Unit II — DOS: Booting, Files & Directories

DOS (Disk Operating System) is a single-user, single-tasking command-line operating system developed by Microsoft (MS-DOS). Although largely replaced by Windows, DOS concepts remain important for understanding OS fundamentals and for exam purposes.

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Booting Sequence

Booting is the process of starting a computer and loading the operating system into RAM. There are two types:

DOS Booting Steps:

1. Power ON → CPU reads BIOS from ROM.
2. BIOS performs POST (Power-On Self Test) — checks RAM, keyboard, disk.
3. BIOS locates the boot device (hard disk, USB).
4. BIOS loads the Master Boot Record (MBR) from the first sector of the boot disk.
5. MBR loads the DOS boot sector → loads IO.SYS and MSDOS.SYS.
6. COMMAND.COM is loaded → provides the command prompt (C:>).
7. AUTOEXEC.BAT and CONFIG.SYS (configuration files) are executed.
8. User sees the DOS prompt: C:>

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File Concepts in DOS

A file is a named collection of related data stored on disk.

DOS File Naming Rules (8.3 Format):

• Filename: up to 8 characters
• Extension: up to 3 characters (after the dot)
• No spaces; no special characters except $, !, @, #, %, &, (, ), -, _
• Not case-sensitive

Examples: REPORT.TXT, PROGRAM.EXE, DATA.BAT

Common File Extensions:

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Directory Concepts

A directory (folder) is a container that holds files and other directories, forming a tree structure.

Directory Tree Example:

C:├── DOS├── WINDOWS└── DOCS    ├── REPORT.TXT
    └── NOTES.TXT

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Wildcards

Key Takeaway: DOS uses an 8.3 filename convention and a hierarchical directory tree. The booting sequence — BIOS → POST → MBR → IO.SYS → COMMAND.COM — is a standard exam question. Understanding absolute vs relative paths and wildcard usage is also frequently tested.