The MS-DOS Boot Sequence
MS-DOS requires three files to boot to a hard disk drive or floppy disk and to load the operating system into memory. These files are:
io.sys, which is the interface between the hardware and the operating system code. This is the lowest interface layer, acting as an intermediary between the various hardware components and the software environment.
msdos.sys, which is the main operating system code. This is the middle layer and only provides both key logic and basic commands for opening and closing files, navigating the file system, and other common tasks performed by the software
command.com, which is the interface between the user and the operating system code. This is the highest interface layer and provides support for the operating system's direct interaction with the user, accepting commands and performing the necessary steps for execution.
These three files would make the operating system operational, however, the computers has an open architecture. This means that no two computers have to be exactly alike as different computer manufacturers can include different hardware components, while the user can upgrade the computer and its components. The operating system must be able to support these variations in hardware levels. The operating system requires custom device drivers and memory configurations to achieve this.
MS-DOS used two startup files for this purpose. These were:
config.sys, which loads the hardware and device drivers that are not built into io.sys.
autoexec.bat, which loads TSR programs specified by the end user and sets up environment variables such as the TEMP directory and PATH locations.
The Windows operating system uses a similar boot process and some versions can also use the autoexec.bat and config.sys files.
When a DOS based computer is booted, the power on self test (POST) is performed. The POST routines include instructions that locate and invoke the operating system by checking for the io.sys and msdos.sys. The operating system processes the config.sys file, which contains information to configure the system environment, including special memory management overlays and hardware drivers. Command.com is then loaded. Thereafter the operating system processes the autoexec.bat file, which loads programs and userdefined settings. If no programs, such as Windows, are invoked by autoexec.bat, command.com presents prompt and waits for a command.