1 files changed, 45 insertions, 0 deletions
diff --git a/src/mmap.c b/src/mmap.c
new file mode 100644
index 0000000..3fe35b5
--- /dev/null
+++ b/src/mmap.c
@@ -0,0 +1,45 @@
+#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;
+}