void init_raminfo (void)

Determine the location and size of available system RAM and initialize the asinfo structure in the system page.

If you know the exact amount and location of RAM in your system, you can replace this library function with one that simply hard-codes the values via one or more add_ram() calls.

If the RAM configuration is known (e.g. set by the IPL code, or the multi-boot IPL code gets set by the gnu utility), then the library version of init_raminfo() will call the library routine find_startup_info() to fetch the information from a known location in memory. If the RAM configuration isn't known, then a RAM scan (via x86_scanmem()) is performed looking for valid memory between locations 0 and 0xFFFFFF, inclusive. (Note that the VGA aperture that usually starts at location 0xB0000 is specifically ignored.)
There's no library default. You must supply your own init_raminfo() function.