问题
I've been searching all over for an explanation of how exactly BIOS works now for quite some time. I have designed a bootloader and have jumped to 32-bit mode with it while successfully initializing the IDT as well as the GDT, but in doing so, I have found "operating systems" seeming to be quite simple, and the feeling as if "BIOS" IS the actual operating system of every computer.
So now I have took on a new challenge of trying to discover how BIOS actually initializes itself, discovers how much RAM is usable, and how/where add-in card ROM gets imported into RAM. To my understanding, the processor, not via a jump, but automatically starts executing code inside RAM at 16-bit segment:offset address 0xFFFF:0x0000. Meaning that all computers technically must have at least 1MB worth of RAM initially in order for them to be able to boot, due to the processors starting location, and due to that knowledge I have been assuming that all BIOS's automatically write themselves into RAM before the processor gets its RST signal. Which I feel is not true, as that is exactly what "Shadow BIOS" is which can be disabled through BIOS I believe. I have been searching everywhere for a "BIOS Designer's Guide" however, I keep coming out empty-handed with every specification I seem to read.
As a programmer, I understand that there are probably numerous ways to actually accomplish what I am actually asking and that there is probably no way in hell to give a decent straight-forward answer, and if I must be more specific, say I was working with a Dell Inspiron 518, or at least a computer containing a G33 chipset (G33 north bridge and ICH9 south bridge) and I wanted to program the initial Pre-POST program, and build my own 16-bit IDT with all the standard interrupts and everything needed that could potentially boot another operating system such as Windows 10 successfully. How does BIOS actually know how much RAM there is? Does it just do a bit-write bit-read test at the highest memory areas and go down from there? And how do add-in cards ROM even get loaded into RAM? To my understanding BIOS builds a very basic list of interrupts and/or "entry points" that add-in card ROMs can utilize and give them the ability to "latch" onto other BIOS's interrupts such as "$PMM"? And how do BIOS manufacturers know what exact anchor strings are needed within their BIOS to be able to boot an operating system like Windows?
Any answers would be very helpful, as well as any recommended specifications and/or any guides able to lead me into the knowledge I've been seeking. Such as maybe a guide to say "the minimum required processes needed to be accomplished by BIOS before handing off to an IPL?" or even an example of source code in C or Assembly with something that can show me what an add-in card's ROM image actually is or looks like would be very helpful.
来源：https://stackoverflow.com/questions/63159663/how-does-bios-initialize-dram