Page 76 - A Practical Guide from Design Planning to Manufacturing

52 Chapter Two

Storing and processing all this data could rapidly use up the com-
puter’s main memory space and processing power. To prevent this all
modern video adapters are also graphics accelerators, meaning they
contain dedicated graphics memory and a graphics processor. The
memory used is the same DRAM chips used for main memory or slight
variations. Graphics accelerators commonly come with between 1 and
32 MB of memory built in.
The Graphics Processor Unit (GPU) can off-load work from the Central
Processing Unit (CPU) by performing many of the tasks used in creat-
ing 2D or 3D images. To display a circle without a graphics processor,
the CPU might create a bitmap containing the desired color of each
pixel and then copy it into the frame buffer. With a graphics processor,
the CPU might issue a command to the graphics processor asking for a
circle with a specific color, size, and location. The graphics processor
would then perform the task of deciding the correct color for each pixel.
Modern graphics processors also specialize in the operations required
to create realistic 3D images. These include shading, lighting, reflections,
transparency, distance fogging, and many others. Because they contain
specialized hardware, the GPUs perform these functions much more
quickly than a general-purpose microprocessor. As a result, for many of
the latest 3D games the performance of the graphics accelerator is more
important than that of the CPU.
The most common bus interfaces between the video adapter and the
Northbridge are the Accelerated Graphics Port (AGP) standards. The
most recent standards, PCI Express, began to be used in 2004. These
graphics bus standards are shown in Table 2-5.
Some chipsets contain integrated graphics controllers. This means the
Northbridge chips include a graphics processor and video adapter, so that
a separate video adapter card is not required. The graphics performance
of these built-in controllers is typically less than the latest separate video
cards. Lacking separate graphics memory, these integrated controllers
must use main memory for frame buffers and display information. Still,

TABLE 2-5 Graphics Bus Standards 1
Memory Transfers Max data
Graphics Bus width bus clock Transfers per second bandwidth
bus (b) (MHz) per clock (MT/s) (MB/s)
AGP 32 66 1 66 267
AGP × 2 32 66 2 133 533
AGP × 4 32 66 4 266 1067
AGP × 8 32 66 8 533 2133
PCI Express × 16 16 2000 1 2000 4000
1
Mueller, Upgrading and Repairing PCs.