DOS commands are divided into internal commands and external commands.
Internal commands are commands hidden inside command.com that can be interpreted and executed by command.com.
No other files (programs) (software) are needed for support.
External commands are actually other programs (files) (software) that are interpreted and executed through command.com.
However, these are standardized software provided by the DOS system, and the software names are conventional. And generally, only one command has one file. The naming of other programs (files) (software) is not specified by DOS, but is named by the software author. It is used as an enhanced software. Sometimes it may be composed of multiple files. Generally, it cannot be called an external command. But its essence is the same. There must be specific executable files, and these files must all be interpreted and executed through command.com.
I don't know if this explanation is clear enough.
Windows has not supported the use of sys since Windows Me.
(Windows Me can only transfer system files from a floppy disk to the C drive, that is, only the usage of a:\sys c:.)
deltree.exe should be said to be an external command. (At least in MSDOS 6.22, it is an external command)
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