Common System Files

There are about a dozen system files you must be familiar with. These include system files used by the operating system to operate, utility files that manage the operating system, and configuration files that explain how the operating system looks at each piece of hardware and software. Although this list is quite extensive, it can never be complete. When you come across a file you don't understand, ask about it rather than assuming you won't use it.

File Type	Description
.ini	Files that contain information about hardware and software settings. It is used for initialization and configuration of the operating system.
.sys	System files. These are the files that contain the code that makes the operating system work.
.vxd	Virtual Device Drivers. These are the driver files that make your hardware and software work with your operating system.
.exe	Executable Files. These files are the actual programs written in machine code that run applications and utilities inside the operating system.
.com	Command Files. These files contain a small set of commands that run programs. The command set is smaller than an .exe file set.
.swp or .par	Memory Swap Files. These files are used to create virtual memory space on the hard drive.
.bat	Batch Files. These are files contain a list of commands to be run one after the other. Batch files allow you to run a series of commands and executables without having to call each one individually.

File Name	Description
io.sys	This file tells the computer information about the start-up of the computer. It is the first file loaded by the computer, and in Window 95 and higher replaces the need for msdos.sys, config.sys, and autoexec.bat.
msdos.sys	This is the DOS operating system file itself. Under Windows 95 and higher, msdos.sys becomes a text file. By editing the "MULTIBOOT" option in msdos.sys, you can boot multiple operating systems.
config.sys	This is a configuration system that, under DOS, told the operating system what hardware was in the computer. Under Windows 95 and up, the config.sys file is only used to run older Windows 3.1 and DOS 6.0 programs.
command.com	This is the file that contains all the DOS commands, as well as creates the DOS prompt. (c:>)
autoexec.bat	Under Windows 3.1 and DOS 6.0, this file loaded the mouse drivers, initialized the serial ports, and ran the memory manager programs. It also was user-editable to allow the user to add files it wanted run on start-up automatically. Like config.sys, Windows 95 and up only keep this file for compatibility, and to allow programs like virus scanners to be run before the main operating system loads.
himem.sys	The High Memory Manager. The file allows DOS and Windows to use Extended Memory.
system.ini	System Initialization File. This file contains all the information on the location and initialization of hardware device drivers.
win.ini	An initiation file that tells the operating system how the desktop should look.
kernel.dll	Contains the main operating systems commands, including memory management, I/O management, and application support.
user32.dll/user.exe	Contains the information about the user information, including mouse, I/O, keyboard, and desktop layout.
gdi32.dll/gdi.exe	Creates the Graphical User Interface (GUI) and controls printing. This WILL be on the test.

In Windows 3.1, the files that created support for hardware and applications were spread among dozens of .ini files. Windows 95 and up bring all these .ini files into one place, referred to as the Registry. The Registry is a database with a complete profile of the applications, hardware, software, and drivers and the files required to run them.

The Registry is composed of 2 files; user.dat and system.dat. The database itself is composed of six major branches, referred to as keys. Each key contains a number of subkeys that are structured in a hierarchical fashion. Each key and subkey is a number of values, each of which has a corresponding value data. Every back-up registry file has the extenstion .DA0.

The 6 Branches Of The Registry
HKEY_CLASSES_ROOT	Contains all file associations and Object Linking and Embedding (OLE) Data.
HKEY_USERS	Contains information about all users registered for that computer. It stores their desktop settings and network connection information.
HKEY_CURRENT_USER	If there are more than a single user on the computer, this contains information about the current user logged into the system. If there is only one registered user, this key is an exact duplicate of HKEY_USERS.
HKEY_LOCAL_MACHINE	Contains the hardware and software information for the computer.
HKEY_CURRENT_CONFIG	Contains the information stored in HKEY_LOCAL_MACHINE and any changes in the hardware and software information from the last reboot. When the computer is rebooted, the HKEY_CURRENT_CONFIG replaces HKEY_LOCAL_MACHINE. This file also contains information about fonts and printers installed on the computer.
HKEY_DYN_DATA	This key stores information about the Plug-and-Play devices and records system performance information.

You'll need to remember that the registry is stored in a hierarchical fashion and is stored as keys. You may see a question that asks what application is used to edit the registry, which is REGEDIT.EXE. Beyond that, don't worry too much about the registry, beyond that editing it is very difficult and can really really REALLY screw up your machine.

As usual, you should also make copies of your Registry on a regular basis. This avoids the dreaded Registry Corruption errors that happen over time in an operating system. By changing USER.DA0 to USER.DAT and SYSTEM.DA0 to SYSTEM.DAT, you can recover a damaged Registry from it's backup files.