IO.SYS is an essential part of MS-DOS

MS-DOS

MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...

 and Windows 9x

Windows 9x

Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced since 1995, which were based on the original and later modified Windows 95 kernel...

. It contains the default MS-DOS device driver

Device driver

In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....

s (hardware interfacing routines) and the DOS initialization program.

Boot sequence

In the PC

Personal computer

A personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...

 bootup sequence, the first sector of the boot disk is loaded into memory and executed. If this is the DOS boot sector

Boot sector

A boot sector or boot block is a region of a hard disk, floppy disk, optical disc, or other data storage device that contains machine code to be loaded into random-access memory by a computer system's built-in firmware...

, it loads the first three sectors of IO.SYS into memory and transfers control to it. IO.SYS then:

1. Loads the rest of itself into memory.
2. Initializes each default device driver
   Device driver
   In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....
   
    in turn (console, disk, serial port
   Serial port
   In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...
   
   , etc..). At this point, the default devices are available.
3. Loads the DOS kernel and calls its initialization routine. The kernel is stored in MSDOS.SYS
   MSDOS.SYS
   MSDOS.SYS an important system file on MS-DOS and Windows 9x systems. It is run after IO.SYS. In MS-DOS, it contains the core operating system code, the kernel...
   
   with MS-DOS and in IO.SYS with Windows 9x. At this point, "normal" file access is available.
4. Processes the MSDOS.SYS file with Windows 9x.
5. Processes the CONFIG.SYS
   CONFIG.SYS
   CONFIG.SYS is the primary configuration file for the DOS, OS/2 as well as similar operating systems. It is a special file that contains setup or configuration instructions for the computer system.- Usage :...
   
   file, in MS-DOS 2.0 and higher and Windows 9x.
6. Loads COMMAND.COM
   COMMAND.COM
   COMMAND.COM is the filename of the default operating system shell for DOS operating systems and the default command line interpreter on Windows 95, Windows 98 and Windows Me...
   
   (or other operating system shell if specified).
7. Displays the bootsplash
   Bootsplash
   Bootsplash is a term for a graphical representation of the boot process of the operating system.Bootsplash can be a simple visualisation of the scrolling boot messages in the console, but it can also present graphics or some combinations of both....
   
    in Windows 9x. If LOGO.SYS
   Logo.sys
   Logo.sys is a system file used by DOS-based Windows versions such as Windows 95, Windows 98 and Windows Me to display its boot-up message.-Logo.sys And Other Screens Displayed:* C:\Logo.sys: the "Starting Windows" message, with the Windows logo...
   
   is present, it is used as the bootsplash. Otherwise, the bootsplash in IO.SYS is used.

IBM PC-DOS and DR DOS uses the file IBMBIO.COM

IBMBIO.COM

IBMBIO.COM is the filename of the DOS-BIOS in many DOS operating systems, and as such part of PC-DOS, earlier versions of MS-DOS, and DR DOS 5.0 and higher...

 for the same purpose. Which in turn loads IBMDOS.COM

IBMDOS.COM

IBMDOS.COM is the filename of the DOS kernel. It exists in DR-DOS and PC-DOS systems, with MS-DOS using MSDOS.SYS. The file is located in the root directory of the drive containing the operating system....

.

Disc layout requirements

• The two first entries of the root directory must be allocated by IO.SYS and MS DOS.SYS, in that order.
• IO.SYS must be the first file stored in the data storage area for files.
• The files IO.SYS + MSDOS.SYS must be contiguous
  Fragmentation (computer)
  In computer storage, fragmentation is a phenomenon in which storage space is used inefficiently, reducing storage capacity and in most cases reducing the performance. The term is also used to denote the wasted space itself....
  
  

However v3.3 allows sector 4 and higher to be fragmented. v5.0 allows the first 3 sectors of IO.SYS to be allocated anywhere. COMMAND.COM can be treated like any ordinary file.

External links

• MS-DOS Device Driver Names Cannot be Used As File Names — includes list of default device drivers.
• SYS.COM Requirements in MS-DOS Versions 2.0-6.0 — includes requirements for IO.SYS to be loaded.