Who loads the BIOS and the memory map during boot-up

Question

For the BIOS, Wikipedia states:

The address of the BIOS's memory is located such that it will be executed when the computer is first started up. A jump instruction then directs the processor to start executing code in the BIOS.

I know that BIOS lives in non-volatile memory. But it would have to be loaded into the RAM for it to be executed. So who loads the BIOS into RAM ?

I have also read that a memory map is loaded at start-up. Does the BIOS load this memory map ? Where is is stored ?

Answer 1

At initial power on, the BIOS is executed directly from ROM. The ROM chip is mapped to a fixed location in the processor's memory space (this is typically a feature of the chipset). When the x86 processor comes out of reset, it immediately begins executing from 0xFFFFFFF0.

However, executing directly from ROM is quite slow, so usually one of the first things the BIOS does is to copy and decompress the BIOS code into RAM, and it executes from there. Of course, the memory controller must be initialized first! The BIOS takes care of that beforehand.

The memory map layout will vary from system to system. At power-on, the BIOS will query the attached PCI/PCIe devices, determine what resources are needed, and place them in the memory map at the optimal location. If everything is working properly, memory-mapped devices should not overlap with RAM. (Note that on a 64-bit system with >3GB of RAM, things get complicated because you need a "hole" in the middle of RAM for your 32-bit PCI/PCIe devices. Some early x64 BIOSes and chipsets had issues with this.)