From 4008fc25e8d5eda4c460133aba68b5380a37902b Mon Sep 17 00:00:00 2001
From: Nigel Barink <nigelbarink@hotmail.com>
Date: Mon, 10 May 2021 21:32:28 +0100
Subject: [PATCH] Added functions to enable MMU with paging

---
 src/kernel/MMU.cpp          | 35 +++++++++++++++++++++++++++++++++++
 src/kernel/MMU.h            | 14 ++++++++++++++
 src/kernel/arch/i386/boot.s | 26 ++++++++++++++++++++++++++
 3 files changed, 75 insertions(+)
 create mode 100644 src/kernel/MMU.cpp
 create mode 100644 src/kernel/MMU.h

diff --git a/src/kernel/MMU.cpp b/src/kernel/MMU.cpp
new file mode 100644
index 0000000..96867f4
--- /dev/null
+++ b/src/kernel/MMU.cpp
@@ -0,0 +1,35 @@
+#include "MMU.h"
+
+
+
+void MMU::enable(){
+
+    //set each entry to not present
+    int i;
+    for(i = 0; i < 1024; i++)
+    {
+        // This sets the following flags to the pages:
+        //   Supervisor: Only kernel-mode can access them
+        //   Write Enabled: It can be both read from and written to
+        //   Not Present: The page table is not present
+        this->page_directory[i] = 0x00000002;
+    }
+
+    // holds the physical address where we want to start mapping these pages to.
+    // in this case, we want to map these pages to the very beginning of memory.
+
+    //we will fill all 1024 entries in the table, mapping 4 megabytes
+    for(unsigned int i = 0; i < 1024; i++)
+    {
+    // As the address is page aligned, it will always leave 12 bits zeroed.
+    // Those bits are used by the attributes ;)
+    first_page_table[i] = (i * 0x1000) | 3; // attributes: supervisor level, read/write, present.
+    }
+
+    // attributes: supervisor level, read/write, present
+    this->page_directory[0] = ((unsigned int)first_page_table) | 3;
+
+
+    loadPageDirectory(this->page_directory);
+    enablePaging();
+}
\ No newline at end of file
diff --git a/src/kernel/MMU.h b/src/kernel/MMU.h
new file mode 100644
index 0000000..cb3b7f8
--- /dev/null
+++ b/src/kernel/MMU.h
@@ -0,0 +1,14 @@
+#pragma once 
+#include <stdint.h>
+
+extern "C" void loadPageDirectory (long unsigned int* addr );
+extern "C" void enablePaging();
+
+class MMU {
+    public:
+    void enable ();
+
+    private:
+    uint32_t page_directory[1024] __attribute__((aligned(4096)));
+    uint32_t first_page_table[1024] __attribute__((aligned(4096)));
+};
\ No newline at end of file
diff --git a/src/kernel/arch/i386/boot.s b/src/kernel/arch/i386/boot.s
index fa5fce2..99fa270 100644
--- a/src/kernel/arch/i386/boot.s
+++ b/src/kernel/arch/i386/boot.s
@@ -21,6 +21,32 @@ stack_bottom:
 stack_top:
  
 
+
+
+.text
+.globl enablePaging
+enablePaging:
+	push %ebp
+	mov %esp, %ebp
+	mov %cr0, %eax
+	or $0x80000000, %eax
+	mov %eax, %cr0
+	mov %ebp, %esp
+	pop %ebp
+	ret
+
+.text
+.globl loadPageDirectory
+loadPageDirectory:
+	push %ebp
+	mov %esp, %ebp
+	mov 8(%esp), %eax
+	mov %eax, %cr3
+	mov %ebp, %esp
+	pop %ebp
+	ret
+
+
 .section .text
 .global _start
 .type _start, @function