The content of this article includes the working process of your computer in brief to give you a basic knowledge about the working process of computer.
What Happens When You Press Your Computer’s Power Button?
When you press the power button on your computer, it sends a signal to the Power Supply Unit (PSU). PSU converts the alternating current (AC) into a direct current (DC) and supply the proper amount of voltage and electricity to the computer and its components.
When the computer and its components receive ample power and the PSU reports no errors it sends a signal (using transistors) to the motherboard and the Computer Processor Unit (CPU). While this is happening, the processor will clear any leftover data in the memory registers and give the CPU program counter a F000 hexadecimal number. This hexadecimal number is the location of the first instruction and tells the CPU that it’s ready to process the instructions which are contained in the basic input/output system (BIOS).
Basic Input/output System (BIOS) and The Power-on Self-Test (POST)
To make sure the components in the computer are present and functioning properly the computer looks at the BIOS at first and it begins the power-on self-test (POST) sequence.
If the computer cannot complete any of these tests or task it will encounter an irregular POST.
Do You Know What Does Irregular POST Mean?
An irregular POST is a beep code which is different from the standard one or two beeps.
For instance, an irregular POST could create a combination of different beeps or no beeps at all to indicate the cause of the failure.
But if the computer can pass the initial POST successfully, it will next look at the first 64-bytes of memory which is located in the Complementary Metal Oxide Semiconductor (CMOS) chip. CMOS is kept alive even when the computer is turned off since CMOS is powered by a replaceable CMOS battery on the motherboard. CMOS contains information (such as, system clock) about all the hardware installed in your computer.
After loading CMOS information, POST will start inspecting and comparing the system settings with what is installed in the computer. If no errors are found while loading CMOS information, then it will start loading the basic device drivers and interrupt handlers for hardware such as the hard drive, floppy drive, keyboard, mouse. These basic drivers permit the CPU to communicate with these hardware devices and permit the computer to continue its booting process.
Then, the POST will check the Real-Time Clock (RTC) and the computer system bus to check both of these are properly working on the computer or not. After the POST has loaded the memory contained on the display adapter, you’ll see a picture on your display.
Next, the BIOS will make sure if it’s currently performing a cold boot or a warm boot (reboot) by looking at the memory address 0000:0472. If the BIOS finds 1234h the BIOS knows that this is a reboot and will skip the remainder of the POST steps.
If 1234h is not found, the BIOS will indicate it as a cold boot and will continue running additional POST steps. Next, it tests Random Access Memory (RAM) which is installed in the computer by writing to each chip.
Finally, the POST will send signals to the computer after that floppy, optical and hard drive to test these drives. If all drives pass the tests successfully, the POST will be completed and then it will instruct the computer to begin the process of loading the operating system.
Booting the Operating System
After completing the POST process, the computer begins booting operation. This process includes what loads the operating system and all of the associated files. Microsoft Windows is the most favorite and commonly used operating system. This section covers the process of loading Microsoft Windows.
The BIOS first hands manage the bootstrap loader. Bootstrap loader looks at the boot sector of the hard drive. Now if your boot sequence in CMOS setup is not set up to look at the hard drive first, and it may look at the boot sector on any inserted floppy disk drive or look optical disc first, before doing this.
In this example, the Microsoft Windows XP, NT Loader (NTLDR) is found on the boot sector and tells the computer where to find the remaining code on the hard drive. After that, Windows loads the ntdetect.com file. This ntdetect.com file displays the Windows splash screen and loads the Windows registry. After loading the registry, Windows starts to load the low-level programs which make up the operating system into memory. So, many of the initially loaded programs are what permit Windows to communicate with the necessary hardware and other programs running on the computer.
After the registry has finished loading the initial basic hardware devices, it starts to load Plug and Play devices, PCI, and ISA devices. After completing loads of all these devices, Windows then begins to load full support of the hard drive, partitions, any other disk drives and then moves to all other drivers that have been installed.
After completing the above steps successfully another additional required services are loaded and Windows starts.
Communication of Hardware Devices with the Computer
After loading the operating system successfully, hardware of the computer must be able to communicate with the CPU. Hardware communication is completed by using an interrupt request (IRQ).
Each time a hardware device requires the attention of the computer, the interrupt controller sends the request (INTR) to the CPU. So it stops temporarily what it is doing to process the request of the hardware device. Anything that was being done by the CPU is currently put on hold and stored as a memory address in the memory heap and is returned to after the interrupt request is processed successfully.