How to define memory bandwidth in video card

In each subsystem, in addition to the speed exchange between the subsystem and the processor, there are different data exchange within the subsystem, in addition to the data exchange between the display chip and the nuclear processor, for the graphics subsystem, There is also the data exchange between the display chip and the video memory. The data exchange speed between the image processing chip and the memory is the bandwidth of the memory, the higher the speed, the faster the exchange speed. If a graphics chip has powerful processing power, but the memory bandwidth is not high, will greatly affect its performance, or, the memory will limit the chip can not achieve its design processing capacity. At present, the performance of the display chip has reached a high degree, its processing capacity is very strong, only large memory bandwidth to ensure its sufficient data input and output. With the multimedia, 3D game hardware requirements are more and more high, in the high resolution, 32-bit true color and high refresh rate of 3D screen, relative to the GPU, the lower video memory bandwidth has become a bottleneck in the performance of the card. Video memory bandwidth is one of the most important factors to determine graphics performance and speed.

Calculation formula

Video memory Bandwidth = working frequency x Memory bit width/8. When the conditions allow, the purchase of video card with large memory bandwidth as much as possible, this is a key choice.

Restrictive factors

Because the video memory bandwidth refers to the graphics processing chip and memory exchange speed, so, the memory interface bus number of the more wide, exchange rate is higher, and the speed of video memory faster, of course, the higher the bandwidth. The influence of the parameter on the bandwidth, first of all, for High-performance graphics card, must use 128bit video memory bus, the use of 64bit bus Savage 4 in performance is significantly behind the other fourth generation card is a good proof. In the same bus speed, the faster the video memory speed, the higher the bandwidth. For Ddr-ram, the bandwidth of the same frequency SDRAM will be produced twice over a period of time due to the transmission of two data in one cycle. But to be sure, This is not to say that the speed of the DDR memory is twice times the SDRAM, because the impact of memory performance is not just data transmission, when the data from the graphics chip transmission to video memory, its implementation speed and SDRAM is the same, so we can not say Ddr-ram speed is SDRAM twice times. Ddr-ram is ultimately a solution to the problem of transmission bottlenecks in low-end solutions.

Impact on the speed of processing

The video memory speed is 200MHz, the bus is 128bit G400max, its video memory bandwidth is 3.2gb/s, but also uses the 128bit bus TNT2 Ultra because uses only 183MHz the memory, therefore its bandwidth is smaller than G400 Max, only is the 2.9gb/s. What effect does the memory bandwidth have on the accelerator card? Let's take a look at the following example, after the graphics chip has received instructions from the CPU, to calculate the need to fill the pixel, and then the pixel through the memory and other channels, the final completion of the digital analog transfer display. Therefore, if the graphics chip and video memory channel transmission number is not enough, the unit time processing of the pixels are limited to video memory bandwidth. The 2.6GB GeForce 256 can achieve the maximum filling rate of 480mt/s, which is exactly the same as the design.