Is BIOS considered an OS? [closed]

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

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.