What is POST in the computer system

BIOS - Basic Input / Output System


The history of the BIOS begins with the first IBM PC, when IBM and Microsoft developed hardware and software (operating system) independently of one another. The question arose as to how the operating system can access the hardware. Therefore an interface between hardware and operating system was designed. The BIOS is this interface. The operating system uses functions in the BIOS to address the connected hardware.
The BIOS is stored on an EEPROM or flash memory chip.

Today the BIOS no longer has the same meaning as it did in the early days of the PC. Modern operating systems only use a few BIOS functions. They address the PC hardware with their own drivers. However, that does not mean that the BIOS is dispensable.

How the BIOS works

The BIOS starts the hardware components after the computer is switched on, i.e. when the system is started. The process is called power-on-self-test (POST). It creates ACPI tables with which the operating system later controls the energy management of the hardware. The BIOS also has the configuration and hardware information. The BIOS determines the time parameters of the memory modules as well as the clock frequency and operating voltage of the main processor. The BIOS then starts the operating system and is no longer relevant.
Starting the operating system is called booting. During the boot process, the hardware is configured using the settings in the BIOS. The parameters (addresses, IRQ, slot, etc.) are taken over in whole or in part by the operating system and managed independently.

Protected mode

Protected mode for the Windows operating system was introduced with the 386 processor (Intel). Since then, the operating system no longer accesses BIOS functions, but has to control the hardware with the help of additional software. These software are called drivers. They still fulfill the same function today. Hardware manufacturers are responsible for their development. Every piece of hardware needs its own driver. Because many components are standardized, there are standard drivers with which you can start up and use almost any piece of hardware, at least in a rudimentary way. Even if there is a special driver from the manufacturer.

Power-on-self-test (POST)

When a PC is switched on, an electrical signal is sent to the processor and random memory contents are reset.
This process is called power-on-self-test (POST) and means: self-test after switching on.
There is a program counter in the processor which indicates the memory address at which the next instruction to be executed is located. When the computer is switched on, the program counter points to a specific address in a memory. The BIOS is stored there.

If the processor uses the address, a series of checks take place in the system:

  1. The processor first checks itself and the self-test program.
  2. It then sends signals over the system bus to ensure that all components are working.
  3. The processor checks the system clock.
  4. The processor examines the memory on the graphics card. After that, something can be seen on the screen for the first time.
  5. The self-test checks the main memory.
  6. The processor checks that the keyboard is properly connected and that any keys are pressed.
  7. The processor sends signals to the various drives to determine which are available.
  8. If the self-test has found new hardware, you have the option of changing the configuration accordingly.
  9. If there are components that have their own BIOS, they are included in the self-test.
  10. After the self-test, the next step is carried out: The operating system is loaded from a data carrier. It's called boot.

boot from a data carrier

After the self-test (POST) has been carried out, the boot program searches in the BIOS for a master boot record (MBR) on the connected data carriers. The order in which the individual data carriers are accessed is stored in the BIOS and can also be changed there. So you can boot the computer from a DVD or USB stick first. The first MBR found will then actually boot.
The master boot record is always in the same place and is only 512 bytes in size. It is loaded into the working memory. There he then takes over further control of the computer. The boot record refers to system files that have different names depending on the operating system. After these system files have been loaded, the boot record becomes superfluous and is overwritten by other data in the main memory.
From here the device drivers that control the memory and the hardware components are loaded. Then the various parts of the operating system that the user needs to work are loaded. When everything is loaded, the boot process is complete.

Boot from flash memory

In special computers where the operating system is not large, the operating system is built into the hardware. It is then loaded from a read-only memory or flash memory. This is the case with older game and home computers (Atari, Amiga, C64) and also with video game consoles. Smartphones and tablets also have an integrated operating system.

Boot from USB drives

Another important function is the support of the boot process from USB mass storage devices. The BIOS inserts an emulation layer here so that the boot loader of the operating system can address the USB drive like a hard disk.

BIOS update

A BIOS is nothing more than software, more precisely a type of firmware that can be renewed by means of an update.
Caution is advised when updating the BIOS. Before updating the BIOS, you have to find out exactly how the update works and what needs to be done. If something goes wrong with the BIOS update, the entire computer will no longer work. And then the BIOS memory needs to be replaced. As a rule, the motherboard manufacturers have taken safety precautions so that nothing can happen.
A little tip on the side: take a picture of all setting options before the BIOS update.

UEFI - Unified Extensible Firmware Interface

The BIOS of a modern PC is basically based on the BIOS introduced in 1982. Since then it has been adapted to modern hardware with patches and extensions. Over time there were hardware and software changes that changed the meaning of the BIOS. The Unified Extensible Firmware Interface (UEFI) does away with the circumstances and enables new functions. The UEFI specification is an embedded system that is easier to use, supports high-resolution graphics cards and is network-compatible.
Even though a PC starts with a UEFI, one still speaks of the BIOS. It is a term that has been learned and used for decades.

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Everything you need to know about computer technology.

Computer technology primer

The computer technology primer is a book about the basics of computer technology, processor technology, semiconductor memory, interfaces, data storage devices, drives and important hardware components.

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Everything you need to know about computer technology.

Computer technology primer

The computer technology primer is a book about the basics of computer technology, processor technology, semiconductor memory, interfaces, data storage devices, drives and important hardware components.

I want that!