Transcript OperatingSystemLectures

Operating Systems
Lecture 2
The BIOS and Boot Process
M. Naghibzadeh
Reference: M. Naghibzadeh, Operating System Concepts and Techniques, iUniverse publisher, 2005.
To order: www.iUniverse.com, www.barnesandnoble.com, or www.amazon.com
The need for BIOS
Main memory, cache, and internal registers of the CPU are
usually volatile. The information that is stored in these devices
is lost when the computer’s power is turned off.
As soon as the computer is turned on, it starts to perform the
fetch-execute cycle over and over again (see Lecture 1).
If there are only volatile memories, there could not be any
instruction in the main memory when the computer is turned
on.
To overcome this problem, a program is designed and stored
in a special memory called Read Only Memory (ROM) and it is
installed on the computer’s motherboard.
This program is called Basic Input Output System (BIOS).
As the computer starts or restarts the BIOS address is
automatically loaded into the PC register and thus the BIOS
starts running.
BIOSes are produced by many factories, but perform the same
basic functions.
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Computer as a layered device
System BIOS
Computer Hardware
(Bare Machine)
“Layer 0”
“Layer 1”
Figure 1: A computer with a BIOS
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BIOS Actions at start/restart
Power-On Self Test

checks to make sure every immediately required
device is connected and functional.
BIOS manufacturer’s logo display

Compaq, Phoenix, Intel, IBM, Award plus, or any
other manufactures.
CMOS and setup modifications

The Complementary Metal Oxide Semiconductor
memory is a battery-supported memory which keeps
modifiable information like date, time, computer
password….
Initial section of booting the operating system.
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Operating System installation
The OS is a giant program that cannot be stored in a
small ROM memory.
In addition, we would like computers to be flexible so
that we are able to run any operating system from
amongst a list of operating systems.
Therefore, the OS has to be installed after the
computer is purchased.
The operating system is usually stored on a CD(s)
which are first transferred to a hard disk drive in order
to become functional because hard disk drives are
much faster than CD drives.
During the transfer other functions, for example
decompression, is performed. This process is called
OS installation.
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Booting the operating system
At any start/restart of the computer, one of the functions of the BIOS is
to begin the process of loading the operating system.
The BIOS usually transfers only one block of data from the first valid
bootable device to the main memory.
This block of data is taken from a specific and fixed place of the
bootable device and is put in a specific and fixed place of main memory.
The size of the block of data is usually 512 bytes and is usually taken
from block zero of track zero of the bootable device.
The block contains a small program that is sometimes called bootstrap.
After loading the bootstrap, control is transferred to this little program.
By doing so, BIOS disqualifies itself from being the internal owner of the
computer.
This little program will complete the boot process, perhaps in more than
one phase.
The BIOS will then become a passive program which can be used by the
operating system to perform many operations.
BIOS contains many other useful procedures (little programs), especially
for interacting with input/output devices.
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Kernel
Eventually, essential parts of the operating system will
be loaded in main memory.
This includes the Operating system Kernel.
Other parts are usually loaded on demand.
The Kernel is the essential part, or the inner part, of
the seed of an operating system.
Kernel is a program that is composed of many
routines for doing activities that are not performed by
any single machine instruction.
Kernel routines are built using machine instructions
and/or BIOS functions.
In addition, kernel is composed of many essential
processes (threads or agents), each designed to carry
out a responsibility of the operating system.
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A Four-layer computer
Other layers of the OS
OS Kernel
System BIOS
…
Computer Hardware
(Bare Machine)
“Layer 0”
…
“Layer 1”
“Layer 2”
Figure 2: A four-layer machine with system BIOS, kernel, and processes
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An actual hierarchical OS
System Services/User Applications: Service Controller, Remote Procedure Call, C++ Compiler, Environmental Subsystems
(Posix, OS/2, Win32), etc.
Input/ Output
Manager
Virtual Memory
Manager
Device Drivers
Process/ Thread
Manager
Security
Reference
Manager
Cache
Manager
Windows
Manager
Windows 2000 Kernel
Hardware Abstraction Layer (HAL)
Hardware and Hardware Interfaces: Buses, CPU, Main Memory, Cache, I/O Devices, Timers, Clock, Direct Memory Access,
Interrupts, Cache Controller, etc.
Figure 3: The structure of Windows 2000
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Protection
programs cannot directly and freely use the facilities within the
lower layers.
The operating system monitors the activities of every running
program.
In the absence of any protection mechanism, strange things
could happen and the system could become unreliable.
In a multi-user system, one person’s process (program) can
change somebody else’s process, making it do something it
shouldn’t.
A process can even modify the operating system itself, hence,
making it useless and even harmful to other programs.
The existence and propagation of viruses, worms, Trojan horses,
etc. in computers is directly related to the existence of weak
security points within the operating system.
Protecting the system from crackers and hackers is a tough job.
Many operating system and network researchers are exclusively
working to make our systems less vulnerable.
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Modes of operations
Every computer has at least two modes of operation:


Kernel mode, or supervisor mode. All machine instructions,
whether privileged or non-privileged, are executable in this
mode of operation.
User mode. Privileged instructions are not usable.
One of the most remarkable features of any operating
system is the way mode transfer is made possible.
During the system startup the system is in kernel
mode.
Right after the startup the mode changes to user
mode.
When a program calls a routine of the kernel the
mode automatically changes to kernel and in return
from the routine, the mode changes back to user.
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Making Your Own Operating System
It should be clear that with the knowledge we
have gathered so far, it is impossible to write
an actual operating system.
However, by thinking of making a little
operating system lots of questions will be
raised, some of which we will talk about in the
following lectures.
I would suggest the following steps in writing
your own little, do nothing useful, operating
system.
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Making Your Own Operating System…
1. Using a running computer, write a mini program to do something
over and over again.
1.
I would prefer this program to repeatedly display the word
“Hello” on computer monitor. The size of the program is
important and its actual machine language size must be less
than 512 bytes. This program will be your operating system. To
write this program you have to know at least one programming
language. Assembly language is preferred, but C or C++ is
acceptable. Let’s suppose you will choose to write the program
with C. After writing the program, compile it and produce an
executable file.
2. An executable file has two sections: (1) the file header and (2)
the actual machine language instructions. The header contains
information about the file, e.g., size of the file. Suppose the file
header is 512 bytes. Write a mini program to change your
executable file into a file without a header by removing the first
512 bytes of the executable file. Let’s assume this file is called a
“com” file.
3. Write a program to transfer the “com” file to sector zero of track
zero of a diskette.
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Making Your Own Operating System…
1.
2.
4.
5.
Prepare a diskette.
Transfer the “com” file to sector zero of track zero of the
diskette.
1.
6.
7.
A com file is a pure machine instruction program without any
attribute sector.
There are numerous file formats and every application program
works with certain file formats. For example, MS-Word produces
and works with document (doc) files, among many others.
You have successfully stored your operating system on the
diskette.
Reset the computer and enter the BIOS setup/modify
program. Make the first floppy drive be the first bootable
device.
Insert your own operating system diskette into the first floppy
drive of the computer and reset the computer again. Wait until
your operating system comes up.
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Summary
The following subjects were discussed
The need for BIOS
BIOS actions
Installing the OS
Booting the OS
Operating system Kernel
Computer as a hierarchical device
Protection
Modes of operation
How to make your own small OS
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Find out
Which company has build your computer’s
BIOS.
The source code of a BIOS from the Internet.
The complete set of privileged instruction of
your computer’s CPU.
A source code for a small OS from the
Internet.
How to make the above source code actually
work.
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Thank you!
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