IO.SYS, MSDOS.SYS, CONFIG.SYS, COMMAND.COM, AUTOEXEC.BAT
“When the DOS system starts, it first reads the two system files IO.SYS and MSDOS.SYS from the boot disk, then looks for and executes the three files CONFIG.SYS, COMMAND.COM, and AUTOEXEC.BAT in the root directory of the boot disk. Among them, IO.SYS, MSDOS.SYS, and COMMAND.COM are all indispensable; if any one of them is missing, the computer will not boot normally.” Quoted from Hongen Online
“When the DOS system starts, it first reads the two system files IO.SYS and MSDOS.SYS from the boot disk, then looks for and executes the three files CONFIG.SYS, COMMAND.COM, and AUTOEXEC.BAT in the root directory of the boot disk. Among them, IO.SYS, MSDOS.SYS, and COMMAND.COM are all indispensable; if any one of them is missing, the computer will not boot normally.” Quoted from Hongen Online



