Disk Operating System (DOS)

Description and functions of individual components of the Disk Operating System: Bios, Kernel, and Command interpreter.

DOS Structure

The structure of DOS (Disk Operating System) breaks down to four distinct components:

  • BIOS Module
  • Kernel
  • Command Processor
  • External Commands

The BIOS Module

The BIOS module includes the default resident drivers for:

  • Console display and keyboard (CON)
  • Line printer (PRN)
  • Auxiliary device (AUX)
  • Date and time (CLOCK)
  • Boot disk device (block device)

Installable drivers are accommodated by DEVICE=driver entries in the CONFIG.SYS file. Driver examples are:

  • MOUSE.SYS (mouse driver)
  • ANSI.SYS (auxiliary console functions)
  • 386EMM.SYS (memory management)
  • CLOCK.SYS (software clock)

The primitive parts of the resident drivers are in the ROM BIOS while the rest are in the IO.SYS (or IBMBIO.COM) system file.

The DOS Kernel

The DOS Kernel performs the following functions:
  • File and record management
  • Memory management
  • Character device input/output
  • Spawning other programs
  • Access to real-time clock

The DOS kernel components are contained in the MSDOS.SYS (or IBMDOS.COM) system file. Programs communicate with the kernel via software interrupts.

The Command Processor

The DOS command processor is COMMAND.COM. A command processor is also known as a shell and a command interpreter. The DOS command processor is not the operating system, but rather a special class of program running under control of the MS-DOS kernel. Its major function is to provide the DOS user interface (UI).

The command processor, COMMAND.COM, is responsible for parsing and carrying out user commands, including both internal and external commands.

Programming code for internal commands is contained within COMMAND.COM itself (see DOS Internal Commands), while code for external commands resides in disk files and must be loaded into primary memory (RAM) before being executed. (See DOS External Commands).


Bruce Miller, 2005, 2014