From 99704be2023f914cba072338aab214012b910dd6 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Felix=20Quei=C3=9Fner?= <git@mq32.de>
Date: Tue, 3 May 2016 09:23:38 +0200
Subject: [PATCH] Implements simple bitmap allocation.

---
 prototypes/base/src/pmm.cpp | 74 ++++++++++++++++++++++++++++++++++++-
 1 file changed, 73 insertions(+), 1 deletion(-)

diff --git a/prototypes/base/src/pmm.cpp b/prototypes/base/src/pmm.cpp
index e0c5834..74af1c5 100644
--- a/prototypes/base/src/pmm.cpp
+++ b/prototypes/base/src/pmm.cpp
@@ -1,16 +1,88 @@
+#include <inttypes.h>
 #include "pmm.hpp"
 
+
+/**
+ * Number stored of pages in the bitmap
+ */
+static const uint32_t BitmapSize = 4096; /* 16 MB */
+
+/**
+ * Number of 32-bit tuples in the bitmap.
+ */
+static const uint32_t BitmapLength = BitmapSize / 32; 
+
+/**
+ * Bitmap that stores the page status.
+ * A set bit marks a used page
+ * An unset bit marks an unused page
+ */
+static uint32_t bitmap[BitmapLength];
+
+static uint32_t ptrcast(void *ptr)
+{
+  return reinterpret_cast<uint32_t>(ptr);
+}
+
+static void *ptrcast(uint32_t ptr)
+{
+  return reinterpret_cast<void*>(ptr);
+}
+
+static bool isAligned(uint32_t ptr)
+{
+  return (ptr % 4096) == 0;
+}
+
 bool PMM::markOccupied(void *page)
 {
-  return false;
+  uint32_t ptr = ptrcast(page);
+  if(!isAligned(ptr))
+    ; // Do something about it!
+  uint32_t pageId = ptr / 4096;
+  
+  uint32_t idx = pageId / 32;
+  uint32_t bit = pageId % 32;
+  
+  bool wasSet = (bitmap[idx] & (1<<bit)) != 0;
+  bitmap[idx] |= (1<<bit);
+  return wasSet;
 }
 
 void *PMM::alloc()
 {
+  for(uint32_t idx = 0; idx < BitmapLength; idx++) {
+    // fast skip when all bits are set
+    if(bitmap[idx] == 0xFFFFFFFF) {
+      continue;
+    }
+    for(uint32_t bit = 0; bit < 32; bit++) {
+      uint32_t mask = (1<<bit);
+      if((bitmap[idx] & mask) != 0) {
+        continue;
+      }
+      
+      // Mark the selected bit as used.
+      bitmap[idx] |= mask;
+      
+      uint32_t pageId = 32 * idx + bit;
+      uint32_t ptr = 4096 * pageId;
+      return ptrcast(ptr);
+    }
+  }
+  // Do something here!
   return nullptr;
 }
 
 void PMM::free(void *page)
 {
+  uint32_t ptr = ptrcast(page);
+  if(!isAligned(ptr))
+    ; // Do something about it!
+  uint32_t pageId = ptr / 4096;
   
+  uint32_t idx = pageId / 32;
+  uint32_t bit = pageId % 32;
+  
+  bitmap[idx] &= ~(1<<bit);
 }
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