mm stuff

kernel/memory/mm.c

@@ -1,13 +1,46 @@
 #include "multiboot.h"
 
+#include <stddef.h>
+
+extern uint8_t __kernel_start;
+extern uint8_t __kernel_end;
+
+#define PAGE_SIZE 4096
+#define MAX_RAM   0xFFFFFFFF // 4GB
+#define BITMAP_SIZE ((MAX_RAM / PAGE_SIZE) / 8)
+
 static uint32_t max_address = 0;
 static uint32_t total_available_memory = 0;
+static uint8_t memory_bitmap[BITMAP_SIZE];
 
-void parse_mmap(void* base, uint32_t limit) {
+static void mark_free(uint32_t addr) {
+    uint32_t page_index = addr / PAGE_SIZE;
+    uint32_t byte_index = page_index / 8;
+    uint32_t bit_index = page_index % 8;
+    memory_bitmap[byte_index] &= ~(1 << bit_index);
+}
+
+static void mark_used(uint32_t addr) {
+    uint32_t page_index = addr / PAGE_SIZE;
+    uint32_t byte_index = page_index / 8;
+    uint32_t bit_index = page_index % 8;
+    memory_bitmap[byte_index] |= (1 << bit_index);
+}
+
+static uint32_t align(uint32_t addr, uint32_t len, uint32_t* usable_start, uint32_t* usable_end) {
+    uint32_t start = addr;
+    uint32_t end = addr + len;
+    *usable_start = (start + 4095) & ~0xFFF;
+    *usable_end = end & ~0xFFF;
+    return *usable_end - *usable_start;
+}
+
+static void parse_multiboot_mmap(void* base, uint32_t limit) {
     multiboot_mmap_entry* entry = (multiboot_mmap_entry*)base;
     void* end = (void*)((uint32_t)base + limit);
-
+    total_available_memory = 0;
     while ((void*)entry < end) {
+        uint32_t entry_size = entry->size + sizeof(entry->size);
         if (entry->type == 1) {
             uint32_t region_start = (uint32_t)(entry->addr);
             uint32_t region_end = (uint32_t)(entry->addr + entry->len);
@@ -15,23 +48,67 @@ void parse_mmap(void* base, uint32_t limit) {
                 max_address = region_end;
             }
 
-            total_available_memory += entry->len;
-
-            // start breaking up the address range
-            // if it's safe (not kernel, not BIOS structures, etc..), find the corresponding page in our PMM, mark it free
-            // need to align start and end to 4kb
-            if (entry->len < 4096) {
+            uint32_t usable_start, usable_end, usable_size;
+            usable_size = align(region_start, entry->len, &usable_start, &usable_end);
+            if (usable_size <=  4096) {
                 continue;
             }
 
-            for (uint32_t addr = region_start; addr < region_end; addr += 4096) {
-                // check for physical kernel address range 
+            for (uint32_t addr = usable_start; addr < usable_end; addr += 4096) {
+                if (addr <= (uint32_t)&__kernel_end && addr >= (uint32_t)&__kernel_start) {
+                    continue;
+                }
 
-                // check if this address range contains our multiboot structure
+                if (addr >= (uint32_t)entry && addr <= (uint32_t)entry + entry_size) {
+                    continue;
+                }
 
-                // don't allow pages under 1MB
+                if (addr <= 0x200000) {
+                    continue;
+                }
+
+                total_available_memory += 4096;
+
+                mark_free(addr);
             }
         }
-        entry = (multiboot_mmap_entry*)((uint32_t)entry + entry->size + sizeof(entry->size));
+        entry = (multiboot_mmap_entry*)((uint32_t)entry + entry_size);
     }
+}
+
+void init_mm(void* mmap_base, uint32_t mmap_limit) {
+    for (int i = 0; i < BITMAP_SIZE; i++) {
+        memory_bitmap[i] = 0xFF;
+    }
+
+    if (mmap_base != NULL && mmap_limit > 0) {
+        parse_multiboot_mmap(mmap_base, mmap_limit);
+    }
+}
+
+uint32_t total_free_memory() {
+    return total_available_memory;
+}
+
+void* alloc_page() {
+    for (uint32_t i = 0; i < BITMAP_SIZE; i++) {
+        uint8_t b = memory_bitmap[i];
+        if (b == 0xFF) {
+            continue;
+        }
+
+        for (int bit = 0; bit < 8; bit++) {
+            if ((b & (1 << bit)) == 0) {
+                uint32_t page_addr = (i * 8 + bit) * PAGE_SIZE;
+                mark_used(page_addr);
+                return (void*)page_addr;
+            }
+        }
+    }
+
+    return 0;
+}
+
+void free_page(void* addr) {
+    mark_free((uint32_t)addr);
 }

kernel/memory/mm.h

@@ -0,0 +1,16 @@
+#ifndef KMM_H
+#define KMM_H
+
+#include <stdint.h>
+
+typedef struct __attribute__((packed)) {
+    uint32_t addr;
+} mem_page_t;
+
+void init_mm(void* mmap_base, uint32_t mmap_limit);
+uint32_t total_free_memory();
+
+void* alloc_page();
+void free_page(void* addr);
+
+#endif