Since DOS was only designed to use 1024K of memory addresses and since DOS assumed that only one application or program would be running at a time, DOS gave the running program direct access to the memory addresses and the data in RAM that they pointed to. Also, since the CPU at the time could only process 16 bits of data at one time, DOS was designed to pass segments of data to the CPU only 16 bits at a time. These standards of operation are collectively referred to as real mode, or MSDOS mode.

Loading And Launching Software In DOS

The major steps that take place in the process of loading and initializing software are:

1. The operating system receives the command to execute the program.

2. The operating system locates the program file for the program.

3. The operating system loads the program file into memory.

4. The operating system gives control to the program.

5. The program requests memory addresses from the operating system for its data.

6. The program initializes itself and possibly requests that data from secondary storage be loaded into memory.

7. The program turns to the user for its first instruction.

If DOS is your operating system, the DOS prompt displays basic information and gives you the opportunity to enter some command for the operating system to perform.

Most of the time, DOS is loaded from the hard drive, which is designated with the letter C.

It is common for computers to provide a DOS prompt that looks like this when the

machine is first turned on:

C:\>

This prompt is called the C prompt. The DOS prompt (C:\>) displayed immediately

after booting means that the operating system was copied from drive C (where the

operating system is stored) when the machine was first turned on.

As part of the startup process, drive C becomes the default drive and default

directory, sometimes called the current drive and directory, which DOS automatically

uses to save and retrieve files.

Sometimes the operating system is copied from a floppy disk rather than the hard drive.

In this case, the default drive and directory will be A:\ or B:\, and the command prompt

will be A:\> or B:\>.

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