Introduction to Disk Operating system

Introduction to Disk Operating system

Disk Operating System (DOS) is a king of system software that comes on disks and act as link between application software and the system’s hardware. Dos must be in the computer’s memory before application software in a computer system's-DOS is an abbreviation of Microsoft Dos. MS-DOS supports both floppy disks and hard disks, MS-DOS comprises of three essential files and many command files. The three essential files are: (for Windows 95, 98, me)

IO.SYS

MS-DOS.SYS

COMMAND.COM

for windows 2000, XP,2003,Vista, ……………….etc.

The first two files IO.SYS and MS-DOS.SYS. Of MS-DOS are hidden files. These files are commonly called system files/ COMMAND.COM is the command interpreter. All these files are stored on a floppy disk or on a hard disk.

A Closer Look at MS-DOS

The operating system provides I/O resources to application programs, as well as an environment that can be used to execute programs or interact with the OS. To accomplish these two tasks, MS - DOS uses three files: IO.SYS, MSDOS.SYS, and COMMAND.COM. Note that the myriad other files shipped with optimize and maintain the system. The following sections examine each of the three core MS-DOS files in more detail. Keep in mind that loading and running an OS properly relies on adequate processing, memory and disk system resources.

IO.SYS

The IO.SYS file provides many of the low-level routines (or drivers) that interact with BIOS. Some versions of IO.SYS are customized by original equipment manufacturers (OEMs) to supplement the particular BIOS for their system. However, OS customization is extremely rare today because it leads to system incompatibilities. In addition to low-level drivers, IO.SYS contains a system initialization routine. The entire contents of the file (expert for the system initialization routine) are kept in low memory throughout system operation. IO.SYS is a file assigned with a hidden-file attribute, so you will not see the file when searching a bootable disk with an ordinary DIR command.

MSDOS.SYS

This is the core of MS-DOS versions up through 6.22. The MSDOS.SYS file is listed second in the boot disk's directory, and is the second file to be loaded during the boot process. It contains the routines that handle OS disk and file access. Like IO.SYS, the MSDOS.SYS file is loaded into low memory where it resides throughout the system's operation. If the file is missing or corrupts, you will see some kind of boot failure message, or the system may lock up.

COMMAND.COM

The COMMAND.COM file serves as the MS-DOS shell and command processor. This is the program that you are interacting with at the command-line prompt. COMMAND.COM is the third file loaded when a PC boots, when it is stored in low memory along with IO.SYS and MSDOS.SYS. The number of commands in normal operation :resident and  transient.

DOS Command

There are two types of Dos Command

Internal Command:

External Command:

Differences between Internal Command and External Command

Internal command	External Command
1. Internal commands are built-in the command.com file.	1.External commands are coordinated by individual command files either having extension .exe or .com.
2. Internal commands can be executed from any DOS prompt.	2. The disk drive must read the content of each file before execution.
3. They are not listed by DIR command since they are parts of command .com file. e.g., DIR. CLS, COPY. etc.	3. They are listed by DIR command since they are individual files. e.g., CHKDSK, MSBACKUP, LABEL, etc.