This system file is the first file loaded by the hardware at boot time for any MS-Dos system.

It essentially is an extension to the bios whose main purpose is to organise the hardware and execute a subroutine called sysinit which loads msdos.sys and other drivers.

On Windows 95/98, IO.SYS also has the boot logo bitmap embedded in it.

When a hard disk partition is made bootable from a Windows 95/98 command prompt using SYS.COM, subsequently booting from that partition shows the Windows logo briefly before the command prompt appears. The logo bitmap is not stored in any separate file but is inside IO.SYS itself.

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