What you're asking for is called DMA. You need to write a driver to reserve this memory.
Yes, I realize you said you didn't want the OS to intervene, and a driver becomes part of the OS, but in absence of a driver's reservation, the kernel believes all memory belongs to it. (Unless you tell the kernel to ignore the memory block, per Aaron's answer, that is.)
Chapter 15 (PDF) of "Linux Device Drivers, 3/e" by Rubini, Corbet and Kroah-Hartmann covers DMA and related topics.
If you want an HTML version of this, I found the second-edition version of the chapter elsewhere online. Beware that the 2nd edition is over a decade old now, having come out when kernel 2.4 was new. There's been a lot of work on the memory management subsystem of the kernel since those days, so it may not apply very well any more.