Hardware Reference
In-Depth Information
The combination of the motherboard ROM, adapter card ROM, and device drivers loaded
from disk into RAM contributed to the BIOS as a whole. The portion of the BIOS con-
tained in ROM chips, both on the motherboard and in some adapter cards, is sometimes
called firmware , which is a name given to software stored in ROM chips rather than on
disk. Of course, after you turned off the system, the drivers in nonvolatile ROM would
remain intact, but those in volatile RAM would instantly vanish. That was not a problem,
however, because the next time the system was turned back on, it went through the boot
process and loaded the necessary drivers from disk all over again.
As the PC has evolved, more and more accessories and new hardware have been devised
to add to the system. This means that more and more drivers have to be loaded to support
this hardware. Adding new drivers to the motherboard ROM is difficult because ROM
chips are relatively fixed (difficult to change) and limited space is available. The PC ar-
chitecture allows only 128KB for the motherboard ROM, and most of it was already used
by the existing drivers, POST, BIOS Setup program, and of course the bootstrap loader.
Also,puttingdriversonadaptercardROMsisexpensive,andonly128KBisallocated for
all adapter card ROMs, not to mention the fact that the video card takes 32KB of that. So,
most companies developing new hardware forthe PC simply wrote drivers designed to be
loaded into RAM during the boot process.
As time went on, more and more drivers were being loaded from diskā€”in some cases,
even drivers that were replacing those in the motherboard. For example, Windows 95
introduced a new hard disk driver that used 32-bit code, which superseded the existing
16-bit driver that existed in the motherboard ROM. In other words, the 16-bit hard disk
driver in the motherboard ROM was used to begin loading the OS and other drivers, and
after the 32-bit driver was loaded into RAM during the boot process, the vector table was
changed to point to the 32-bit driver in RAM instead of the 16-bit driver in ROM. Win-
dows 95, 98, and Me allowed the use of both 16-bit and 32-bit drivers, easing the trans-
ition to full 32-bit operation.
When modern 32-bit and 64-bit versions of Windows are run, corresponding 32-bit and
64-bit drivers are loaded from disk to replace all the drivers in the motherboard ROM.
Modern OSs cannot use any of the 16-bit drivers found in either the motherboard ROMs
or any adapter card ROMs and must use only 32-bit or 64-bit drivers, depending on the
version.The16-bitcodeinthemotherboardROMisusedonlytogetthesystemfunction-
ing long enough to get the drivers and OS loaded, at which point they take over. In other
words, once Windows is loaded, the BIOS (meaning all the drivers) essentially resides
entirely in RAM. The motherboard ROM exists only to get the system started, to initial-
ize specific hardware, to offer security in the way of power-on passwords and such, and
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