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src/kernel/pmm.c

@@ -0,0 +1,89 @@
+#include "pmm.h"
+#include "../arch/arch.h"
+#include <stddef.h>
+#include <stdio.h>
+#include <string.h>
+
+#define BITMAP_SIZE 32768 // Supports up to 1GB of RAM
+#define USABLE_REGION_SIZE 32
+
+static uint8_t page_bitmap[BITMAP_SIZE];
+static uint64_t total_pages = 0;
+static uint64_t free_pages = 0;
+
+void pmm_init()
+{
+    memset(page_bitmap, 0xFF, BITMAP_SIZE);
+
+    for (size_t i = 0; i < memory_map.num_regions; i++)
+    {
+        memory_region_t region = memory_map.regions[i];
+
+        uint64_t start_page = region.base_address / PAGE_SIZE;
+        uint64_t num_pages = region.length / PAGE_SIZE;
+
+        for (uint64_t page = start_page; page < start_page + num_pages; page++)
+        {
+            if (page < BITMAP_SIZE * 8)
+            {
+                page_bitmap[page / 8] &= ~(1 << (page % 8));
+                free_pages++;
+                total_pages++;
+            }
+            else
+            {
+                printf("System has more ram than the bitmap allows!\n");
+                break;
+            }
+        }
+    }
+
+    // Reserve first 64MB which is reserved by boot code
+    for (uint64_t page = 0; page < (64 * 1024 * 1024) / PAGE_SIZE; page++)
+    {
+        if (page < BITMAP_SIZE * 8)
+        {
+            if (!(page_bitmap[page / 8] & (1 << (page % 8))))
+            {
+                page_bitmap[page / 8] |= (1 << (page % 8));
+                free_pages--;
+            }
+        }
+    }
+}
+
+// Returns the address of first free physical page
+uint64_t pmm_alloc_page()
+{
+    for (size_t i = 0; i < BITMAP_SIZE; i++)
+    {
+        if (page_bitmap[i] != 0xFF)
+        {
+            for (int bit = 0; bit < 8; bit++)
+            {
+                if (!(page_bitmap[i] & (1 << bit)))
+                {
+                    page_bitmap[i] |= (1 << bit);
+                    free_pages--;
+                    return ((i * 8 + bit) * PAGE_SIZE);
+                }
+            }
+        }
+    }
+
+    return 0;
+}
+
+// Frees the physical page at the specified address
+void pmm_free_page(uint64_t addr)
+{
+    uint64_t page = addr / PAGE_SIZE;
+    if (page < BITMAP_SIZE * 8)
+    {
+        if (page_bitmap[page / 8] & (1 << (page % 8)))
+        {
+            page_bitmap[page / 8] &= ~(1 << (page % 8));
+            free_pages++;
+        }
+    }
+}