Architecture of Windows 9x

To start up or boot, Windows 95 and other Windows 9x operating systems require the following set of files:[1] 32-bit shell and command line interpreter: Windows 95 core: Registry and other configuration files: Virtual Machine Manager and configuration manager: Installable file System Manager: Device drivers: The system may also use CONFIG.SYS, which contains settings and commands executed before loading the command interpreter and AUTOEXEC.BAT, which is a batch file automatically executed after loading COMMAND.COM.

However, these two files are not critical to the boot process, as IO.SYS contains a default setting for both, in case of absence from the system.

The ROM BIOS starts the execution at the physical memory address 000FFFF0h.

During this phase, BIOS first executes the Power-on self-test, then checks for the existence of a boot disk on drive A.

If it is not found in drive A, the ROM BIOS checks for a hard disk.

The boot sector contains the disk boot program and BIOS Parameter Block table which searches for the location of the root directory and IO.SYS file, which then loads the IO.SYS file into memory.

IO.SYS initialises the minimal File Allocation Table driver and loads MSDOS.SYS into memory.

It then displays "Starting Windows" depending on the Boot-Delay line in the MSDOS.SYS file.

Windows ME ignores this step, as Real Mode DOS support is disabled and TSRs being loaded can compromise system stability.

After initialisation of the display driver, Windows switches to graphical mode.