Added I would just note that there was a comment the poster may have found useful, that in Device Manager if you do view..resources by type then double click memory. And you get a memory map. On an XP machine I just checked, it started from 0 and went to 9FFFF for system board and PCI bus. With Graphics starting at A0000. On another it showed Graphics starting at A0000 but didn't show what led up to that. Suffice it to say, you get a memory map, sometimes from start to finish.

Furthermore. In another comment, allquixotic mentioned a book the OP may find useful. The famous Windows Internals by Mark Russinovich(of sysinternals fame)