Boot Process for Operating Systems DOS BOOT PROCESS 1. Upon powering powering on on the machine machine a POST POST (Power (Power on Self Test) is is executed, executed, this this does a quick test on things such as the memory and power supply of a machine. POST transfers control to CMOS (Complementary Metal Oxide Semiconductor) which contains the BIOS (Basic Input Output System). The BIOS loads various low level settings such as boot-order and network n etwork settings. The BIOS is also a very useful tool for testing memory or hard drives, as well as finding out basic information such as serial numbers and asset tags. Note if POST fails it is usually identified by various light or audible beep codes which often can be searched for by model on the manufacturers' website. 2. The Master Master Boot Record (MBR) (MBR) is is then read read which which is a set set of tasks tasks to to load the the operating system. It will check the floppy, CD-ROM, or hard drive (Not necessarily in that order). The MBR points to the boot sector, which locates IO.SYS and loads the file into RAM. 3. IO.SYS IO.SYS is a file file that loads loads low level level drivers drivers for hardware hardware devices devices like the keyboard, serial and parallel ports, floppy drive, and hard drive. Next, IO.SYS will run Sysinit which in turn loads Msdos.sys into RAM. 4. MSDOS.SYS MSDOS.SYS in turn turn helps manage the the input and output output for the hard disk. It It processes the commands in Config.sys. Config.sys loads all the other device drivers and manages memory for optimization. 5. Command.com Command.com is is loaded loaded into RAM. RAM. Command.co Command.com m is a file file that that stores stores all all your internal commands. 6. Command.com Command.com then then processes processes Autoexec.bat Autoexec.bat which which will will set the initial initial configuration of DOS. The user can add a dd what programs to start each time the computer boots. 7. The last last step step will will get you you the the command command promp prompt. t.
WIN95/98/ME BOOT PROCESS (Similar to DOS) 1. POST POST is is perf perfor orme med. d. 2. Master Master Boot Record Record (MBR) (MBR) is is loaded loaded into RAM RAM and locates locates the boot sector. sector. The boot sector then locates the Io.sys. 3. The Io.sys Io.sys runs Sysin Sysinit it and loads Msdos.sys Msdos.sys into RAM. RAM. 4. Io.sys Io.sys then loads loads System. System.dat, dat, but does not process process at this this time. time. 5. Io.sys Io.sys then loads loads Config.sys Config.sys and and Autoexec.bat Autoexec.bat.. These are not not necessary necessary to load load the operating system, but are needed when 16 bit (real mode) drivers are used. 6. Io.sys Io.sys then loads loads Win.com Win.com which which starts starts the the operating operating system. system. 7. Win.com Win.com loads loads drivers drivers specified specified by the the regist registry. ry. 8. Win.com Win.com processes processes the the System.in System.inii and Win.ini Win.ini files. files. System.i System.ini ni is used to to configure the memory cache and buffers. Win.ini is used for 16-bit applications. Otherwise, they are run by the registry.
9. Win.com Win.com will will start start KERNEL32. KERNEL32.DLL, DLL, KERNEL3 KERNEL386.EXE, 86.EXE, GDI.EXE, GDI.EXE, and GDI32.EXE. The Kernel files are the operating systems. The GDI files are responsible for the graphical functions and display. 10. The Kernel files will will load the Explorer user interface, interface, which includes the Desktop, Taskbar, and Start Menu. 11. The Kernel will then process process anything in the Startup Startup folder and restore network connections. 12. The user can now interact with the the operating system. system.
WIN2000/NT/XP BOOT PROCESS 1. 2. 3. 4. 5. 6.
BIOS BIOS runs uns POS POST T. Computer Computer finds finds the boot boot device device and loads loads Master Master Boot Boot Record Record (MBR) into RAM. RAM. MBR looks looks for the active active partiti partition on on the boot boot device device and loads loads the boot boot sector. sector. The boot sector sector in turn turn starts starts the Ntldr Ntldr file file which will will load the the operating operating system. system. Ntldr configu configures res the compute computer’s r’s processo processorr to recognize recognize all of the memory. memory. Ntldr will will start start the file file system, system, and the the operating operating system system will will load, load, whether it it is FAT or NTFS. 7. Ntldr checks checks the the Boot.ini Boot.ini file to to find the the operating operating system( system(s) s) and to create create the boot boot menu. 8. After the the operating operating system system is select selected, ed, Ntldr will run run Ntdetect.co Ntdetect.com m which will will make the hardware list. 9. Ntldr will will load load Ntoskrnl.ex Ntoskrnl.exee which is the the kernel of of the operating operating syste system. m. 10. The operating system system will start start the graphical interface. 11. Ntoskrnl.exe loads Smss.exe Smss.exe which is the Session Manager. 12. The user user logs logs on. on.
WINDOWS VISTA BOOT PROCESS 1. 2. 3. 4.
BIOS BIOS run POST. OST. Computer Computer finds finds the boot boot device device and loads loads Master Master Boot Boot Record Record (MBR) into RAM. RAM. MBR looks looks for the active active partiti partition on on the boot boot device device and loads loads the boot boot sector. sector. The boot boot sect sector or in in turn turn starts starts the bootmgr file which will load the operating system.
