IO.SYS

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Script error: No such module "Distinguish". Script error: No such module "Unsubst". Template:Short description Script error: No such module "Unsubst". Template:Use list-defined references <templatestyles src="Mono/styles.css" />IO.SYS is an essential part of MS-DOS and Windows 9x. It contains the default MS-DOS device drivers[1] (hardware interfacing routines) and the DOS initialization program.

Boot sequence

In the PC bootup sequence, the first sector of the boot disk is loaded into memory and executed. If this is the DOS boot sector, it loads the first three sectors of <templatestyles src="Mono/styles.css" />IO.SYS into memory and transfers control to it. <templatestyles src="Mono/styles.css" />IO.SYS then:

  1. Loads the rest of itself into memory.
  2. Initializes each default device driver in turn (console, disk, serial port, etc..). At this point, the default devices are available.
  3. Loads the DOS kernel and calls its initialization routine. The kernel is stored in <templatestyles src="Mono/styles.css" />MSDOS.SYS with MS-DOS and in <templatestyles src="Mono/styles.css" />IO.SYS with Windows 9x. At this point, "normal" file access is available.
  4. Processes the <templatestyles src="Mono/styles.css" />MSDOS.SYS file with Windows 9x.
  5. Processes the <templatestyles src="Mono/styles.css" />CONFIG.SYS file, in MS-DOS 2.0 and higher and Windows 9x.
  6. Loads <templatestyles src="Mono/styles.css" />COMMAND.COM (or other operating system shell if specified).
  7. Displays the bootsplash in Windows 9x. If <templatestyles src="Mono/styles.css" />LOGO.SYS is present, it is used as the bootsplash. Otherwise, the bootsplash in <templatestyles src="Mono/styles.css" />IO.SYS is used.

The <templatestyles src="Mono/styles.css" />IO.SYS filename was also used by Template:Ill (DCP), an MS-DOS derivative by the former East-German VEB Robotron.[2][3]

IBM PC DOS and DR DOS use the file <templatestyles src="Mono/styles.css" />IBMBIO.COM for the same purpose; it in turn loads <templatestyles src="Mono/styles.css" />IBMDOS.COM.

In Windows 9x, the <templatestyles src="Mono/styles.css" />IO.SYS not only contains the DOS BIOS, but also holds the DOS kernel, which previously resided in <templatestyles src="Mono/styles.css" />MSDOS.SYS. Under some conditions, Windows 9x uses the alternative filenames <templatestyles src="Mono/styles.css" />WINBOOT.SYS[4] or <templatestyles src="Mono/styles.css" />JO.SYS instead. When Windows 9x is installed over a preexisting DOS install, the Windows file may be temporarily named <templatestyles src="Mono/styles.css" />IO.W40 for as long as Windows' dual-boot feature has booted the previous OS. Likewise, the <templatestyles src="Mono/styles.css" />IO.SYS of the older system is named <templatestyles src="Mono/styles.css" />IO.DOS for as long as Windows 9x is active.

DR-DOS 7.06 (only this version) also follows this scheme and the <templatestyles src="Mono/styles.css" />IO.SYS filename in order to become bootable via MS-DOS boot sectors.

Similarly, FreeDOS uses a combined system file as well, but names it <templatestyles src="Mono/styles.css" />KERNEL.SYS.

Disk layout requirements

  • The two first entries of the root directory must be allocated by <templatestyles src="Mono/styles.css" />IO.SYS and <templatestyles src="Mono/styles.css" />MSDOS.SYS, in that order.[5][6]
  • <templatestyles src="Mono/styles.css" />IO.SYS must be the first file stored in the FAT directory table for files.[5][6]
  • The files <templatestyles src="Mono/styles.css" />IO.SYS and <templatestyles src="Mono/styles.css" />MSDOS.SYS must be contiguous.[5][6]

However, MS-DOS version 3.3 allows sector 4 and higher to be fragmented; version 5.0 allows the first 3 sectors of <templatestyles src="Mono/styles.css" />IO.SYS to be allocated anywhere (as long as they are contiguous).[nb 1][7][5][8]

<templatestyles src="Mono/styles.css" />COMMAND.COM can be treated like any ordinary file.

See also

Notes

<templatestyles src="Reflist/styles.css" />

  1. ↑ The MS-DOS 5.0 manual incorrectly states that the system files no longer need to be contiguous. However, for the boot process to work the system files still need to occupy the first two directory entries and the first three sectors of IO.SYS still need to be stored contiguously. SYS continues to take care of these requirements.

Script error: No such module "Check for unknown parameters".

References

<templatestyles src="Reflist/styles.css" />

  1. ↑ Script error: No such module "citation/CS1".
  2. ↑ Script error: No such module "citation/CS1".
  3. ↑ Script error: No such module "citation/CS1".
  4. ↑ Script error: No such module "citation/CS1". [1] (NB. Also has a few bits on how other DOSes are booted.)
  5. ↑ a b c d Script error: No such module "citation/CS1".
  6. ↑ a b c Script error: No such module "citation/CS1". (xix+1570 pages; 26 cm) (NB. This edition was published in 1988 after extensive rework of the withdrawn 1986 first edition by a different team of authors. [2])
  7. ↑ Script error: No such module "citation/CS1". (xxvi+738+iv pages, 3.5"-floppy [3][4]) Errata: [5][6][7]
  8. ↑ Script error: No such module "citation/CS1".

Script error: No such module "Check for unknown parameters".

Template:DOS system files

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