Looking at the Video RAMs

Looking at the Video RAMs

The memory (or RAM) on the video card is also called the frame buffer, because it holds graphic instructions about each scene or frame to be displayed on the monitor. The first form of video RAM was standard DRAM, the same kind of RAM found in early PCs. Because DRAM requires constant electrical refreshing to hold its contents, it didn't work well for video memory. For one reason, it could be accessed while it was being refreshed and the video system had to wait, which meant that its performance suffered. Since then, several memory technologies have been and are being used, each one increasingly more efficient and faster than the last.

The most common RAM technologies used with video cards are:

l Dynamic Random Access Memory (DRAM): The same DRAM used on early PCs.

l Extended Data Output DRAM (EDO DRAM): Provides a higher bandwidth and handles read/write cycles better than standard DRAM.

l Video RAM (VRAM): VRAM, not to be confused with the generic "VRAM," is a special type of DRAM that doesn't need to be refreshed quite as often. VRAM is dual-ported, which means it has two access portals, and the processor and RAMDAC (RAM digital to analog converter) can both be accessing it at the same time. As the saying goes, two doors are better than one, and dual porting doubles the memory's speed.

l Windows RAM (WRAM): Called Windows Accelerator Card RAM on the A+ Core Hardware blueprint, WRAM is a dual-ported memory that runs a bit faster than VRAM.

l Synchronous DRAM (SDRAM): SDRAM is EDO DRAM that it is synchronized to the video card's processor and chipset. SDRAM is single-ported (one door) memory that is very common on video cards.

l Double Data Rate SDRAM (DDR SDRAM): DDR SDRAM has twice the data transfer speed of standard SDRAM. DDR memories are becoming more commonplace on video cards, especially on 3-D graphics accelerators.

l Synchronous Graphics RAM (SGRAM): An improvement on standard SDRAM adds features such as block writes and write-per-bit that support faster graphics performance. Block write is used to copy the contents of a color register into memory in a single clock cycle. Write per bit allows a single bit of a data block to be changed without the need to rewrite the entire data block.

SGRAM, which is a single-ported memory, is found only on video cards with chipsets that support it.

l Double Data Rate SGRAM (DDR SGRAM): DDR does for SGRAM exactly what it did for SDRAM: doubles its data transfer rates.

l Direct Rambus DRAM (RDRAM): A newer general-purpose memory type that is used for the PC's main memory (as well as on video cards) that runs about 20 times faster than conventional DRAM. RDRAM includes bus mastering and dedicated channels between memory devices. Bus mastering allows the video card to take control of the PC's system bus and transfer data into and out of system RAM. This improves the performance of some video operations that use primary RAM for certain calculations, such as 3-D acceleration.

l Unified Memory Architecture (UMA): Many lower-cost systems intended for home use integrate graphics support and the video system into the motherboard. UMA is so named because it uses system RAM for video memory. This technology almost always produces inferior graphics performance.