Introduction to DOS

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Introduction to DOS ➡️

DOS stands for (Disk operating system), and it is a name for the basic IBM PC operating system. DOS is command User/Line interface (CUI) Operating system software. Several varieties of DOS are available , including Microsoft's versions of DOS (MS-DOS) , IBM's version (PC-DOS) , and several others.

            There are actually several levels to DOS. At  the lowest level is the BIOS  (Basic input/output system)  which is responsible for managing devices like the keyboard and disk drivers at simplest possible lavel e.g. the BIOS let's you say things like ( get me sector 5 of track 3 from disk drive 1). This is done partly by software in ROM (Read-only memory) and partly by BIOS extension, which are loaded when the system first starts up (with MS-DOS, these are in a file called IO.SYS; on PC-DOS they're in IBMBIO.COM).

    The second layer provides a set of higher level services implemented using the low-level BIOS services; you can now refer to disk drive 1 as (drive A) and instead of refering to spacific sectors or tracks you can refer to files by name (e.g. LETTER.TXT.). You can also treat devices as if they were named files  so that for example  you can use the name PRN: to refer to the printer. In other words, this level provides a file system (you can refer to files by name and let dos worry about translating the name into a physical  location) as well as some device independence (you don't have to differentiate storing text in a file from sending it to the printer). This layer of the system is implemented by another file, which is loaded when the system first starts up (the file is called MS-DOS.SYS on MS-DOS systems, and IBMDOS.COM on PC-DOS systems).

   The third layer is command interpreter (or shell), which is what most people think of as DOS (it's not, but it's what you interact with when you use the computer, so it's an understandable confusion). This is contained in another file called  COMMAND.COM, which is just an ordinary program that is started automatically. The shell's job is to display a COMMAND PROMPT in the screen to let you know you're supposed to type something, then to read a line of text that you type, and to interpret it as a command, which usually involves loading another program into memory and running it. When it's finished doing this it displays another prompt and waits for you to type in another command.



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