How a Graphics Card Works - Page 8

At this point we've got a basic three dimensional scene that's been constructed by the GPU, and then converted into a two dimensional representation (via rasterisation) that's stored in the frame buffer, ready to be displayed on the monitor. But how did the information needed to do all this, the mapping images and geometry data, get into the graphics card memory in the first place?

Most modern 3D games use a game engine (as mentioned earlier) which runs on the system CPU/RAM. This engine takes care of updating the data stored on the graphics card, ready for the next frame to be rendered. Usually this data is originally stored on the hard disk, the CPU then loads the relevant data from the disk into system memory (the loading delay you see when starting a game or a new level).

The CPU will usually need to modify geometry data on the fly, as with cars or human players moving in the scene (they also need to be in the right place at the right time, or else the realism of the scene will be compromised).

Changes to the scene since the last frame are sent to the graphics card memory via the graphics bus. The graphics bus (PCI-Express or AGP) is a high speed, dedicated link between the system memory and the graphics card memory (see below).

The game engine also instructs the GPU on 'how' to process the geometry, so that, for instance, the game is more realistic (or however the games visual designers wanted the scene to look). The game engine also decides which shader programs the GPU should use for different surfaces.

Interfaces
The interface on the bottom of a graphics card provides a high-speed connection to the rest of the computer, as previously mentioned this is usually through either an AGP port, or a PCI-Express (PCIe) port.

AGP: the older of the two standards. It comes in several speed grades up to 8x. In an AGP system there will be only one AGP slot. The peak transfer rate for an 8x AGP device is 2.1GB/s.

The AGP and PCI (not to be confused with PCIe) ports on a motherboard

PCI-Express: Most new systems now use PCIe. Like AGP, PCIe slots also come in different speeds, up to 32x. A PCIe motherboard will usually have a mix of the different types (e.g. one 16x slot for the graphics card, and four 1x PCIe slots). Most current graphics cards are 16x.

The PCIe ports on a motherboard. This motherboard has two 16x PCIe ports, so it can run more than one graphics card at the same time.

PCIe is much more advanced than AGP, and allows for transfer rates of up to 8GB/s, along with the ability to transmit and receive data at the same time (unlike AGP). This simultaneous transmission allows multiple devices to communicate with the system without interrupting one another (and reducing performance).

PCIe also has another advantage - the ability to put more than one graphics card in the PC to increase rendering speeds (such as SLi by NVIDIA or Crossfire by ATI). This operating mode allows the system to effectively split the work between two (or more recently four) graphics cards, allowing for higher frame rates and better quality settings to be used.

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Article Navigation:

• Introduction
• The Basics
• Engines, Their Drivers and A Little History
• Telling Your Buffers from Your Outputs
• Feeding the Frame Buffer
• Surface Colours and Maps
  • MIP Maps
  • Bump Mapping
• Lights, Camera, Action!
• On The Buses
  • Interfaces
• Shaders
• Pipelines
• The Rendering Process - An Overview
• Improving Image Quality - AA/AF/HDR
  • Antialiasing
  • Anisotropic Filtering
  • High Dynamic Range
• End Frame
  • Links