Article ID: 66530 - View products that this article applies to.
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:
IO.SYS and MSDOS.SYS
A disk can only be made bootable in MS-DOS by using the FORMAT /S, DISKCOPY, or SYS command.
IBMBIO.COM and IBMDOS.COM
SYS Original FunctionalityThe 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 command.com c:\
Changes to SYSChanges 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 action. 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.
Article ID: 66530 - Last Review: May 12, 2003 - Revision: 2.0