Disk Operating System - Introduction, Functioning, Internal and External Commands

An Operating System (OS) is a fundamental software component that manages a computer's physical hardware. It serves as a critical intermediary between the human user and the complex hardware components of the machine.

Purpose of an Operating System

The primary objective of an operating system is to establish an environment where a user can execute various programs in a manner that is both convenient and efficient. To achieve this, the hardware must include mechanisms that ensure correct operation and prevent user-level programs from interfering with the overall stability of the system.

Three Primary Functions:

1. Convenience: It simplifies the user experience, making the computer more accessible.
2. Efficiency: It manages system resources (like CPU time and memory) to ensure they are used effectively.
3. Ability to Evolve: A well-designed OS is modular, allowing for the development, testing, and implementation of new functions without disrupting existing services.

Understanding the Disk Operating System (DOS)

A Disk Operating System (abbreviated DOS) is an operating system specifically designed to reside on and utilize disk storage devices. These devices traditionally include:

• Floppy disks.
• Hard disk drives.
• Optical discs.

Operational Requirements and Context

A DOS must inherently provide a file system. This system is responsible for organizing, reading, and writing data files onto the storage media. While the term "DOS" is strictly used for older generations of operating systems and does not accurately describe modern versions of Microsoft Windows, it remains a foundational concept in computing history.

How DOS Works?

The operation of DOS follows a specific cycle of communication between the user and the hardware:

1. Command Input: Users enter commands via the keyboard or mouse.
2. Translation: DOS translates these human-readable inputs into electronic signals that the computer's circuitry can interpret.
3. Processing: The Central Processing Unit (CPU) receives these signals, processes them, and calculates the result.
4. Feedback: The system returns the final result to be displayed on the monitor.
5. Hardware Driving: The "System Unit," which contains the brain of the PC (the CPU and hard disk), is driven by DOS, which in turn follows the specific directions provided by the user's typed commands.

Classification of DOS Commands

DOS commands are split into two categories: Internal and External. The primary difference lies in how they are stored and loaded into the computer's memory.

1. Internal Commands

Internal commands are considered "resident DOS" because they are loaded into the computer's internal memory (RAM) the moment the system is powered on. DOS does not need to load these from a disk before execution; they are always ready. They do not require any additional files to run. To execute them, the user simply types the command at the prompt.

Detailed Internal Command Reference

These commands are always accessible to the user without needing extra files.

Pro-Tip: Subdirectory Shortcuts

Every subdirectory contains a "dot-dot" (..) file. This is a linkage to the parent directory. The root directory is the only one without a dot-dot file because it has no parent. You can use CD .. as a shortcut to move up one level without needing to type the parent directory's name.

2. External Commands

External commands are not part of the resident DOS and are stored individually on the disk. Each command is saved in a specific "command file". On MS-DOS systems, these files carry a .COM or .EXE extension. Before execution, DOS must locate the file (usually in a specific DOS directory) and load it into the internal memory.

These commands are version-specific. Using an external command file from a different DOS version than the one installed can result in an "Incorrect version" error message.

Detailed External Command Reference

These commands require specific files (like .EXE or .COM) to be present on the disk to function.