Hardware Reference
In-Depth Information
The POST cards have a two-digit hexadecimal display used to report the number of the
currentlyexecutingtestroutine.Beforeeachtestisexecuted,ahexadecimalnumericcode
is sent to the port, and then the test is run. If the test fails and locks up the machine, the
hexadecimal code of the last test being executed remains on the card's display.
Many tests are executed in a system before the video display card is enabled, especially if
the display is EGA orVGA. Therefore, many errors can occur that would lock upthe sys-
tem before the system could possibly display an error code through the video system. Be-
causenotalltheseerrorsgeneratebeepcodes,tomostnormaltroubleshootingprocedures,
a system with this type of problem (such as a memory failure in Bank 0) would appear
completely “dead.” By using one of the commercially available POST cards, however,
you can often diagnose the problem.
These codes are completely BIOS dependent because the card does nothing but display
the codes sent to it. Some BIOSs have more detailed POST procedures and therefore send
more informative codes. POST cards can be purchased from JDR Microdevices or other
sources and are available in both ISA and PCI bus versions.
For simple but otherwise fatal errors that can't be displayed onscreen, most of the BIOS
versions also send audio codes that can be used to diagnose such problems. The audio
codes are similar to POST codes, but they are “read” by listening to the speaker beep
rather than by using a special card.
The following section details the text error codes for all the popular BIOS versions.
For detailed lists of the BIOS POST beep codes,
see
Figure 20.1
,
Chapter 20
,
p.
963
.
BIOS Boot Error Messages
During the boot process, the bootstrap loader routine in the motherboard ROM BIOS
reads the first physical sector of each of the bootable drives or devices, which is cylinder
0, head 0, sector 1 in CHS mode or logical block address 0 in LBA mode. The code from
thefirstsectorisloadedintoRAM,andthelasttwobytesarechecked toseewhetherthey
