unsigned x86_scanmem (paddr_t beg,
paddr_t end)
An x86-only function that scans memory between beg and end looking for RAM, and returns the total amount of RAM found. It scans memory performing a R/W test of 3 values at the start of each 4 KB page. Each page is marked with a unique value. It then rescans the memory looking for contiguous areas of memory and adds them to the asinfo entry in the system page.
A special check is made for a block of memory between addresses 0xB0000 and 0xBFFFF, inclusive. If memory is found there, the block is skipped (since it's probably the dual-ported memory of a VGA card).
The call x86_scanmem (0, 0xFFFFFF) would locate all memory in the first 16 megabytes of memory (except VGA memory). You may make multiple calls to x86_scanmem() to different areas of memory in order to step over known areas of dual-ported memory with hardware.