From 8773024d1b4e756fe5d3494479247c64c7ad8491 Mon Sep 17 00:00:00 2001
From: Fabian Imhof <fabian.imhof@ost.ch>
Date: Tue, 8 Oct 2024 09:51:26 +0000
Subject: begin implementing frame allocator

---
 arch/x86_64/src/memory/frame_allocator.cpp | 70 +++++++++++++++++++++++++++++-
 1 file changed, 69 insertions(+), 1 deletion(-)

(limited to 'arch/x86_64/src/memory')

diff --git a/arch/x86_64/src/memory/frame_allocator.cpp b/arch/x86_64/src/memory/frame_allocator.cpp
index b9d26a5..b8f53be 100644
--- a/arch/x86_64/src/memory/frame_allocator.cpp
+++ b/arch/x86_64/src/memory/frame_allocator.cpp
@@ -2,8 +2,76 @@
 
 namespace teachos::arch::memory
 {
-  auto area_frame_allocator::allocate_frame() -> std::optional<frame> {}
+  auto area_frame_allocator::allocate_frame() -> std::optional<frame>
+  {
+    /*
+     * Only try to allocate memory if current_area is not null, because
+     * the current_area is null if there is no more available memory.
+     */
+    if (current_area)
+    {
+      frame frame{next_free_frame.frame_number};
+
+      struct frame current_area_last_frame = {
+          frame::containing_address(current_area->base_address + current_area->area_length - 1)};
+
+      if (next_free_frame > current_area_last_frame)
+      {
+        // All frames of current area are used, switch to next area
+        choose_next_area();
+      }
+      else if (frame >= multiboot_start && frame <= kernel_end)
+      {
+        // `frame` is used by the kernel or multiboot information structure
+        next_free_frame = arch::memory::frame{kernel_end.frame_number + 1};
+      }
+      else
+      {
+        // Frame is unused, increment `next_free_frame` and return it
+        next_free_frame.frame_number += 1;
+        return frame;
+      }
+
+      // `frame` was not valid, try it again with the updated `next_free_frame`
+      return allocate_frame();
+    }
+
+    // no free frames left
+    return std::nullopt;
+  }
 
   auto area_frame_allocator::deallocate_frame(frame frame) -> void {}
 
+  namespace
+  {
+    auto area_frame_allocator::choose_next_area() -> void
+    {
+      current_area = std::nullopt;
+
+      // TODO: Fix this loop as soon as you've created an area iterator
+      for (const auto & area : areas)
+      {
+        std::size_t address = area.base_addr + area.length - 1;
+        if (frame::containing_address(address) >= next_free_frame)
+        {
+          // The `next_free_frame` address is smaller than the last address of the current area
+          if (!current_area || area.base_addr < current_area->base_address)
+          {
+            current_area = area;
+          }
+        }
+      }
+
+      if (current_area)
+      {
+        // Update the `next_free_frame` according to the new memory area
+        frame start_frame = frame::containing_address(current_area->base_address);
+        if (next_free_frame < start_frame)
+        {
+          next_free_frame = start_frame;
+        }
+      }
+    }
+
+  }  // namespace
 }  // namespace teachos::arch::memory
-- 
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