Datasheet

“main” (Installation and Administration) — 2004/6/25 — 13:29 — page 204 — #230
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8.1 Booting a PC
After turning on your computer, the first thing that happens is that the
BIOS (basic input output system) takes control, initializes the screen and
keyboard, and tests the main memory. At this point, no storage media or
external devices are known to the system.
After that, the system reads the current date and time as well as informa-
tion about the most important peripheral devices from the CMOS setup.
After reading the CMOS, the BIOS should recognize the first hard disk, in-
cluding details such as its geometry. It can then start to load the operating
system (OS) from there.
To load the OS, the system loads a 512-byte data segment from the first
hard disk into main memory and executes the code stored at the beginning
of this segment. The instructions contained in it determine the rest of the
boot process. This is why the first 512 bytes of the hard disk are often called
the master boot record (MBR).
Up to this point (loading the MBR), the boot sequence is independent of
the installed operating system and is identical on all PCs. Also, all the PC
has to access peripheral hardware are those routines (drivers) stored in the
BIOS.
8.1.1 Master Boot Record
The layout of the MBR always follows a standard that is independent of the
operating system. The first 446 bytes are reserved for program code. The
next 64 bytes offer space for a partition table for up to four partitions (see
Section 3.9 on page 134). Without the partition table, no file systems exist
on the hard disk — the disk would be virtually useless without it. The last
two bytes must contain a special magic number (AA55). An MBR containing
a different number would be considered invalid by the BIOS and any PC
operating system.
204 8.1. Booting a PC