October 9th, 2017

Why does NTVDM create empty IO.SYS and MSDOS.SYS files?

Raymond Chen
Raymond Chen

A:-)Brunuś asks why NTVDM creates empty IO.SYS and MSDOS.SYS every time it starts up.

For compatibility, of course.

Specifically, it was added for one particular program that searched for the MS-DOS system files, and if they are missing, it got confused and corrupted your CONFIG.SYS file.

This reminds me of another program we discovered in Windows 95 that parsed your SYSTEM.INI file, and if any line in that file was longer than 80 characters, it deleted your SYSTEM.INI file outright. That’ll teach ya.

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Old New Thing
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Raymond Chen

Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

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