Ms dos boot process

MS-DOS BOOT PROCESS

Zahra Sadeghi

MS-DOS BOOT PROCESS

Summary of the DOS Boot Sequence

POST

MBR

IO.SYS

MSDOS.SYS

CONFIG.SYS

COMMAND.COM

AUTOEXEC.BAT

Command prompt

BOOTSTRAP

Boot is short for bootstrap, which in olden days was a strap attached to the top of your boot that you could pull to help get your boot on.

Boot: the starting-up of a computer, which involves

loading the operating system and other basic software .

A cold boot is when you turn the computer on from an off position.

A warm boot is when you reset a computer that is already on

CPU's initialization

starts with the special boot program software that is in the system BIOS ROM

Location: at the memory register FFFF0h of the

BIOS. This memory register location is at the end of system memory.

Containing: a jump instruction that moves execution

to the location of the BIOS start-up program. This

program runs a Power-On Self Test (POST)

Bios check

BIOS (Basic Input Output System) :

is a set of OS-independent programs in ROM that

configure the system, seek and boot an operating system (OS)

The BIOS typically looks to the CMOS chip to tell it where to find the OS.

The first of these BIOS tasks = the Power On Self Test

(POST)

Power-On Self Test (POST) Note: The system will skip the POST if you perform a

"warm" boot by pressing CTRL-ALT-DEL)

POST checks the hardware including:

Video Test: initializes the display adapter, tests the video card and video memory, and displays configuration information or

any errors.

BIOS Identification: displays the BIOS version, date, and manufacturer.

Memory Test: tests the memory chips. A running sum of the amount of memory tested will display.

error message :indicates a faulty memory chip that will require

replacement.

If the POST does not detect a battery failure, it then continues

to initialize the CPU:

PCI Initialization – If you have a PCI bus the system will now initialize the cards on the bus.

Configuration Display – The BIOS now tests and displays the system configuration, including the status of:

CPU Floppy drives Mouse and keyboard ports Serial ports Parallel ports Cache memory information

If not finding these things it gives an error: 1. on screen 2. by emitting a series of beeps through the pc built-in

speaker. There are different varieties of BIOS, and each has its own BEEP codes, which are used

to diagnose problems during the boot process If everything works, your computer will beep once and continue booting. A "fatal" error will cause the BIOS to halt the boot process.

CMOS Check

CMOS (complementary metal oxide semiconductor).

settings that are held on a bettery-powered chip.

CMOS settings are configurable by the user, and can be accessed at the beginning of the boot process by pressing a special key, such as F1 or Delete.

CMOS is a memory chip (only 64 bytes in size)

containing information such as date and time, hard disk settings, etc.

A CMOS memory chip requires a very small amount of current to retain its settings. A small

battery inside the computer supplies current. Oftentimes, the first sign of a failure of the

computer’s battery will be the inability of the computer to remember date and time settings.

Settings in CMOS Settings in CMOS are accessed through a system setup

program included on the BIOS chip.

The motherboard manual will list the keystrokes

necessary to invoke the setup program. Most BIOS programs will also list the keystrokes onscreen during the boot process in the form of:

PRESS <DEL> TO ENTER SETUP

The order of drives that the CMOS looks to in order to locate the OS is called the boot sequence, which can be changed by altering the CMOS setup.

Note:You need to be quick with these keystrokes, since the setup program can usually be initiated only during the few seconds that

the boot process takes to complete.

a bit about Hard Disk Drives In order to better manage the ever-increasing capacities of a hard-drive, a

scheme called partitionaning is created. Partitioning: physically splits up the hard drive into chunks, or

"Partitions". Each partition has a 512byte chunk of data at the beginning called the Boot Record.

Boot Record is the very first sector of the disk for every disk formatted

under DOS Master Boot Record : MBR is a small 512 bytes partition which

is at the first physical sector of the hard disk. The location is denoted as CHS 0,0,1 meaning 0th Cylinder, 0th Head and 1st sector

MBR contains a small program known as bootstrap

program which is responsible for booting into any OSes.

MBR

For DOS systems, the MBR will point the computer to two hidden start-up files:

1. IO.SYS (or for IBM computers, IBMBIO.COM).

This must be the very first file (physically) on your boot disk or your disk will not boot.

2. MSDOS.SYS (or IBMDOS.COM).

This must be the second file (physically) on the boot disk or it

will not boot.

This is why you cannot create a boot disk by simply copying thse files to

blank floppy disk. They would be on the disk, but they would not be in the proper position for the boot process to find them.

To create a "system disk", you need to use a particular command that tells the computer to place these files in the precise position needed for booting to occur

Boot code The BIOS, now, loads the boot code found in the master boot record

and then transfers control of the system to it.

the boot code searches the root directory of the device being booted for the operating system files that contain the operating system. For MS-

DOS, these are the files "IO.SYS", "MSDOS.SYS" and "COMMAND.COM". The first place the BIOS will look is in the boot record for the drive listed

in CMOS as the first bootable drive (usually drive A:).

If no operating system files are found, the boot program will display an

error message similar to:

"Non-system disk or disk error - Replace and press any key when ready". It means that the BIOS examined the floppy disk for example and just

rejected it because it couldn't boot an operating system.

If there is no disk in drive A: the system will then turn to the

next bootable device listed in CMOS (usually drive C:).

IO.SYS & MS-DOS.SYS

IO.SYS (or IBMBIO.COM) contains the interface between MS-DOS (or PC-DOS) and the I/O subsystem.

This allows the OS to communicate with the basic peripherals and perform input and output tasks.

it contains more BIOS software

MSDOS.SYS (or IBMDOS.COM) is the Kernel of the OS. It interacts

with the hardware and helps process the various DOS functions contains: software to manage files, run applications software and interface

with hardware Both IO.SYS and MSDOS.SYS are hidden files, which means that you

won't see them in a DOS dir comand normally , and

they would not appear in a directory listing. This is to protect the OS from someone mistakenly deleting these key files

and rendering the computer unable to boot.

CONFIG.SYS

The third file to be loaded

this file contains information that helps to configure your system. CONFIG.SYS is a text

file This is where you would install devices and

drivers :mouse, sound card, and CD-ROM drive installed here.

contains commands that tell DOS: 1. (FILE=) how many files it can open at any one time 2. (BUFFERS=) how many file buffers to create 3. (DRIVERS=)commands to load device drivers (small programs

that tell your computer how to communicate with devices such as printers)

Device driver

any program which controls a device

are made permanently resident

are loaded as part of the CONFIG.SYS

E.x:The programs that make access to CD-ROM drives and

sound cards

their extension can be anything the developer wants.

virtually all commercially available device drivers have an extension

of SYS, COM, or EXE.

Some drivers are available with the extension DEV

The syntax for loading a device driver in your

CONFIG.SYS is:

device=

if you have a device HIMEM.SYS in the directory

C:\DOS, it will take a numeric parameter to set the size of some buffer

you might load it with:

device=C:\dos\himem.sys 128

Parameters of devices

The question of which parameters a

given device driver allows or whether it allows

any: at all depends on the driver and should be dealt with in the documentation for the program in question

ANSI.SYS takes no parameters

VDISK.SYS takes parameters explained

in the DOS manual.

BUFFERS diskette access & hard disk has access times are much slower than RAM

access times.

Disk caching sets aside some RAM to keep a copy of the most recently accessed disk information

for example, if a database is continually accessing a disk, the first time the disk is really read but the next time the copy in cache memory will be read instead

DOS comes with some free rudimentary disk caching included. It keeps N buffers of 512 bytes each.

By default N = eight (on most DOS systems)

You should certainly make this number larger by including the line: BUFFERS=N ,10<N<25

Examples of Device Drivers

Device: a virtual file which can typically be written to and read from

Despite the name, the device command can load other programs which do not control devices and

physical "devices" may not be devices in the sense of setting up a virtual file. A mouse is a good example of something which is not a device in

this technical sense

The most common example :"con" which you typically read from when you issue the command "copy con filename".

Devices can only be installed via the CONFIG.SYS. The hidden file MSDOS.SYS sets up several devices even if you have no

CONFIG.SYS: con, prn, aux, lpt1, lpt2, lpt3, com1, com2. LPTn and COMn are names for the parallel and serial ports

Con= short for console, is a combined keyboard/monitor device,

prn =printer

The FILES command file handle :When DOS opens a file, it keeps certain information

in memory to be able to quickly access the file.

During boot up, memory is put aside for these file handles so a limit is placed on the number of files that can be open at one time.

The default is eight may seem adequate since programs normally close files when they

are done allowing the file handles to be reused

is often not adequate. DOS uses four of the handles itself for "files" like con and prn. Thus there are four available for your programs

If DOS is asked to open a file and a handle is not available,

DOS issues an error message and the running program may even

abort.

place the line : FILES=40 in your CONFIG.SYS file.

If you are running Microsoft Windows then set the files=80.

COMMAND.COM The fourth file to be loaded

is loaded into memory at the beginning of the boot process, and stays resident in memory at all times.

This file does not need to be in any particular physical location on the

disk, but does need to be in the root directory.

This file is a command interpreter: In Unix (and Linux)= shell. it interprets all of the Internal Commands= the commands that are used in writing

batch files: time , rename , path , cd , exit , cls

External commands: some commands do not need to be loaded into memory every time you boot,

because they are used infrequently, or possibly because they are large and take up a lot of space. These commands are stored as separate files in your C:\DOS directory : debug , help , find , edit

consists of 3 parts: 1. more code to manage Input/Output (I/O), 2. internal DOS commands such as COPY and DIR, 3. and a short program that looks for AUTOEXEC.BAT.

AUTOEXEC.BAT The fifth, and last file to be loaded. stands for "automatically executed batch" program. 1. holds a list of DOS commands that are automatically executed

each time DOS loads. 2. batch file:it contains a series of commands that are executed whenever

the file is run. If there is no AUTOEXEC.BAT file present, a default version is run. if the

default version is run you will be asked to enter the date and the time. After all of this, if it all works correctly, you will finally

see the prompt, which will most likely look like one of these:

A:\> C:\>

Booting windows

If any of the Windows 95/98/ME versions were being started, the above would only be the beginning of the startup process.

For loading these Windows versions, there are many more routines for boot process: reading the system registry, initializing hardware devices and starting the graphical user interface and …

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