Hardware Reference
In-Depth Information
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 Power Good Signal , p. 836 ( Chapter 18 , Power Supplies ”).
4. The microprocessor begins executing the ROM BIOS code, starting at memory ad-
dressFFFF:0000.Becausethislocationisonly16bytesfromtheveryendoftheavail-
able ROM space, it contains a JMP (jump) instruction to the actual ROM BIOS start-
ing address.
5. The ROM BIOS performs a test of the central hardware to verify basic system func-
tionality. 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/out-
put (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, Win-
dows queries the PnP BIOS for device information and then queries each PnP device
for its configuration.
10. TheBIOSperformsavideoROMscanofmemorylocationsC000:0000-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
thevideoadapterandacursorappearsonscreen.Ifthechecksumtestfails,thefollow-
ing message appears:
C000 ROM Error
11. IftheBIOSfindsnovideoadapterROM,itusesthemotherboardROMvideodrivers
to initialize the video display hardware, and a cursor appears onscreen.
12. The motherboard ROM BIOS scans memory locations C800:0000-DF80:0000 in
2KBincrementsforanyotherROMslocatedonanyotheradaptercards(suchasSCSI
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