One of two hidden system files that made up Microsoft's MS-DOS. The other was IO.SYS. These two system files were loaded into memory when the computer was booted. They processed the instructions in CONFIG.SYS, then loaded COMMAND.COM and finally processed the instructions in AUTOEXEC.BAT. The PC-DOS counterparts of these system files were IBMBIO.COM and IBMDOS.COM.
From Executable to Text
In Windows 95/98, MSDOS.SYS was changed to a text configuration file rather than an executable program. It determined among other things whether the computer booted into DOS or Windows. IO.SYS was still a binary executable.
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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. On Windows 9x systems, the DOS kernel code is merged into IO.SYS, and MSDOS.SYS is an ASCII file containing configuration directives.
By default, the file is located in the root directory of the bootable drive/partition (normally C:\) and has the hidden, read-only, and system file attributes set.
IBM PC-DOS and DR DOS used the file IBMDOS.COM for the same purpose.
Windows NT-based operating systems (NT 3.1-4, 2000, XP, and 2003) use the NTLDR file and NT 6+ ( Vista, 2008, and 7) uses bootmgr instead, as they have a different boot sequence.
MSDOS.SYS in Windows 9x (95/98/ME):
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