explanation of __va() and __pa() macros.

Hi, My source is from 2.6 git tree. In LDD3 its said that a logical address is mapped to physical address and vice versa by the macros __va() and __pa(). In my source tree they are defined as follows: <include/asm-generic/page.h> #define __va(x) ((void *)((unsigned long) (x))) #define __pa(x) ((unsigned long) (x)) I'm not actually understanding how these "cute" looking casting is performing "address mapping". I thought address mapping involves more complex operations. cheers. aft