System files are used to boot MS-DOS. For safety, the system files are
marked with the hidden, system, and read-only attributes, and cannot
be changed using the ERASE, DEL, or COPY command. For a disk to boot
MS-DOS, these files must occupy a specific location on the disk. The
names of these system files vary among original equipment
manufacturers (OEMs). The most common names used are:
IO.SYS and MSDOS.SYS
IBMBIO.COM and IBMDOS.COM
A disk can only be made bootable in MS-DOS by using the FORMAT /S,
DISKCOPY, or SYS command.
SYS Original Functionality
The following is a brief overview of the SYS command's original
SYS.COM copies the hidden system files from the default drive to the
specified destination drive and updates the boot sector of the
destination drive. Each version of SYS only recognizes system files
that have the same filenames as its version of MS-DOS. The MS-DOS boot
sequence requires that the system files be the first files in the disk
data area, that they be contiguous (that is, not fragmented), and that
they occupy the first two entries in the root directory.
SYS copies the files to the destination if there is empty space for
them at the beginning of the disk. If the destination disk is already
bootable and its system files have the same names, SYS replaces the
existing system files. SYS does not copy COMMAND.COM. To make a
bootable C drive, boot with the original MS-DOS disk, and enter the
following at the drive A prompt:
copy command.com c:\
Changes to SYS
Changes to SYS include the following (by MS-DOS version number):
Version Description of Changes
3.3 MS-DOS allows system files to be noncontiguous
(fragmented). However, the system files must still occupy
the first two directory entries, and the first three
sectors of IO.SYS (or IBMBIO.COM) must be located
in the first three sectors of the disk data area.
4.0 When either or both of the following conditions are met,
- The first three sectors are allocated, and not to the
first system file, but there is enough free space on
the drive to copy the system files over.
- The SYS command does not recognize the first two files
in the root directory as system files, but the root
directory is not full.
the SYS command will, if necessary, move the first two
directory entries and/or the data located in the first
three sectors of the disk to another location, and then
copy the system files. This allows you to use SYS to
change from PC-DOS to MS-DOS; however, the PC-DOS system
files will not be deleted.
The SYS command will execute properly in earlier versions
of MS-DOS if the drive formats are compatible; however,
this is not guaranteed in all circumstances. If it is
impossible to boot MS-DOS 4.0 before running SYS, using
the SYS command on a floppy drive is the safest course of
5.0 MS-DOS 5.0 SYS.COM recognizes both the Microsoft system
file names (IO.SYS and MSDOS.SYS) and the IBM system file
names (IBMBIO.COM and IBMDOS.COM) as system files. So,
when using the SYS command to transfer MS-DOS to a system
that booted with PC-DOS, IBMBIO.COM and IBMDOS.COM will
The first three sectors of IO.SYS do not need to be in
the first three sectors of the data area; however, they
must be contiguous.
SYS copies COMMAND.COM to the destination disk.
An optional second parameter to specify the source of the
system files is supported when running in MS-DOS 5.0.
Again, running the SYS command without booting MS-DOS 5.0
is not recommended. If you cannot boot MS-DOS 5.0, use
the SYS command on a disk in drive A, and use that disk
to boot MS-DOS 5.0.
6.0 SYS copies DBLSPACE.BIN to the destination disk, as well
as copying IO.SYS, MSDOS.SYS, and COMMAND.COM.