Booting process in DOS
The booting
process in DOS (Disk Operating System) involves multiple steps to initialize
the hardware, load essential system files, and prepare the computer for user
interaction. Here's an in-depth explanation of each stage:
1. Power-On
Self-Test (POST)
- Definition: When the computer is powered
on, the BIOS (Basic Input/Output System), embedded in the system's
firmware, initiates the POST process.
- Purpose:
- Verifies the integrity and
functionality of essential hardware components like:
- Memory (RAM)
- Processor
- Keyboard
- Storage devices (hard drives,
floppy drives, etc.)
- Video card
- Ensures that the system can
proceed with loading the operating system.
- Process:
- If hardware passes the tests,
the system continues booting.
- If errors are detected:
- Beep codes or error messages
are displayed.
- The system may halt until the
issues are resolved.
2.
Locating a Bootable Device
- BIOS Role:
- BIOS checks the boot sequence
(configured in the BIOS settings) to find a bootable device, such as:
- Floppy disk
- Hard drive
- CD-ROM
- USB drive
- The first device with a valid
boot sector is selected.
- Master Boot Record (MBR):
- The BIOS reads the first sector
(512 bytes) of the bootable device, called the MBR.
- The MBR contains:
- A small bootloader program.
- A partition table that
identifies the structure of the storage device.
- The bootloader program in the
MBR is executed to load the operating system.
3.
Loading DOS System Files
DOS requires
specific system files to function. These files are read from the boot device in
the following order:
- IO.SYS:
- Acts as the bridge between DOS
and the computer’s hardware.
- Initializes low-level drivers
for input and output devices (keyboard, disk drives, etc.).
- Loads a basic set of device
drivers into memory.
- MSDOS.SYS:
- The core of DOS.
- Contains essential routines for
file and disk management, memory handling, and interfacing with hardware.
- Works in conjunction with
IO.SYS to provide the foundation for DOS operations.
4. System
Configuration
After the
core system files are loaded, DOS executes configuration files to prepare the
system environment:
- CONFIG.SYS:
- A text file that configures the
system and loads device drivers.
- Common entries include:
- DEVICE=driver.sys: Loads a
specific device driver.
- FILES=n: Sets the maximum
number of files that can be opened simultaneously.
- BUFFERS=n: Defines the number
of disk buffers to optimize file access.
- DOS=HIGH: Allows DOS to load
into the high memory area to free up conventional memory.
- AUTOEXEC.BAT:
- A batch file containing
commands that run automatically during the boot process.
- Used to:
- Set the system path with the
PATH command.
- Define environment variables.
- Load Terminate and Stay
Resident (TSR) programs like MSCDEX.EXE (for CD-ROM support).
5.
Loading the Command Interpreter
- The command interpreter, COMMAND.COM,
is the last essential file loaded.
- Responsibilities:
- Provides the user interface for
executing commands.
- Interprets and processes input
from the keyboard.
- Executes built-in DOS commands
like DIR, COPY, DEL, etc.
- Starts external programs when
instructed.
- Error Handling:
- If COMMAND.COM is missing or
corrupted, DOS cannot proceed and displays an error.
6.
Finalizing the Boot Process
- After COMMAND.COM is loaded, DOS
completes its initialization.
- The system displays the DOS prompt
(e.g., C:\>), indicating that it is ready for user commands.
- Users can now:
- Execute commands.
- Run batch files.
- Launch applications or
programs.
Key
Characteristics of DOS Booting
- Sequential Loading:
- DOS follows a linear process,
loading one component at a time.
- Each step depends on the
successful completion of the previous step.
- Single-Tasking Environment:
- DOS is a single-tasking
operating system, allowing only one program to run at a time.
- Customization:
- Users can modify CONFIG.SYS and
AUTOEXEC.BAT to:
- Load specific device drivers.
- Optimize memory usage.
- Automate repetitive tasks.
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