By default the shell is COMMAND.COM on the boot disk, but this can be overridden by the SHELL directive in CONFIG.SYS. These are loaded at boot-up they read CONFIG.SYS for configuration, and once all the device drivers have loaded and initialised, they load the shell and start running it. In DOS, the “kernel” is provided by two files, typically IO.SYS and MSDOS.SYS (or IBMBIO.COM and IBMDOS.COM). They’re “standard” programs which handle whatever the input and output system is, and provide their features with no extra special sauce. Most shells have two modes of operation, interactive mode where they manage the command line and execute commands entered interactively, and batch mode where they run scripts. Traditionally, operating shells are relatively independent of the operating system’s function and the operating system can operate without a shell.
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