A computer's graphical subsystem communicates with the CPU via one of two types of channels, the PCI (Periphral Component Interconnect) bus in older systems or via the AGP (Accelerated Graphics Port) bus. (To learn about a computer system's various buses, click _____) The problem with sending graphical information through the PCI bus is simply bandwidth. The PCI bus is not a dedicated bus. It is shared with other components within your computer. Your USB (Universal Serial Bus), IDE and other components all communicate to the CPU via the PCI bus. Graphical information is very large and needs to be processed very quickly otherwise it will bog down the system. So AGP was invented. Now the graphics card had its very own channel to the CPU, one with which it shared with nothing else. Ontop of this, the AGP bus is much faster than the PCI bus, allowing large graphical content to be transfered to the CPU very quickly. On a motherboard (to learn about motherboards, click ____) with a standard architecture, the AGP channel goes straight to the front-side bus (or FSB) wich then goes to the CPU. The only other thing in the computer that communicates via the FSB is the system memory (to learn more about memory, or RAM click here). This is important because it allows the intense graphical content not only a direct pipe to the CPU but also to the memory. Since graphical content is so large and diffucult to process it requires it's own memory that it can use to render anything from a simple screen-saver to complex 3D images from a 3D game. The amount of memory the GPU uses depends on what it is rendering. Lets say you have a 17" monitor that has a maximum resolution of 1280x1024 (this number refers to the amount of rows of pixels, the smallest unit of a picture, per side of the monitor.) . We will say you are running your monitor at 1024x768 ( a very common and popular resolution) and we will say you are running 16-bit color. That translates into almost 1.6MB of memory used to simply render your desktop (the desktop is a term that refers to your workspace within your operating system. It is the place where all of your icons might be.) Let's say you increase the color depth to 24-bit, the amount of memory used goes all the way up to almost 2.4MB. The desktop is also 2D and is a still picture, not a video image. Lets say you wanted to run that 17" monitor at 1280x1024 at 24-bit and you wanted to make a 2D image move across the screen at a rate of 30 frames per second (or FPS, this refers to how often the GPU can render a video image on the screen. The faster the better and typically anything over 30 FPS is considered TV quality.) you can see that the graphics card has a lot of work to do, and that example wasn't even 3D. Obviously, memory is important to a graphical subsystem and their are two ways in which memory is reserved for graphics. In systems with a integrated graphics setup, a certain amount of system memory is reserved for use by the graphics accelerator.
