The architecture of Windows 9x

Windows 9x

Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced since 1995, which were based on the original and later modified Windows 95 kernel...

 series OS

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

 kernel

Kernel (computing)

In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources...

 is monolithic

Monolithic kernel

A monolithic kernel is an operating system architecture where the entire operating system is working in the kernel space and alone as supervisor mode...

. The basic code is considered similar in function to MS-DOS

MS-DOS

MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...

 - as a 16/32 bit hybrid, it requires MS-DOS support to operate.

Critical files

• IO.SYS
  IO.SYS
  IO.SYS is an essential part of MS-DOS and Windows 9x. It contains the default MS-DOS device drivers and the DOS initialization program.- Boot sequence :...
  
   - executable handling all of the basic functions, such as I/O
  I/O
  I/O may refer to:* Input/output, a system of communication for information processing systems* Input-output model, an economic model of flow prediction between sectors...
  
   routines.
• MSDOS.SYS
  MSDOS.SYS
  MSDOS.SYS an important system file on MS-DOS and Windows 9x systems. It is run after IO.SYS. In MS-DOS, it contains the core operating system code, the kernel...
  
   - contains some low-level boot
  Booting
  In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
  
   setting such as disabling double-buffering or the GUI
  Gui
  Gui or guee is a generic term to refer to grilled dishes in Korean cuisine. These most commonly have meat or fish as their primary ingredient, but may in some cases also comprise grilled vegetables or other vegetarian ingredients. The term derives from the verb, "gupda" in Korean, which literally...
  
   logo.
• COMMAND.COM
  COMMAND.COM
  COMMAND.COM is the filename of the default operating system shell for DOS operating systems and the default command line interpreter on Windows 95, Windows 98 and Windows Me...
  
   - command line shell
  Shell (computing)
  A shell is a piece of software that provides an interface for users of an operating system which provides access to the services of a kernel. However, the term is also applied very loosely to applications and may include any software that is "built around" a particular component, such as web...
  
   executable.
• WIN.COM
  WIN.COM
  WIN.COM is the executable file used to load versions of Windows that run from DOS. In Windows 3.1 and its predecessors, it is executed either manually from the DOS prompt or as a line in AUTOEXEC.BAT. In Windows 95 and onward it is automatically invoked by IO.SYS after AUTOEXEC.BAT is processed...
  
   - responsible for loading the GUI and the operating system itself.
• LOGO.SYS
  Logo.sys
  Logo.sys is a system file used by DOS-based Windows versions such as Windows 95, Windows 98 and Windows Me to display its boot-up message.-Logo.sys And Other Screens Displayed:* C:\Logo.sys: the "Starting Windows" message, with the Windows logo...
  
   - contains RGB encoded bitmap
  Bitmap
  In computer graphics, a bitmap or pixmap is a type of memory organization or image file format used to store digital images. The term bitmap comes from the computer programming terminology, meaning just a map of bits, a spatially mapped array of bits. Now, along with pixmap, it commonly refers to...
  
   displayed during the boot sequence

The system may also use CONFIG.SYS

CONFIG.SYS

CONFIG.SYS is the primary configuration file for the DOS, OS/2 as well as similar operating systems. It is a special file that contains setup or configuration instructions for the computer system.- Usage :...

 (which contains settings and commands executed before loading the command interpreter) and AUTOEXEC.BAT

AUTOEXEC.BAT

AUTOEXEC.BAT is a system file found originally on DOS-type operating systems. It is a plain-text batch file that is located in the root directory of the boot device...

 (which is a batch file

Batch file

In DOS, OS/2, and Microsoft Windows, batch file is the name given to a type of script file, a text file containing a series of commands to be executed by the command interpreter....

 automatically executed after loading COMMAND.COM), but these two files aren't critical to the boot process, as Windows 9x IO.SYS contains a default setting for both, in case they're absent in the system.

Boot sequence

The boot sequence of Windows 9x relies heavily on MS-DOS. The order of the boot sequence is:

1. IO.SYS loads, using boot parameters from MSDOS.SYS,

2. CONFIG.SYS commands get parsed by IO.SYS,

3. COMMAND.COM loads, allowing processing of AUTOEXEC.BAT (if it's present in the system),

4. WIN.COM loads the GUI (if the GUI is not disabled manually by boot menu or MSDOS.SYS setting).

Kernel

The Windows 9x kernel is a MS-DOS kernel with additional support for security in memory and virtual memory

Virtual memory

In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...

. The kernel has very basic support for device interfacing. Everything that is not under the DOS 16-bit kernel is run in User space

User space

A conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...

.