DDR SDRAM) is the most common video RAM technology on recent video cards. It is designed to transfer data at speeds twice that of conventional SDRAM by transferring data on both the rising and falling parts of the processing clock cycle. Today's mid-range and low-end video cards based on chipsets such as NVIDIA's GeForce FX and ATI's RADEON 9xxx series use DDR SDRAM for video memory.
DDR-II SDRAM The second generation of DDR SDRAM fetches 4 bits of data per cycle, instead of 2 as with DDR SDRAM. This doubles performance at the same clock speed. The first video chipset to support DDR-II was NVIDIA's GeForce FX, which became the top of NVIDIA's line of GPUs in late 2002. DDR-II (also spelled DDR-2) is also used by ATI's high-end graphics cards.
GDDR-3 SDRAM GDDR-3 SDRAM, which began appearing on NVIDIA's high-end graphics cards in early 2004, is based on DDR-II memory, but with two major differences:
GDDR-3 separates reads and writes with a single-ended unidirectional strobe, whereas DDR-II uses differential bidirectional strobes. This method enables much higher data transfer rates.
GDDR-3 uses an interface technique known as pseudo-open drain , which uses voltage instead of current. This method makes GDDR-3 memory compatible with GPUs designed to use DDR or DDR-II memory.
For more information about DDR, DDR-II, and GDDR-3, see Chapter 6, "Memory."
Next: Video RAM Speed >>
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