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What is IO.SYS?IO.SYS is a hidden executable binary file that processes instructions that tell the operating system (OS) how the computer is set up when it is booted or started. IO.SYS was an essential part of MS-DOS and up to Windows 9x but is not used in newer versions of Windows. In MS-DOS, IO.SYS supplied default device drivers and the instructions needed to initialize DOS. Windows 95 changed the role of IO.SYS by combining older IO.SYS and MSDOS.SYS files into a single file also called IO.SYS. The newer IO.SYS contains all the information needed to start a computer and eliminates the need for the CONFIG.SYS and autoexec.bat files. However, both files still come with Windows for older applications and drivers that need them. What does IO.SYS do?IO.SYS is a critical part of a computer's boot sequence for Windows 9x and earlier versions of Windows and works in the following ways:
In the early days of personal computing, the default boot sector of DOS and early versions of Windows would start loading IO.SYS -- which was always the first entry of a disk's root directory -- into memory. The disk begins loading IO.SYS into memory to start the OS boot. IO.SYS takes over to finish loading into memory and processes a series of startup steps that include the following:
What are the differences between IO.SYS, MSDOS.SYS and CONFIG.SYS?Traditionally, DOS relied on the following three key files to configure the computer and initiate the OS startup:
As DOS matured and became the default foundation for early Windows versions, IO.SYS and MSDOS.SYS files evolved and merged to some extent. For example, Windows 95 was based on MS-DOS 7.x and Windows ME was based on MS-DOS 8.x, and these OS versions basically combined the functionality of IO.SYS and MSDOS.SYS into the IO.SYS file. This left the MSDOS.SYS file as a plain text file containing boot directions. MSDOS.SYS was not executed if WINBOOT.INI was present on the system. CONFIG.SYS remained largely unchanged as a text file with configuration details about the specific computer, and was finally replaced by the CONFIG.NT file with 32-bit Windows versions such as early Windows NT. Generally, end users have the ability to open, see and edit text files such as CONFIG.SYS. Editing these files, however, should only be attempted by expert computer users with a clear knowledge of configuration settings and their implications. How to edit IO.SYS behaviorIn computers using DOS or up to Windows 9x, the IO.SYS is loaded with the MSDOS.SYS file when the computer is booted. MSDOS.SYS is a text file that determines whether DOS or Windows is loaded. Because IO.SYS is a sequence of executable code rather than readable text, it is not editable like MSDOS.SYS, CONFIG.SYS or AUTOEXEC.BAT. To change default values that come with IO.SYS, a user must change the CONFIG.SYS file. In loading IO.SYS, the system picks up any changed values that are present in the CONFIG.SYS file. The CONFIG.SYS file can be altered using any simple text editor and supports an array of commands that influence memory use, shell selection, opens files and buffers, and other options. Any changes to the CONFIG.SYS file must be saved, and the system rebooted before those changes are reflected in memory. Learn about the new features of Windows 11 Enterprise and how this version differs from Windows 10. This was last updated in April 2022 Continue Reading About IO.SYS
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