Answer to Question #309092 in C++ for Mwanza

Question

Explain the start-up process of a computer clearly indicating the purpose of the POST, BIOS, and CMOS

Answer 1

The process of starting a computer can be divided into several stages:

1. Turn on the computer, POST, BootMonitor

The initial stage of loading the operating system after turning on the computer begins in the BIOS (Basic Input / Output System). In the BIOS settings, we specify a boot device, or a set of boot devices in order of priority. Various boot options and their combinations are possible: from a hard disk, CD / DVD - disk, USB-flash and others, and first of all, the BIOS program checks the subsystems, this procedure is called POST - Power On Self Test. The test can be abbreviated or full, this is set in the BIOS settings.

The abbreviated test includes:

1) Checking the integrity of the BIOS programs in the ROM using the checksum.

2) Detection and initialization of the main controllers, system buses and connected devices (graphic adapter, drive controllers, etc.), as well as the execution of programs included in the BIOS of devices and providing their self-initialization.

3) Determining the size of RAM and testing the first segment (64 kilobytes).

Full POST rules:

1) Checking all processor registers;

2) Verification of the checksum of the ROM;

3) Checking the system timer and audio alarm port (for IBM PC - IC i8253 or equivalent);

4) DMA controller test;

5) RAM regenerator test;

6) Test of the lower area of ​​RAM for projecting resident programs in the BIOS;

7) Loading of resident programs;

8) Standard graphics adapter test (VGA or PCI-E);

9) RAM test;

10) Test of the main input devices (NOT manipulators);

11) CMOS test

12) Test of main LPT/COM ports;

13) Test of floppy disk drives (FDD);

14) Test of hard disk drives (HDD);

15) Self-diagnostics of BIOS functional subsystems;

16) Transfer control to the bootloader.

Immediately after passing the POST (Power-On Self-Test), the computer's BIOS will begin to enumerate the specified boot devices one by one until it finds a suitable special entry on one of them, which contains information about further actions.

2. Loader 1st level. Master Boot Record

Master Boot Record - The master boot record is located in the first physical sectors of bootable storage devices. It contains a Partition Table and executable code.

The main task of the program written in the MBR is to find the active system partition of the disk and transfer control to its boot sector. Thus, this stage can be called preparatory, due to the fact that the actual loading of the OS itself does not yet occur.

It is customary to call the system disk partition (storage device) on which the operating system files responsible for the OS boot process are located (the operating system itself can be located in another partition). In principle, there can be several system partitions, so one of them is marked as active. That is what the program loaded from the MBR is looking for.

3. Loader 2nd level. Partition Boot Sector

The next step in booting the computer is to transfer control to the executable code written in PBS. The PBS is located in the first sector(s) of the corresponding disk partition. The PBS code contains the file name of the operating system loader, to which control is transferred at this stage.

4. The initial stage of loading the operating system. OS boot manager

The original Linux bootloader was LILO (Linux Loader). Due to its shortcomings, the main of which was the inability to understand the file systems used in Linux, the GRUB (GRand Unified Bootloader) bootloader was later used, in which the shortcomings of LILO were corrected.

For pre-Vista versions of Windows, such as Windows XP, Ntldr will be loaded. It, in turn, reads information from the Boot.ini text file, which contains information about installed operating systems.

5. Loading the operating system kernel

The final stage of loading the operating system is loading the OS kernel and transferring control to it.