Hardware Reference
In-Depth Information
to perform some basic initial configuration. After the OS is loaded, a whole new set of
drivers takes over.
A PC system can be described as a series of layers—some hardware and some soft-
ware—that interface with each other. In the most basic sense, you can break a PC down
into four primary layers, each of which can be broken down further into subsets. Figure
5.1 shows the four layers in a typical PC.
Figure 5.1 PC system layers.
The purpose of the layered design is to enable a given OS and applications to run on dif-
ferent hardware. Figure 5.1 shows how two machines with dissimilar hardware can use
different sets of drivers (BIOS) to interface the unique hardware to a common OS and
applications. Thus, two machines with different processors, storage media, video display
units, and so on can run the same OS and applications.
In this layered architecture, the application software programs talk to the OS via what is
called an application programming interface (API) . The API varies according to the OS
you are using and consists of the various commands and functions the OS can perform
for an application. For example, an application can call on the OS to load or save a file.
This prevents the application itself from having to know how to read the disk, send data
to a printer, or perform any other of the many functions the OS can provide. Because the
application is completely insulated from the hardware, you can essentially run the same
applicationsondifferentmachines;theapplicationisdesignedtotalktotheOSratherthan
the hardware.
The OS then interfaces with or talks to the BIOS or driver layer. The BIOS consists of all
the individual driver programs that operate between the OS and the actual hardware. As
such, the OS never talks to the hardware directly; instead, it must always go through the
 
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