From d43d7caf15a01dd9c2ef1d9e975df3ef7c4e9204 Mon Sep 17 00:00:00 2001
From: Clyne Sullivan <clyne@bitgloo.com>
Date: Fri, 27 Sep 2024 06:02:49 -0400
Subject: wip: initial commit

---
 memory.cpp | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100644 memory.cpp

(limited to 'memory.cpp')

diff --git a/memory.cpp b/memory.cpp
new file mode 100644
index 0000000..2cc15be
--- /dev/null
+++ b/memory.cpp
@@ -0,0 +1,87 @@
+#include "textoutput.hpp"
+
+#include <array>
+#include <cstdint>
+
+struct PageDirectory
+{
+    static constexpr std::uint32_t NotPresent = 0x2;
+
+    PageDirectory(): value(NotPresent) {}
+    PageDirectory(void *addr): value(reinterpret_cast<std::uint32_t>(addr) | 3) {}
+
+    std::uint32_t value;
+};
+static_assert(sizeof(PageDirectory) == sizeof(std::uint32_t));
+
+extern std::uint32_t lowerMem;
+extern std::uint32_t upperMem;
+extern TextOutput& term;
+
+static std::uintptr_t lowerFree = 0x400;
+static std::uintptr_t upperFree = 0x100000;
+
+alignas(4096)
+static std::array<PageDirectory, 1024> pageDirectory;
+
+alignas(4096)
+static std::array<std::uint32_t, 1024> pageTable;
+
+void memory_initialize()
+{
+    lowerMem -= 1024;
+
+    const auto totalKb = (lowerMem + upperMem) / 1024u;
+
+    term.write("Claiming ");
+    term.write(totalKb);
+    term.write(" kB for allocations...\n");
+
+    std::uint32_t addr = 0;
+    for (auto& p : pageTable) {
+        p = addr | 3; // supervisor, r/w, present
+        addr += 0x1000;
+    }
+
+    pageDirectory[0] = PageDirectory(pageTable.data());
+
+    asm volatile(R"(
+        mov %%eax, %%cr3
+        mov %%cr0, %%eax
+        or $0x80000000, %%eax
+        mov %%eax, %%cr0
+    )" :: "a"(pageDirectory.data()));
+
+    term.write("Paging enabled.\n");
+}
+
+static void *memory_alloc(std::size_t size)
+{
+    void *ret = nullptr;
+
+    if (lowerMem > size) {
+        ret = reinterpret_cast<void *>(lowerFree);
+        lowerFree += size;
+        lowerMem -= size;
+    } else if (upperMem > size) {
+        ret = reinterpret_cast<void *>(upperFree);
+        upperFree += size;
+        upperMem -= size;
+    } else {
+        // Uh oh!
+        term.write("!!! Kernel allocation failed !!!");
+    }
+
+    return ret;
+}
+
+void *operator new(std::size_t size)
+{
+    return memory_alloc(size);
+}
+
+void *operator new[](std::size_t size)
+{
+    return memory_alloc(size);
+}
+
-- 
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