Is BIOS considered an OS? [closed]

This question comes to mind when dealing with RAID arrays, but is not necessarily applicable to the subject.

How is that the BIOS is able read from a hard drive to effectively load the OS, when many OS's need drivers to be able to do the same thing? Does the BIOS have a list of drivers it pulls from like one of the mainstream OS's do like Windows or Linux?

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4 Answers

First off, BIOS is the Basic Input/Output System, not technically an Operating System... As for loading RAID and such, what happens during the boot is the machine queries any add in cards asking if they have a BIOS to load. Mr RAID card yells out "Ya, I got something to do first", then he fires up his "BIOS" (in this case, an OS), does some calculations itself, then passes control back to the main BIOS ("Hey, I'm done, here is all the information you need to know on a need to know, you know?").

Once the main BIOS gets control again, it can go through as say "Ahhh, I see Mr RAID has a bootable volume, I shall cycle through that if possible, then try my other tricks".

So not "technically" is the BIOS an operating system, but it is an Input/Output system to prepare for the real operating system

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The BIOS, literally a "basic input/output system", is a set of small programs hard-coded into a computer's motherboard (usually stored on an EEPROM). These programs include the ability to modify settings, write text and take user input from basic I/O devices (the motherboard has to emulate them for advanced VGA and USB mouse/keyboard devices), and most importantly, provides the functionality to find and boot operating systems present on any attacked storage devices.

By itself, the BIOS is not an operating system. The BIOS is a small program to actually load an OS. While it is possible in some operating systems to invoke the various BIOS sub-systems, this requires the CPU to switch back to real-mode, since the BIOS doesn't provide virtual memory support, task switching (and thus the ability to run programs), or device driver support (and thus, cannot directly access hardware out of the direct-map accessible range, nor anything not covered in the BIOS interrupt calls).

As Linus Torvalds said:

Not that I'd ever claim that the BIOS is wonderful either, but at least everybody knows that the BIOS is just a bootloader, and doesn't try to make it anything else.

The absolutely biggest advantage of a BIOS is that it's so inconvenient and obviously oldfashioned, that you have to be crazy to want to do anything serious in it [...] don't get any ideas about it being some grandiose framework for anything else than "just load the OS and get the hell out of there".

Finally, it should be noted that a BIOS is significantly different than a UEFI (Unified Extensible Firmware Interface), the latter being significantly closer to providing a basic operating system interface (providing device drivers and application support). It is still questionable as to whether or not even the UEFI can be considered a true operating system on it's own, although it is significantly closer to being considered an OS than a BIOS would.

What is or isn't an "operating system" is a matter of judgment. Arguably (and I argued this in 1980), the original PC/MS DOS was not an "operating system" (despite its name), since it provided only I/O services (and a crude command analyzer) and no true system management facilities.

OTOH, many BIOS implementations now include some degree of hypervisor function which manages system resources in a fairly sophisticated way.

As to how BIOS manages to load from disk, etc, without drivers, by convention/standard "bootable" devices include a set of operations that can be used without having to set up a sophisticated management infrastructure. In some cases this is only the ability to read a few sectors at a relatively fixed location, but that's all that's needed in most cases.

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To deal with the many comments and points brought up, I have edited the answer yet again.

Is the BIOS an Operating System?

Well, if an operating system is defined as software that allows a user to interact with the hardware of a “computer” then yes… technically and literally it is an Operating System.

Does Task Switching define what makes an Operating System?

Considering that MS-DOS was a valid Operating System, and up until version 5 Task Switching was not supported, task switching does not affect whether or software is an Operating System.

Does Virtualization of Memory define what makes an Operating System?

Again, using MS-DOS as an example, although support could be added for memory virtualization by running extenders during the loading process, it was not necessary to use them. Thus, memory virtualization also is not a prerequesite to what makes up an Operating System.

If it is stored in Firmware, is it an OS?

Some would argue that a router doesn’t use an Operating System. For example, there appears to be contention as to whether DD-WRT is considered an Operating System. Are there devices that store an Operating System in Firmware? Modern phones, iPods, and more store complex operating systems that can have programs added to them in Firmware. So, just because an OS is loaded into Firmware, this does not exclude the software in question from being considered an Operating System.

If you can’t add programs, it’s not an Operating System.

Step away from a modern smartphone. Look at a cheap disposable cell phone. It has an Operating System stored in Firmware, but you cannot add programs. It runs as it is, and only with the functionality that is offered. You navigate through the menus as you would any other OS, you choose what you want to do (play games, etc) and with a good number of them from a few years back, they did not have the ability to add any additional software.

If it is not modern, is it not an Operating System?

The point in history at which the OS was created and launched has no bearing on whether or not it is an Operating System.

So, the BIOS may not be pretty, and it may not offer you extreme functionality. However, it is still an Operating System.

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