How the Computer Boots Up

Booting Up in DOS

1. Power comes on.

This is pretty easy, just flip the switch!

2. POST Program Runs.

POST stands for Power On Self Test. It is located in the ROM-BIOS (Read Only Memory - Basic Input-Output System) chip and performs a series of checks on the system:

a) Checks itself
The POST Program checks itself to make sure that it is functioning properly.
b) Checks bus lines and video ROM
Bus lines are the pathways that carry data from one component of the computer to another. The POST Program sends signals along the bus lines to various hardware components to make sure they're working. Some of these components, like a video card, also have they're own ROM-BIOS, which is loaded into memory with the original ROM-BIOS.
c) Primary Memory is tested
Primary memory, also called main memory or RAM (Random Access Memory), is checked and a running total is displayed on the monitor.
d) Keyboard connection is tested
The POST Program checks to see if the keyboard is properly connected. If it isn't, a message asking you to check the connection and press the F1 key is displayed on the screen.
e) Diskette Drives and Hard Drives Tested
Any diskette drives and hard drives connected to your computer are tested to ensure that they are connected and working properly.
f) Results of POST compared to CMOS
The results of the POST are compared with the permanent record in the CMOS.
 

CMOS stands for Complementary Metal-Oxide Semi-Conductor and refers to an area of the system that stores information about how the hardware on your computer works.

3. Operating System Loads.

This is when the actual operating system is loaded onto the system. In order for this to happen, 3 files are needed. They are IO.SYS, MSDOS.SYS and COMMAND.COM. IO.SYS and MSDOS.SYS are hidden system files so they don't show up in a directory listing. They are like a contractor and a foreman on a construction site - they determine what tasks need to be done on the system and then get those tasks done using the system's resources. COMMAND.COM is where the internal DOS commands are located.

Another piece of code in the ROM BIOS looks for the boot sector where the boot program is located, and for the 2 system files. First, it will look on Drive A: If they aren't found there, it will look on Drive C: If you have accidentally placed a disk in Drive A: that isn't a boot disk, you will get a "Non-System Disk" Error and must remove the disk from the drive, or replace it with a bootable disk. When the boot program and files have been found, the boot program loads IO.SYS into memory. This file contains a subroutine called SYSINIT that takes care of the rest of the process. SYSINIT loads MSDOS.SYS into memory. The commands in CONFIG.SYS, if it exists, are executed. COMMAND.COM is found and loaded into memory. AUTOEXEC.BAT is found and, if it exists, its commands are executed.

Booting Up in Windows95

 The boot process for Windows95 is slightly different than for DOS:

1. Power On and POST.

When you first boot the machine, the POST occurs just as it does in a DOS-based environment.

2. BIOS Allocates Resources to Devices

Windows95 uses a different BIOS, called a Plug-N-Play BIOS. This BIOS will automatically allocate the necessary resources to devices (like the CD-ROM drive, keyboard and sound card) so that they will work properly with the rest of the system.

3. Operating System Loads.

The BIOS searches for the boot sector, just as it does in DOS. When it is found, IO.SYS is loaded. SYSINIT locates and executes the commands in CONFIG.SYS, which performs the same function as with DOS even though with Plug-N-Play in use it is not really necessary. CONFIG.SYS is optional with Windows95.

Next, MSDOS.SYS is located. In Windows95, MSDOS.SYS is simply a plain text file that a few instructions that configures Windows95 (such as locations of the Windows95 directory, the Windows95 startup files, and the type of startup desired).

Just as with DOS, COMMAND.COM is loaded and AUTOEXEC.BAT is executed, if it exists.

4. Windows95 Desktop Appears.

With the boot process complete, the Windows95 desktop appears on the screen, ready to be used.

The Autoexec.Bat and Config.Sys Files

Config.Sys The Config.Sys file is a configuration file located in the root directory. It contains special commands to customize how DOS works with certain parts of your computer, such as:

 FILES   This determines the number of files your system can have open at any one time.
Example:  FILES=25
BUFFERS   This determines the number of buffers to create. Buffers are temporary storage areas in memory that hold data while it waits to go to a device like a hard drive or a printer.
Example:  BUFFERS=40
DEVICE   Loads a specific device driver into memory. A device driver is a special software program that allows the operating system to work with a device attached to your system, such as a mouse or CD-ROM drive. Some device drivers also help to manage your computer's memory.
Example:  DEVICE=C:\WINDOWS\HIMEM.SYS

Autoexec.Bat The Autoexec.Bat file is a batch file located in the root directory that is executed during system boot-up. Most commands make certain changes to your DOS environment but you can also use your autoexec.bat to run programs automatically when you boot up (most systems with Windows 3.x do this). If you have a DOS-based system and don't have an autoexec.bat file, your computer will simply run the TIME and DATE commands when it boots up.

Well, that should get you started with basic operating systems. If you are worried about understanding all this information, don't worry! Many of these concepts will be touched upon again in future sections.

Lab Exercises

The following projects from your textbook should be completed in the lab:

Chapter 1: