Loading the OS If no problems are detected in the disk’s DOS volume boot sector, IO.SYS (or IBM-BIO. COM) is loaded and executed. If Windows 95 is on the system, IO.SYS might be renamed WINBOOT.SYS, which will be executed instead. IO.SYS contains extensions to BIOS that start low-level device drivers for such things as the keyboard, printer, and block devices. Remember that IO.SYS also contains initialization code that is only needed during system startup. A copy of this initialization code is placed at the top of conventional memory which takes over initialization. The next step is to load MSDOS.SYS (or IBM-DOS. COM, which is loaded so that it overlaps the part of IO.SYS containing the initialization code. MSDOS.SYS (the MS-DOS kernel) is then executed to initialize base device drivers, detect system status, reset the disk system, initialize devices (such as the printer and serial
port, and setup system-default parameters. The MS-DOS essentials are now loaded and control returns to the IO.SYS/WINBOOT.SYS initialization code in memory. Establishing the Environment If a CONFIG.SYS file is present, it is opened and read by IO.SYS/WINBOOT.SYS. The DEVICE statements are processed first in the order they appear, then INSTALL statements are processed in the order they appear. A SHELL statement is handled next. If no SHELL statement is present, the COMMAND.COM processor is loaded. When COMMAND. COM is loaded, it overwrites the initialization code leftover from IO.SYS (which is now no longer needed. Under Windows 95, COMMAND.COM is loaded only if an AUTOEXEC.BAT file is present to process the AUTOEXEC.BAT statements. Finally, all other statements in CONFIG.SYS are processed, and WINBOOT.SYS also looks for the SYSTEM.DAT registry file. When an AUTOEXEC.BAT file is present, COMMAND.COM (which now has control of the system) will load and execute the batch file. After the batch-file processing is complete, the familiar DOS prompt will appear. If there is no AUTOEXEC.BAT in the root directory, COMMAND.COM will request the current date and time, then show the DOS For Windows 95 systems, IO.SYS (or WINBOOT.SYS) combines the functions of IO.SYS and MSDOS.SYS.