MSDOS.SYS

MSDOS.SYS is a system file in MS-DOS and Windows 9x operating systems. In versions of MS-DOS from 1.1x through 6.22, the file comprises the MS-DOS kernel and is responsible for file access and program management. is loaded by the DOS BIOS  as part of the boot procedure. In some OEM versions of MS-DOS, the file is named.

In Windows 95 (MS-DOS 7.0) through Windows ME (MS-DOS 8.0), the DOS kernel has been combined with the DOS BIOS into a single file,  (aka  ), while   became a plain text file containing boot configuration directives instead. If a  file exists, the system will retrieve these configuration directives from   rather than from. When Windows 9x is installed over a preexisting DOS install, the Windows file may be temporarily named  for as long as Windows' dual-boot feature has booted the previous OS. Likewise, the  of the older system is named   for as long as Windows 9x is active.

Some DOS utilities expect the  file to have a minimal file size of at least 1 KB. This is the reason why a large dummy comment is typically found in the   configuration file since Windows 95.

By default, the file is located in the root directory of the bootable drive/partition (normally  for hard disks) and has the hidden, read-only, and system file attributes set.

The MS-DOS derivative Disk Control Program (DCP) by the former East-German VEB Robotron used a DCDOS.SYS filename instead.

IBM PC DOS as well as DR-DOS since 5.0 (with the exception of DR-DOS 7.06) used the file  for the same purpose, whereas DR DOS 3.31 to 3.41 used   instead. FreeDOS uses the file  for the same purpose.

Windows NT-based operating systems (NT 3.1–4, 2000, XP, and 2003) use the NTLDR file and NT 6+ operating systems (Vista, 2008, 7, 8, 8.1, and 10) use bootmgr instead, as they have a different boot sequence.