Disambiguation - If you are looking for the music group, see IOSYS.
IO.SYS is a major system file for MS-DOS and Windows 9x operating systems. It contains information about device drivers and the DOS initialization program.
During the boot sequence of the PC, the first boot disk space (MBR) sector is loaded in memory and executed. If it is the DOS boot sector, then the first three sectors of IO.SYS are in memory and the control is transferred to it. IO.SYS then performs the following steps:
In the first Italian version of Windows 95 OSR2.5, this file contained a serious bug: the HIMEM.SYS file, occupying 45 kilobytes of base memory more than expected, prevented the IO.SYS file from happening properly, blocking the launch of some programs, and in some cases the same operating system.
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