In the Windows 95 operating system, IO.SYS is an executable binary file (in other words, a program) that, when the operating system is booted (started), processes instructions that tell the operating system how the computer is set up. In Windows 95, IO.SYS replaced the older MS-DOS system files, IO.SYS and MSDOS.SYS. The newer IO.SYS contains all the information needed to start your computer and eliminates the need for the CONFIG.SYS and autoexec.bat files because the values in CONFIG.SYS and AUTOEXEC.BAT are in IO.SYS instead. Both of these files still come with Windows for older applications and drivers that need them.
In computers using DOS or Windows 3.x, 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, and AUTOEXEC.BAT.
To change default values that come with IO.SYS, you 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.