Hardware Reference
In-Depth Information
occur during the boot process.
The Hardware Boot Process: Operating System Independent
If you have a problem with your system during startup and can determine where in this sequence of
events your system has stalled, you know which events have occurred and probably can eliminate
each of them as a cause of the problem. The following steps occur in a typical system startup
regardless of which operating system you are loading:
1.
You switch on electrical power to the system.
2.
The power supply performs a self-test (known as the POST). When all voltages and current
levels are acceptable, the supply indicates that the power is stable and sends the Power_Good
signal to the motherboard. The time from switch-on to Power_Good is normally between 0.1
and 0.5 seconds.
3.
The microprocessor timer chip receives the Power_Good signal, which causes it to stop
generating a reset signal to the microprocessor.
See the
Chapter 18
section, “
The Power Good Signal
,
”
p.
848
.
4.
The microprocessor begins executing the ROM BIOS code, starting at memory address
FFFF:0000. Because this location is only 16 bytes from the very end of the available ROM
space, it contains a JMP (jump) instruction to the actual ROM BIOS starting address.
5.
The ROM BIOS performs a test of the central hardware to verify basic system functionality.
Any errors that occur are indicated by audio beep codes because the video system has not yet
been initialized. If the BIOS is Plug and Play (PnP), the following steps are executed; if not,
skip to step 10.
6.
The PnP BIOS checks nonvolatile random access memory (NVRAM) for input/output (I/O)
port addresses, interrupt request lines (IRQs), direct memory access (DMA) channels, and
other settings necessary to configure PnP devices on the computer.
7.
All PnP devices found by the PnP BIOS are disabled to eliminate potential conflicts.
8.
A map of used and unused resources is created.
9.
The PnP devices are configured and reenabled, one at a time. If your computer does not have a
PnP BIOS, PnP devices are initialized using their default settings. These devices can be
reconfigured dynamically when Windows starts. At that point, Windows queries the PnP BIOS
for device information and then queries each PnP device for its configuration.
10.
The BIOS performs a video ROM scan of memory locations C000:0000-C780:0000 looking
for video adapter ROM BIOS programs contained on a video adapter found either on a card
plugged into a slot or integrated into the motherboard. If the scan locates a video ROM BIOS, it
is tested by a checksum procedure. If the video BIOS passes the checksum test, the ROM is
executed; then the video ROM code initializes the video adapter and a cursor appears onscreen.
If the checksum test fails, the following message appears:
C000 ROM Error
11.
If the BIOS finds no video adapter ROM, it uses the motherboard ROM video drivers to
initialize the video display hardware, and a cursor appears onscreen.
12.
The motherboard ROM BIOS scans memory locations C800:0000-DF80:0000 in 2KB
