The autoexec.bat and config.sys files are not required, but are retained for backward compatibility with older 16 bit programs.
After ROM BIOS runs the POST, the bootstrap loader looks to the boot sector for the primary active partition, for the instructions to identify which operating system will be loaded, if there are more than two operating systems.
Io.sys is located and loaded, and the PC starts in real mode, which sets segment addressing for conventional memory, and the real mode drivers are loaded.
Io.sys then locates ms.dos which is a read only plain ASCII file, which is also a hidden system file.
