src/mmap.c


1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45

#include "mmap.h"
#ifdef DEBUG
#include <stdio.h>
#endif

__attribute__((section(".multiboot.text"))) unsigned
multiboot2_mmap(const struct multiboot_mmap_entry entries[], unsigned entry_count, unsigned bitmap[1024 * 32])
{
  // clear out the bitmap
  for (unsigned i = 0; i < 1024 * 32; ++i) bitmap[i] = 0;
  unsigned avail_frames = 0;

  // loop through all the mmap_entry structures where type is MULTIBOOT_MEMORY_AVAILABLE
  for (unsigned i = 0; i < entry_count; ++i) {
    if (entries[i].type != MULTIBOOT_MEMORY_AVAILABLE) continue;

    // number of frames in this entry
    unsigned n_frames = entries[i].len / 4096;
    avail_frames += n_frames;

#ifdef DEBUG
    printf("mmap_entry: 0x%16llx\tlen=%12llu\t%d frames (%d blocks + %d)\n", entries[i].addr, entries[i].len, n_frames,
           n_frames / 32, n_frames % 32);
#endif

    // the bitmap is an array of blocks, each holding 32 (2^5) values
    unsigned table_idx = (entries[i].addr >> 17); // get the upper 15 bits

    while (n_frames != 0) {
      if (n_frames >= 32) {
        bitmap[table_idx] = 0xffffffff;
        table_idx++;
        n_frames -= 32;
      }
      else {
        unsigned block = bitmap[table_idx];
        for (unsigned l = 0; l < n_frames; ++l) block |= (1 << l);
        bitmap[table_idx] = block;
        n_frames = 0;
      }
    }
  }

  return avail_frames;
}