Chapter 2

Computer Architecture and the

Boot Process

2.1 The Boot Process

Now, we begin our journey.
When we reboot our computer, it must start up again, initially without any notion of
an operating system. Somehow, it must load the operating system --- whatever variant
that may be --- from some permanent storage device that is currently attached to the
computer (e.g. a floppy disk, a hard disk, a USB dongle, etc.).
As we will shortly discover, the pre-OS environment of your computer offers little in
the way of rich services: at this stage even a simple file system would be a luxury (e.g.
read and write logical files to a disk), but we have none of that. Luckily, what we do have
is the Basic Input/Output Software (BIOS), a collection of software routines that are
initially loaded from a chip into memory and initialised when the computer is switched
on. BIOS provides auto-detection and basic control of your computer’s essential devices,
such as the screen, keyboard, and hard disks.
After BIOS completes some low-level tests of the hardware, particularly whether or
not the installed memory is working correctly, it must boot the operating system stored
on one of your devices. Here, we are reminded, though, that BIOS cannot simply load a
file that represents your operating system from a disk, since BIOS has no notion of a file-
system. BIOS must read specific sectors of data (usually 512 bytes in size) from specific
physical locations of the disk devices, such as Cylinder 2, Head 3, Sector 5 (details of
disk addressing are described later, in Section XXX).
So, the easiest place for BIOS to find our OS is in the first sector of one of the disks
(i.e. Cylinder 0, Head 0, Sector 0), known as theboot sector. Since some of our disks may
not contain an operating systems (they may simply be connected for additional storage),
then it is important that BIOS can determine whether the boot sector of a particular
disk is boot code that is intended for execution or simply data. Note that the CPU does
not differentiate between code and data: both can be interpreted as CPU instructions,
where code is simply instructions that have been crafted by a programmer into some
useful algorithm.

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