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Table Of Contents  CertiGuide to A+ (A+ 4 Real)
 9  Chapter 1: What are Operating Systems and How Do They Work?
      9  CDI System Commands

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“8.3“ File Naming Limit
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DOS - Where it all Begins

We will begin our review of DOS by analyzing how it starts up and becomes in a ready-to-use state. When a PC is first turned on, the BIOS sets up the system for booting and attempts to find a valid source to boot from (such as a floppy disk or hard drive). When a source is found, the first program launched is known as the bootstrap loader, or boot loader. The boot loader then looks for the DOS startup files, or any other operating system that is at the beginning of a diskette or hard drive. Assuming that the DOS startup files are there, the second step occurs, which is actually loading DOS.

Once successfully loaded (“booted”), a capital letter relating to the boot drive will be displayed on screen, followed by a greater than sign. This is known as the “DOS prompt”. The prompt is literally telling you DOS is ready for you to type in a command.

There are three major components of DOS:

  • MSDOS.SYS

  • IO.SYS

  • COMMAND.COM

The first two files usually have the hidden and systems attributes set, and are the core drivers for the operating system. The third program is visible, and actually replaceable. The purpose of COMMAND.COM is to provide the user interface for DOS. It takes verbose commands in “native” human language, and interprets them into something that one of the hidden sub-programs can use. This mechanism is why COMMAND.COM is sometimes referred to as a command interpreter.

COMMAND.COM has a number of useful commands built into it. These commands are effectively sub-programs that provide basic operating system functionality, such as creating and deleting directories, copying files and displaying files stored on disks. Because these commands are built into the interpreter, they are known as internal commands. To extend the functionality of DOS, various external commands are also available. These provide tools that are more comprehensive for the user to work with, such as robust file copying, disk partitioning and formatting.


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CDI System Commands
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“8.3“ File Naming Limit
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