Sungju's Slow Life

I wondered where linux get information about memory capacity in the machine. In i386 machine, we can get information from BIOS or e820 which is shorthand for BIOS function name. You can find some information from whikipedia : http://en.wikipedia.org/wiki/E820

Anyway, after checking basic memory map information, kernel call following function to get max_pfn value.

/*
 * Find the highest page frame number we have available
 */
void __init find_max_pfn(void)
{
        int i;

        max_pfn = 0;
        if (efi_enabled) {
                efi_memmap_walk(efi_find_max_pfn, &max_pfn);
                return;
        }

        for (i = 0; i < e820.nr_map; i++) {
                unsigned long start, end;
                /* RAM? */
                if (e820.map[i].type != E820_RAM)
                        continue;
                start = PFN_UP(e820.map[i].addr);
                end = PFN_DOWN(e820.map[i].addr + e820.map[i].size);
                if (start >= end)
                        continue;
                if (end > max_pfn)
                        max_pfn = end;
        }
}