(^268) PC Hardware: A Beginner’s Guide
The Video BIOS
The function of the video BIOS (Basic Input/Output System) is very much like that of the
system BIOS. It provides an interface between the system BIOS, the PC’s operating system,
and any application programs running on the PC to the video card and ultimately to the
monitor. The issues that impact the video card at the BIOS level are video interfaces, sys-
tem resource requirements, and video drivers.Video System Interfaces
A large amount of data must be moved between the video card and the PC’s CPU and
RAM. The video system interface is the pathway over which this data travels. This path-
way connects the GPU and video RAM to the PC. Because of the amount of data to be
transferred, the video system interface requires more bandwidth than any other periph-
eral device on the PC.
One common mistake made by users is to assume that the number of bits used on the
video card is also the number of bits required in the video system interface. But a 64-bit
or 128-bit video card only uses this bandwidth internally between its onboard compo-
nents. The width in bits of the interface to the CPU and memory will be either 16 bits
(ISA/EISA cards) or 32 bits (VL-Bus, PCI, or AGP). There is a 64-bit PCI bus available on
newermotherboardsandvideocardsthatusea64-bitinterface,butthereisnota128-bit
interface—not yet, anyway.
ThetwomostpopularvideosysteminterfacesinusetodayarethePCIandAGPbuses: Peripheral Component Interconnect (PCI) Support for the PCI interface
bus is included in the system chipset on all Pentium-class computers. PCI is
commonly used for 2D graphics cards, sound cards, network interface cards,
and other expansion cards that attach directly to the motherboard. Of course,
a PCI card slot is required. PCI is a bus structure and as such may support
a number of different devices. PCI slots, shown in Figure 12-6, are found on
virtually all Pentium-class motherboards boards.
AGP (Accelerated Graphics Port) This interface was designed specifically
for use as a video system interface. AGP, which runs twice as fast as the PCI
interface, creates a high-speed link between the video card and the PC’s
processor. The AGP interface is also directly linked to the PC’s system memory,
which makes it possible for 3D images to be stored in main memory and for 2D
systems to use system RAM for some calculations. All AGP video cards require
the motherboard to have an AGP slot. AGP is a port and as such can support
only a single device. There is usually only one AGP slot on a motherboard (see
Figure 12-6), and it is reserved for the graphics card.AGP is fast replacing PCI as the interface of choice for video cards because of its faster
transfer rates. In fact, AGP has evolved into several standard versions, each noting its
multiple of the original standard. For example, AGP 1X has a data transfer rate of
266MBps (compared to PCI’s 133MBps), AGP 2X supports 533 MBps, and AGP 4X trans-
fers data at 1.07GBps.