Well, to be completly accurate. On an x86 (including x86-64) IBM compatible system:
When the computer FIRST starts up, the BIOS cannot use the RAM immediatly. The BIOS's first task is usually to detect the RAM so it can continue to perform it's other functions. For this, the BIOS uses chiptset provided features all of which use it's own memory. This can be seen in plain english fairly well here: http://wiki.osdev.org/System_Initialization_%28x86%29 .
You can verify this for yourself by looking through the source code of http://www.coreboot.org .
In theory, you could replace the bios with a very small OS that works completly from ROM. But you still wont be able to access any memory outside of what the chipset might provide and the registers within the CPU itself.