SYS is an external command of DOS and MicrosoftWindows 9x operating systems that is used to make an already formatted medium bootable. It will therefore install a boot sector capable of booting the operating system into the first logical sector of the volume. Further, it will copy the principal DOS system files, that is, the DOS-BIOS (IO.SYS or IBMBIO.COM) and the DOS kernel (MSDOS.SYS or IBMDOS.COM) into the root directory of the target. Due to restrictions in the implementation of the boot loaders in the boot sector and DOS' IO system, these two files must reside in the first two directory entries and be stored at the beginning of the data area under MS-DOS and PC DOS. Depending on version, the whole files or only a varying number of sectors of the DOS-BIOS (down to only three sectors in modern issues of DOS) will have to be stored in one consecutive part. SYS will try to physically rearrange other files on the medium in order to make room for these files in their required locations. This is why SYS needs to bypass the filesystem driver in the running operating system. Other DOS issues such as DR-DOS do not have any such restrictions imposed by the design of the boot loaders, therefore under these systems, SYS will install a DR-DOS boot sector, which is capable of mounting the filesystem, and can then simply copy the two system files into the root directory of the target. SYS will also copy the command lineshell (COMMAND.COM) into the root directory. The command can be applied to hard drives and floppy disks to repair or create a boot sector.
Although an article on Microsoft's website says the SYS command was introduced in MS-DOS version 2.0, this is incorrect. SYS actually existed in 86-DOS 0.3 already. According to The MS-DOS Encyclopedia, the command was licensed to IBM as part of the first version of MS-DOS, and as such it was part of MS-DOS/PC DOS from the very beginning (IBM PC DOS 1.0 and MS-DOS 1.25).