Hardware Reference
In-Depth Information
adapters). If any ROMs are found, they are checksum-tested and executed. These ad-
apter ROMs can alter existing BIOS routines and establish new ones.
13. Failure of a checksum test for any of these ROM modules causes the message
XXXX ROM Error
to appear, where the address XXXX indicates the segment address of the failed ROM
module.
14. TheROMBIOSchecksthewordvalueatmemorylocation0000:0472toseewheth-
er this start is a cold start or a warm start. A word value of 1234h in this location is a
flag that indicates a warm start, which causes the BIOS to skip the memory test por-
tion of the POST. Any other word value in this location indicates a cold start, and the
BIOS performs the full POST procedure. Some system BIOSs let you control various
aspects of the POST procedure, making it possible to skip the memory test, for ex-
ample, which can be lengthy on a system with a lot of RAM.
15. If this is a cold start, the full POST executes; if this is a warm start, a mini-POST
executes, minus the RAM test. Any errors found during the POST are reported by
a combination of audio and displayed error messages. Successful completion of the
POST is indicated by a single beep (with the exception of some Compaq computers,
which beep twice).
16. The ROM BIOS searches for a boot record at cylinder 0, head 0, sector 1 (the very
first sector) on the default boot drive. At one time, the default boot drive was always
the first floppy disk (or A: drive). However, the BIOSs on today's systems often en-
ableyoutoselectthedefaultbootdeviceandtheorderinwhichtheBIOSwilllookfor
other devices to boot from if necessary, using a floppy disk, hard disk, or even a CD-
ROM drive in any order you choose. This sector is loaded into memory at 0000:7C00
and tested.
If a disk is in the drive but the sector can't be read, or if no disk is present, the BIOS
continues with step 19.
Booting from Optical or Floppy
Ifyouwanttobootfromanopticaldrive,besurethedriveislistedbeforetheharddiskinthe
boot devices menu in your BIOS setup. If you have a floppy drive installed, you can set that
to come before the hard disk in the sequence as well. This enables you to always be ready
for an emergency. So long as you do not start up the system with a floppy or bootable optical
disc loaded, the BIOS bypasses both the optical and floppy drives and boots from the hard
drive instead.
Refer to Chapter 11 , Optical Storage ,” for information on how to make a bootable disc.
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