The Hermit Hermit's Free Library  MS-DOS

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

Four parts of the DOS Structure

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

The BIOS Module

The BIOS module includes the default resident drivers for:

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

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:

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.

COMMAND.COM is not the operating system, but rather a special class of program running under control of the MS-DOS kernel. The major function of COMMAND.COM is to provide the DOS user interface (UI), or shell.

The DOS command processor includes the command interpreter, which is responsible for parsing user input and carrying out user commands.

The programming code for all the DOS internal commands is included within COMMAND.COM. Hence, unlike the DOS external commands, DOS internal commands are always present in primary memory

See DOS Internal Commands.

The External Commands

The less-frequently used DOS External Commands (format.exe for example) are sometimes referred to as disk-bound commands.

Until needed, programming code for the external commands resides on disk in discrete files - one file for each command. Hence, external commands must be read from disk and loaded into RAM each time they are used, imposing a time penalty.

See DOS External Commands.