Question

if we turned on computer and OS is not Installed in it then which instructions executes??​

Answer 1

Answer:

Without the operating system, then it’ll just be the system firmware, which will detect no operating system or bootloader and either halt on an error like “No system disk found” or run the firmware configurator.

The operating system is FAR from the first thing to run on your PC. It’s not even the second or third thing to run on your PC.

When you first power on the average PC, the first thing that loads is a reset vector which, in turn, loads up the system firmware and then executed that. That, in turn, will run any possible option ROMs to set up hardware connected to your computer all before running POST. By the time it reaches the bootloader, your computer’s likely ran 5–6 completely distinct programs. And even STILL your operating system is still not running yet. On some systems that bootloader is broken down into 2–3 programs, and it’s often up to 11–12 distinct programs run by the time the operating system kernel actually executes and starts loading other software.

As to what happens in, say, a computer system where it’s JUST a CPU and no programs in any part of memory anywhere, no ROM or anything? Chances are the CPU will still try to load a reset vector and run what it THINKS would be instructions in that location in memory, which may simply result in the CPU doing nothing or just executing a bunch of illegal opcodes. It won’t be doing any meaningful work.