SYS.COM Requirements in MS-DOS versions 2.0-6.0 (66530)

The information in this article applies to:
    Microsoft MS-DOS operating system 2.11
    Microsoft MS-DOS operating system 3.1
    Microsoft MS-DOS operating system 3.2
    Microsoft MS-DOS operating system 3.21
    Microsoft MS-DOS operating system 3.3
    Microsoft MS-DOS operating system 3.3a
    Microsoft MS-DOS operating system 4.0
    Microsoft MS-DOS operating system 4.01
    Microsoft MS-DOS operating system 5.0
    Microsoft MS-DOS operating system 5.0a
    Microsoft MS-DOS operating system 6.0
    Microsoft MS-DOS operating system 6.2
    Microsoft MS-DOS operating system 6.21
    Microsoft MS-DOS operating system 6.22

This article was previously published under Q66530


The following is a description of the requirements needed to use the MS-DOS SYS command (SYS.COM) to transfer the MS-DOS system files. Changes that have been made to the SYS command since its introduction in MS-DOS version 2.0 are described below.


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:
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 functionality:

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:
   sys c:
   copy 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
             be deleted.

             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.

Modification Type: Major Last Reviewed: 5/12/2003
Keywords: KB66530