DOS is made up of three parts: IO.SYS, MSDOS.SYS, and COMMAND.COM. IO.SYS works with ROM BIOS to control the computer’s input/output functions; MSDOS.SYS(sometimes called the kernel) manages files, runs programs, and performs basic system functions; COMMAND. COM performs all the DOS commands. In addition,
COMMAND.COM functions as the overall “manager” for the computer. During the boot process, the three parts of the operating system are loaded into memory one at a time.
After IO.SYS is loaded, it checks to see if all the system components are responding properly. MSDOS.SYS is loaded next so it can perform the commands in the CONFIG.SYS (a special configuration file).
COMMAND.COM is loaded last and is kept in memory so that DOS commands (such as COPY) can be executed when needed. COMMAND.COM then executes the commands in AUTOEXEC.BAT, which completes the boot process.
