A Hidden Windows Configuration File

The core (or kernel) of the DOS operating system consisted of three files. These files are IO.SYS which contained the extensions to the BIOS required by DOS, MSDOS.SYS which contained the DOS loader, and COMMAND.COM which contained the base commands built into the Operating System. The first two of these three files has the hidden attribute set on by default.

With Windows 95 the functionality of the IO.SYS and MSDOS.SYS files were combined into the IO.SYS file making the MSDOS.SYS file available to be used for something else.

The something else that Microsoft decided to use the file for is as a file containing some very basic configuration options that they wanted to keep well hidden from Windows users.

This file is broken up into three parts. The first part starts with a line that reads [Paths] and contains information about where the operating system is installed. The second part starts with a line that reads [Options] and contains some options that control the way that Windows works. Finally, there is a long section of comments labelled Do not remove that is there to make sure that the length of the file is above 1024 bytes. This last section is required because the previous use of the file as a DOS loader was this size and some code within the operating system still hasn't been amended to reflect the change in the use to which this file is being put.


There are three statements that you will find in this section. They are:


There are a lot more option parameters than there are path parameters and you will not necessarily find all of the following listed in your file.

There are a number of other parameters that can be entered in this section of the file but in customising your system, the ones listed are the ones I would think you would be most likely to be interested in.


This article written by Stephen Chapman, Felgall Pty Ltd.

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