Most boards used to have an option in the BIOS to configure this behavior. It was typically called shadowing, and it was usually enabled by default. I don't think many boards bother giving you the option these days and just always shadow. The reason is because RAM is faster than ROM, so it speeds things up to copy it to RAM and run it from there.
Note that the copy isn't done by some magic circuitry, it is just done by the bios itself when it starts executing out of ROM initially, it just copies itself to RAM and then continues executing from there.
A BIOS is a hardware dependent piece of code stored on the motherboard itself. Every different motherboard needs a custom BIOS written for it, so it would be impossible to have a generic BIOS/OS all-in-one (although the BIOS is technically just stored code, so you could theoretically write an OS for one particular motherboard). As you mentioned, the purpose of the BIOS is to do the following:
When the PC starts up, the first job for the BIOS is the power-on
self-test, which initializes and identifies system devices such as the
CPU, RAM, video display card, keyboard and mouse, hard disk drive,
optical disc drive and other hardware.
Note that you can still start a computer without any external storage - which is why the BIOS is a requirement for a computer. In other terms, the BIOS provides a common software interface to allow a stored computer program to communicate with various hardware devices connected to the motherboard.
For example, if I have two different motherboards with two different SATA controllers, the BIOS allows me to write a piece of code that can work with both, without my knowledge of how the motherboard actually sends commands to the SATA device. I just have to tell the computer "read sector X from this SATA device", and the BIOS is responsible for actually sending those commands to the hardware.
Where it actually gets the "read sector X" information from is a stored program contained within the BIOS, which usually directs the computer to start reading from a bootloader stored in a common location. These common locations are agreed upon by various software and hardware developers, and usually provided to the public to allow for more compatibility between systems.
Once a basic level of interfacing (again, logical interfacing through software) is established, the operating system itself builds a common interface with your various hardware devices (usually by using "device drivers"), and the operating system can then control the hardware.
Finally, it should be noted that the BIOS is also used to make modifications to the computer hardware configurations, and store them in the on-board EEPROM (so your computer remembers the changes next time you start it up). However, as I previously stated, once the operating system is loaded, it has full control of the computer.
This allows for motherboard manufacturers to develop software allowing you to make these changes from within your operating system, as opposed to having to reboot into the BIOS. Again, this is very hardware and software dependent, but goes to show that all computer interfacing is relative. The BIOS is exactly what it's name implies - a basic input/output system, to allow a common software interface for a more advanced program ("operating system") to take control of the machine.
Best Answer
As I wrote in my answer to that question the received folk wisdom on the subject — as unfortunately exemplified by other answers there (and elsehere in SuperUser) — is stuck in the world as it was around 1991, despite the wealth of technical references available explaining how it is now otherwise.
You wouldn't have been this confused if you had read my answer, because you wouldn't be asking about "BIOS loaded from ROM" in the first place.
Your "BIOS chip" is not ROM; there is no machine code between processor startup and the first instruction in firmware; and the "M" in both "RAM" and "ROM" means "memory".
As I wrote before, in modern PCs, the machine firmware is held in non-volatile RAM. It's not ROM as it used to be. See the previous answer for details of the NVRAM chip connected to the LPC bus. (For example: On a machine sitting disassembled beside me as I type this, the NVRAM holding the firmware is a Pm49FL004T, an LPC Flash RAM chip.)
32-bit CPUs don't start up in real mode, and don't start with an address that is below the 1MiB line. That's decades out of date rubbish from the times of 16-bit x86 processors. They start in what is colloquially known as unreal mode, and again in my prior answer I gave the details of what has actually been the case since the advent of the 80386. They load their first instruction from an address that is in fact right at the top of the 32-bit address space,
FFFFFFF0.In my prior answer I told you in detail where the machine firmware is principally mapped into physical address space on 32-bit and 64-bit x86 machines. Remember: Both RAM and ROM are memory. Physical addresses are memory addresses, on the system bus. They can address either RAM or ROM. (They can even address other things as well, but that's just complicating this discussion.) The physical address
FFFFFFF0is 16 bytes below the top of the 512KiB range where the top 512KiB of the firmware, in non-volatile RAM, is always mapped on the system bus by the "chipset".There's no "loading" from some mythical ROM chip that goes on at processor initialization or reset. The chip holding the firmware is non-volatile RAM. It retains its contents, written when it is "flashed", across power cycles. And the CPU just reads the firmware instructions and data from it, over the system bus and over an LPC bus (and possibly an LPC/FWH bridge) connected to the system bus via the chipset, using a physical memory address.
Further reading