From 15386c43baca7bcc7f547ee94d675473f6d33970 Mon Sep 17 00:00:00 2001
From: Arnau478 <arnauxabia@gmail.com>
Date: Sat, 18 Jun 2022 01:53:28 +0200
Subject: [PATCH] Implemented GDT

---
 src/kernel/arch/i686/gdt.asm | 31 ++++++++++++
 src/kernel/arch/i686/gdt.c   | 91 ++++++++++++++++++++++++++++++++++++
 src/kernel/arch/i686/gdt.h   |  6 +++
 src/kernel/hal/hal.c         |  2 +
 4 files changed, 130 insertions(+)
 create mode 100644 src/kernel/arch/i686/gdt.asm
 create mode 100644 src/kernel/arch/i686/gdt.c
 create mode 100644 src/kernel/arch/i686/gdt.h

diff --git a/src/kernel/arch/i686/gdt.asm b/src/kernel/arch/i686/gdt.asm
new file mode 100644
index 0000000..0071e50
--- /dev/null
+++ b/src/kernel/arch/i686/gdt.asm
@@ -0,0 +1,31 @@
+[bits 32]
+
+global i686_gdt_load
+i686_gdt_load:
+    ; New call frame
+    push ebp
+    mov ebp, esp
+
+    ; Load GDT
+    mov eax, [ebp + 8]
+    lgdt [eax]
+
+    ; Reload code segment
+    mov eax, [ebp + 12]
+    push eax
+    push .reload_cs
+    retf
+
+.reload_cs:
+    ; Reload data segments
+    mov ax, [ebp + 16]
+    mov dx, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+    mov ss, ax
+
+    ; Restore old call frame
+    mov esp, ebp
+    push ebp
+    ret
diff --git a/src/kernel/arch/i686/gdt.c b/src/kernel/arch/i686/gdt.c
new file mode 100644
index 0000000..c1603e7
--- /dev/null
+++ b/src/kernel/arch/i686/gdt.c
@@ -0,0 +1,91 @@
+#include <stdint.h>
+#include "gdt.h"
+
+typedef struct{
+    uint16_t limit_low;
+    uint16_t base_low;
+    uint8_t base_middle;
+    uint8_t access;
+    uint8_t flags_limit_high;
+    uint8_t base_high;
+} __attribute__((packed)) gdt_entry;
+
+typedef struct{
+    uint16_t limit;
+    gdt_entry *ptr;
+} __attribute__((packed)) gdt_descriptor;
+
+typedef enum{
+    GDT_ACCESS_CODE_READABLE = 0x02,
+    GDT_ACCESS_DATA_WRITEABLE = 0x02,
+    
+    GDT_ACCESS_CODE_CONFORMING = 0x04,
+    GDT_ACCESS_DATA_DIRECTION_NORMAL = 0x00,
+    GDT_ACCESS_DATA_DIRECTION_DOWN = 0x04,
+
+    GDT_ACCESS_DATA_SEGMENT = 0x10,
+    GDT_ACCESS_CODE_SEGMENT = 0x18,
+
+    GDT_ACCESS_DESCRIPTOR_TSS = 0x00,
+    
+    GDT_ACCESS_RING0 = 0x00,
+    GDT_ACCESS_RING1 = 0x20,
+    GDT_ACCESS_RING2 = 0x40,
+    GDT_ACCESS_RING3 = 0x60,
+
+    GDT_ACCESS_PRESENT = 0x80,
+} GDT_ACCESS;
+
+typedef enum{
+    GDT_FLAG_64BIT = 0x20,
+    GDT_FLAG_32BIT = 0x40,
+    GDT_FLAG_16BIT = 0x00,
+
+    GDT_FLAG_GRANULARITY_1B = 0x00,
+    GDT_FLAG_GRANULARITY_4K = 0x80,
+} GDT_FLAGS;
+
+// Helpful macros
+#define GDT_LIMIT_LOW(limit) ((limit) & 0xFFFF)
+#define GDT_BASE_LOW(base) ((base) & 0xFFFF)
+#define GDT_BASE_MIDDLE(base) (((base) >> 16) & 0xFF)
+#define GDT_FLAGS_LIMIT_HIGH(limit, flags) ((((limit) >> 16) & 0xF) | ((flags) & 0xF0))
+#define GDT_BASE_HIGH(base) (((base) >> 24) & 0xFF)
+
+#define GDT_ENTRY(base, limit, access, flags) { \
+    GDT_LIMIT_LOW(limit), \
+    GDT_BASE_LOW(base), \
+    GDT_BASE_MIDDLE(base), \
+    access, \
+    GDT_FLAGS_LIMIT_HIGH(limit, flags), \
+    GDT_BASE_HIGH(base), \
+}
+
+static gdt_entry g_gdt[] = {
+    // Null descriptor
+    GDT_ENTRY(0, 0, 0, 0),
+
+    // Kernel 32-bit code segment
+    GDT_ENTRY(
+        0,
+        0xFFFF,
+        GDT_ACCESS_PRESENT | GDT_ACCESS_RING0 | GDT_ACCESS_CODE_SEGMENT | GDT_ACCESS_CODE_READABLE,
+        GDT_FLAG_32BIT | GDT_FLAG_GRANULARITY_4K
+    ),
+
+    // Kernel 32-bit data segment
+    GDT_ENTRY(
+        0,
+        0xFFFF,
+        GDT_ACCESS_PRESENT | GDT_ACCESS_RING0 | GDT_ACCESS_DATA_SEGMENT | GDT_ACCESS_DATA_WRITEABLE,
+        GDT_FLAG_32BIT | GDT_FLAG_GRANULARITY_4K
+    ),
+};
+
+static gdt_descriptor g_gdt_descriptor = {sizeof(g_gdt) - 1, g_gdt};
+
+void __attribute__((cdecl)) i686_gdt_load(gdt_descriptor *descriptor, uint16_t code_segment, int16_t data_segment);
+
+void i686_gdt_initialize(){
+    i686_gdt_load(&g_gdt_descriptor, i686_GDT_CODE_SEGMENT, i686_GDT_DATA_SEGMENT);
+}
diff --git a/src/kernel/arch/i686/gdt.h b/src/kernel/arch/i686/gdt.h
new file mode 100644
index 0000000..1ea9b51
--- /dev/null
+++ b/src/kernel/arch/i686/gdt.h
@@ -0,0 +1,6 @@
+#pragma once
+
+#define i686_GDT_CODE_SEGMENT 0x08
+#define i686_GDT_DATA_SEGMENT 0x10
+
+void i686_gdt_initialize();
diff --git a/src/kernel/hal/hal.c b/src/kernel/hal/hal.c
index ab8602c..e995c42 100644
--- a/src/kernel/hal/hal.c
+++ b/src/kernel/hal/hal.c
@@ -1,6 +1,8 @@
 #include "hal.h"
+#include "../arch/i686/gdt.h"
 #include "../arch/i686/idt.h"
 
 void hal_initialize(){
+    i686_gdt_initialize();
     i686_idt_initialize();
 }