2.1 What is Disk Operating System (DOS)?

DOS (Disk Operating System) was the first widely installed operating system in personal computers. (Earlier, the same name had been used for an IBM operating system for a line of business computers.) The first personal computer DOS, called Personal Computer - Disk Operating System, was developed for IBM by Bill Gates and his new Microsoft Corporation. He retained the rights to market a Microsoft version, called MS-DOS. PC-DOS and MS-DOS are almost identical and most users have referred to either of them as just "DOS." DOS was (and still is) a non-graphical line-oriented command-driven operating system, with a relatively simple interface but not overly "friendly" user interface. Its prompt to enter a command looks like this: C:\>The first Microsoft Windows operating system was really an application that ran on top of the MS-DOS operating system. Today, Windows operating systems continue to support DOS (or a DOS-like user interface) for special purposes by emulating the operating system. In the 1970s before the personal computer was invented, IBM had a different and unrelated DOS (Disk Operating System) that ran on smaller business computers. It was replaced by IBM's VSE operating system.

2.2 How Is MS DOS Organized

MS- DOS is partitioned into several layers that serve to isolate the kernel logic of the operating system, and the user’s perception of the system, from the hardware it is running on. These layers are:

·         The BIOS (Basic Input/Output System)

·         The DOS kernel

·         The Command Processor (Shell)

We’ll discuss the functions of each of these layers separately.

The BIOS (Basic Input/Output System)

The BIOS is specific to the individual computer system and is provided by the manufacturer of the system. It contains the default resident hardware dependent drivers for the following devices:

·         Console display and keyboard (CON)

·         Line printer (PRN)

·         Auxiliary device (AUX)

·         Date and time (Clocks)

·         Boot disk device (block device)

The MS-DOS kernel communicates with these device drivers through I/O request packets; the drivers then translate theses request into the proper commands for various hardware controllers. In many MS-DOS systems, including the IBM PC, The most primitive parts of the Hardware drivers are located in read-only memory (ROM) so that they can be used by stand-alone applications, diagnostic, and the system start-up program.

The terms resident and installable are used to distinguish between the drivers built into the BIOS and the drivers installed during system initialization by DEVICE commands in the CONFIG.SYS file.

The BIOS is read into random-access memory (RAM) during system initialization as part of a file named IO.SYS.

The DOS Kernel

The DOS kernel implements MS-DOS as application programs see it. The kernel is a proprietary program supplied by Microsoft Corporation and provides a collection of hardware-independent services called system functions. These functions include the following:

·         File and Record management

·         Memory management

·         Character-device input/output

·         Spawning of other programs

·         Access to the real-time clock

Programs can access system functions by loading registers with function specific parameters and then transferring to the operating system by means of a software interrupt. The DOS kernel is read into memory during system initialization from the MSDOS.SYS file on the boot disk.

The Command Processor

The command processor, or shell, is the use’s interface to the operating system. It is responsible for parsing and carrying out user commands, including the loading and execution of other programs from a disk or other mass-storage device.

The default shell that is provided with MS-DOS is found in a file called COMMAND.COM. Although COMMAND.COM prompts and responses constitute the ordinary user’s complete perception of MS-DOS, it is important to realize that COMMAND.COM is not operating system, but simply a special class of program running under the control of MS-DOS.

COMMAND.COM can be replaced with a shell of the programmer’s own design by adding a SHELL directive to the system-configuration file (CONFIG.SYS) on the system start-up disk. The product COMMAND PLUS from ESP systems is an example of such an alternative shell.

Utilities

·         Filec - Filename completion Filec completes partially typed filenames, and takes up less than 3k of memory. Filename completion speeds up DOS navigation no end.

·         DZ - See space directories take up DZ by displays the space taken up by directories as a neat bar chart.

·         ACD - Fast directory switching There are loads of replacement utilities for the CD command on the internet all offering faster directory switching.

·         MaxDIR - Best DIR replacement MaxDIR displays files of different types in different colours, letting you see at a glance which files are executable, which are documents, directories etc. It also shows true disk usage by files, and percentage of disk used. Once you're used to it you wont want to use DIR. This one is freeware too.

·         OneDIR - Another great DIR replacement Like MaxDIR, OneDIR displays files of different types in different colours. OneDIR also supports long file names under Windows 95 and you can customize the colours allocated to filetypes. It is however shareware.

·         Bookmark - Directory Bookmarks My 'Bookmark' utility provides a fast and simple way of switching/changing between directories in DOS by letting you 'bookmark' directory paths. It also gives you a quick way of copying files between directories and provides a useful facility for other batch files. Requires ansi.sys.

·         PKZip - DOS zip/unzip utility You'll need this to zip/unzip things in DOS.

·         RAR - DOS rar/unrar utility You'll need this to rar/unrar things in DOS.

Internal and External Commands

The Internal Commands allow you to manage the files on a disk. These commands are loaded as part of COMMAND.COM and are always present in memory until the computer is turned off. They do not need to be loaded from disk every time you want to use them.

Examples: BREAK, CDCHDIR, CHCP, CLS, COPY, CTTY, DATE, DELERASE, DIR, MDMKDIR, PATH, PROMPT, RDRMDIR, SET, TIME, TYPE, VER, VOL, VERIFY, LOCK, UNLOCK, LOADHIGH, EXIT, TRUENAME

External Commands are actually programs that are just bundled with the OS. 

Examples:  DELTREE, DISKCOMP, DISKCOPY, DOSKEY, DEBUG, EDIT, EDLIN, EXE2BIN, FASTOPEN, FC, FDISK, FIND, FORMAT, GRAFTABL, GRAPHICS, JOIN, KEYB, LABEL, LINK, MEM, MODE, MORE, MOVE, NLSFUNC, PRINT, RECOVER, REPLACE RESTORE, SCANDISK, SELECT, SETVER, SHARE, SMARTDRV, SORT, SUBST, TREE, SYS, XCOPY, XCOPY32.