From 8e579133cc27ef87c14a44b133407b483875cb96 Mon Sep 17 00:00:00 2001
From: Alex <enderice2@users.noreply.github.com>
Date: Sat, 5 Nov 2022 06:31:45 +0200
Subject: [PATCH] Updated GDT code

---
 .../amd64/cpu/GlobalDescriptorTable.cpp       | 67 +++++++++++++++++--
 Architecture/amd64/cpu/gdt.hpp                | 34 ++++++++++
 2 files changed, 95 insertions(+), 6 deletions(-)

diff --git a/Architecture/amd64/cpu/GlobalDescriptorTable.cpp b/Architecture/amd64/cpu/GlobalDescriptorTable.cpp
index 239fe09..764df07 100644
--- a/Architecture/amd64/cpu/GlobalDescriptorTable.cpp
+++ b/Architecture/amd64/cpu/GlobalDescriptorTable.cpp
@@ -8,12 +8,67 @@
 namespace GlobalDescriptorTable
 {
     static GlobalDescriptorTableEntries GDTEntries = {
-        {.Length = 0x0000, .BaseLow = 0x0000, .BaseMiddle = 0x00, .Access = 0b00000000, .Flags = 0b00000000, .BaseHigh = 0x00},    // null
-        {.Length = 0x0000, .BaseLow = 0x0000, .BaseMiddle = 0x00, .Access = 0b10011010, .Flags = 0b00100000, .BaseHigh = 0x00},    // kernel code
-        {.Length = 0x0000, .BaseLow = 0x0000, .BaseMiddle = 0x00, .Access = 0b10010010, .Flags = 0b00000000, .BaseHigh = 0x00},    // kernel data
-        {.Length = 0x0000, .BaseLow = 0x0000, .BaseMiddle = 0x00, .Access = 0b11111010, .Flags = 0b00100000, .BaseHigh = 0x00},    // user code
-        {.Length = 0x0000, .BaseLow = 0x0000, .BaseMiddle = 0x00, .Access = 0b11110010, .Flags = 0b00000000, .BaseHigh = 0x00},    // user data
-        {.Length = 0, .Low = 0, .Middle = 0, .Flags1 = 0b10001001, .Flags2 = 0b00000000, .High = 0, .Upper32 = 0, .Reserved = 0}}; // tss
+        // null
+        {.Length = 0x0,
+         .BaseLow = 0x0,
+         .BaseMiddle = 0x0,
+         .Access = 0b00000000,
+         .Flags = 0b00000000,
+         .BaseHigh = 0x0},
+
+        // kernel code
+        {.Length = 0x0,
+         .BaseLow = 0x0,
+         .BaseMiddle = 0x0,
+         .Access = RING0 |
+                   CODE_READABLE |
+                   CODE_SEGMENT |
+                   PRESENT,
+         .Flags = _64BITS,
+         .BaseHigh = 0x0},
+
+        // kernel data
+        {.Length = 0x0,
+         .BaseLow = 0x0,
+         .BaseMiddle = 0x0,
+         .Access = RING0 |
+                   DATA_WRITEABLE |
+                   DATA_SEGMENT |
+                   PRESENT,
+         .Flags = 0b00000000,
+         .BaseHigh = 0x0},
+
+        // user code
+        {.Length = 0x0,
+         .BaseLow = 0x0,
+         .BaseMiddle = 0x0,
+         .Access = CODE_READABLE |
+                   CODE_SEGMENT |
+                   RING3 |
+                   PRESENT,
+         .Flags = _64BITS,
+         .BaseHigh = 0x0},
+
+        // user data
+        {.Length = 0x0,
+         .BaseLow = 0x0,
+         .BaseMiddle = 0x0,
+         .Access = DATA_WRITEABLE |
+                   DATA_SEGMENT |
+                   RING3 |
+                   PRESENT,
+         .Flags = 0b00000000,
+         .BaseHigh = 0x0},
+
+        // tss
+        {.Length = 0x0,
+         .Low = 0x0,
+         .Middle = 0x0,
+         .Flags1 = 0b10001001,
+         .Flags2 = 0b00000000,
+         .High = 0x0,
+         .Upper32 = 0x0,
+         .Reserved = 0x0}};
 
     GlobalDescriptorTableDescriptor gdt = {.Length = sizeof(GlobalDescriptorTableEntries) - 1, .Entries = &GDTEntries};
 
diff --git a/Architecture/amd64/cpu/gdt.hpp b/Architecture/amd64/cpu/gdt.hpp
index ed3abda..7bc1d9c 100644
--- a/Architecture/amd64/cpu/gdt.hpp
+++ b/Architecture/amd64/cpu/gdt.hpp
@@ -5,6 +5,40 @@
 
 namespace GlobalDescriptorTable
 {
+    /* https://github.com/nanobyte-dev/nanobyte_os/blob/master/src/kernel/arch/i686/gdt.c */
+    /* https://wiki.osdev.org/Global_Descriptor_Table */
+    typedef enum
+    {
+        CODE_READABLE = 0b00000010,
+        DATA_WRITEABLE = 0b00000010,
+
+        CODE_CONFORMING = 0b00000100,
+        DATA_DIRECTION_NORMAL = 0b00000000,
+        DATA_DIRECTION_DOWN = 0b00000100,
+
+        DATA_SEGMENT = 0b00010000,
+        CODE_SEGMENT = 0b00011000,
+
+        DESCRIPTOR_TSS = 0b00000000,
+
+        RING0 = 0b00000000,
+        RING1 = 0b00100000,
+        RING2 = 0b01000000,
+        RING3 = 0b01100000,
+
+        PRESENT = 0b10000000
+    } AccessFlags;
+
+    typedef enum
+    {
+        _64BITS = 0b00100000,
+        _32BITS = 0b01000000,
+        _16BITS = 0b00000000,
+
+        GRANULARITY_1B = 0b00000000,
+        GRANULARITY_4K = 0b10000000
+    } GDTFlags;
+
     typedef struct _TaskStateSegmentEntry
     {
         uint16_t Length;