It's even more interesting than that. One of the things that the BIOS has to do, if I recall correctly, is to set up the memory controller. Until it does that,

It's even more interesting than that. One of the things that the BIOS has to do, if I recall correctly, is to set up the memory controller. Until it does that, it's only able to execute out of registers and cache, no RAM. If you're interested, Coreboot (an open source BIOS replacement) has interesting information on this.