How do OS installers works if there isn’t any OS that could manage applications in an empty computer?
In: Technology
A CPU is a machine for executing machine code instructions. You put machine code instructions at a location in memory, set the CPU’s instruction pointer to the starting point of those instructions, and let it run.
The OS, once installed, has to be on a piece of persistent storage like a hard drive. So it would have to be copied into RAM before the CPU could run it. Copying data into RAM from a hard drive is a *very* complicated task, relative to what a CPU knows how to do on its own. So instead, a little bit of code is built into your motherboard that knows how to
1. Find a storage device
2. Copy the first 512 bytes of that storage device into RAM at an architecture-defined address
3. Set the instruction pointer to that address
The true way to run software on your machine is therefore to put code in the first 512 bytes of a USB stick or something, plug it in, tell the BIOS to use that drive, and then let it boot. Usually the code in these 512 bytes just copies *more* code from the rest of the drive, and points the IP there.
Just to avoid confusion should you choose to research this a bit more:
The BIOS is no more. The technology had quite a number of restrictions. Intel developed something called the “Extensible Firmware Interface” (EFI) way back together with a now failed hardware plattform (Itanium).
It took a while until this became widely adopted (I think, Apple first used it on the – at the time – new Intel-based Macs). Nowadays, basically every modern PCs (and other platforms) come with the Universal EFI (UEFI) technology on board which, aside from starting up the whole computer, supports a lot more (e.g. to remotely manage computers in enterprise environments etc.)
But regarding your question: Today, on a modern computer, the UEFI does what the BIOS did – just not in such a very basic manner anymore.
Most computers still allow “legacy booting” with the old BIOS interface but that will disappear in time (and has disappeared for a lot of “made for business” PCs e.g. from Dell or HP)
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