Hardware Reference
In-Depth Information
appropriate drivers. This provides a consistent way to talk to the hardware. It is usually
the responsibility of the hardware manufacturer to provide drivers for its hardware. Be-
cause the drivers must act between both the hardware and the OS, the drivers typically
areOSspecific.Thus,thehardwaremanufacturermustofferdifferentdriversfordifferent
OSs. Because many OSs use the same internal interfaces, some drivers can work under
multiple OSs.Forexample, driversthat workunder32/64-bitversions ofWindows7usu-
ally work under the corresponding versions of Vista; drivers that work under Windows
XP usually workunder Windows 2000and NT; and drivers that work under Windows Me
usually work under Windows 98 and 95. This is because Windows 7 and Vista are essen-
tially variations on the same core OS, as are Windows XP, Windows 2000, and NT, and
Windows 95, 98, and Me. Although Windows 7 and Vista were based on Windows NT,
the driver model has changed enough that they generally can't use the same drivers as XP
and earlier NT-based OSs.
Because the BIOS layer looks the same to the OS no matter what hardware is above it (or
underneath, depending on your point of view), the same OS can run on a variety of sys-
tems. For example, you can run Windows on two systems with different processors, hard
disks, video adapters, and so on, yet Windows will look and feel pretty much the same to
the users on both of them. This is because the drivers provide the same basic functions no
matter which specific hardware is used.
Asyoucanseefrom
Figure5.1
,theapplicationandOS'slayerscanbeidenticalfromsys-
tem to system, but the hardware can differ radically. Because the BIOS consists of drivers
that act to interface the hardware to the software, the BIOS layer adapts to the unique
hardware on one end but looks consistently the same to the OS at the other end.
The hardware layer is where most differences lie between the various systems. It is up to
theBIOStomaskthedifferencesbetweenuniquehardwaresothatthegivenOS(andsub-
sequently the application) can be run. This chapter focuses on the BIOS layer of the PC.
BIOS and CMOS RAM
Some people confuse BIOS with the CMOS RAM in a system. This confusion is aided by
the fact that the Setup program in the BIOS is used to set and store the configuration settings
in the CMOS RAM. They are, in fact, two separate components.
The BIOS on the motherboard is stored in a fixed ROM chip. Also on the motherboard is
a chip called the
RTC/NVRAM chip
, which stands for real-time clock/nonvolatile memory.
This is where the settings in the BIOS Setup are stored, and it is actually a clock chip with a
few extra bytes of memory thrown in. It is usually called the
CMOS chip
because it happens
to be made using CMOS (complementary metal-oxide semiconductor) technology.
The first example of this ever used in a PC was the Motorola MC146818 chip, which had
64 bytes of storage, of which 14 bytes were dedicated to the clock function, leaving 50 bytes
