From 0787b1a1fc4a7ee003418c4ee56c00b0ded6bd34 Mon Sep 17 00:00:00 2001
From: starnakin <camille@chauvet.pro>
Date: Mon, 9 Sep 2024 18:00:17 +0200
Subject: [PATCH] add: gdt (peut etre y'a un gdt on sait pas)

---
 headers/gdt.h  | 18 +++++++++++++
 src/gdt/gdt.c  | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++
 src/gdt/sgdt.s | 12 +++++++++
 src/kernel.c   |  3 ++-
 4 files changed, 102 insertions(+), 1 deletion(-)
 create mode 100644 headers/gdt.h
 create mode 100644 src/gdt/gdt.c
 create mode 100644 src/gdt/sgdt.s

diff --git a/headers/gdt.h b/headers/gdt.h
new file mode 100644
index 0000000..53b61e3
--- /dev/null
+++ b/headers/gdt.h
@@ -0,0 +1,18 @@
+#pragma once
+
+#include <stdint.h>
+
+struct gdt_entry {
+    uint32_t limit;
+    uint32_t base;
+    uint8_t access_byte;
+    uint8_t flags;
+};
+
+// https://wiki.osdev.org/Global_Descriptor_Table#GDTR
+struct gdt_descriptor {
+    uint16_t size;
+    uint32_t base;
+} __attribute__((packed));
+
+void initGdt();
\ No newline at end of file
diff --git a/src/gdt/gdt.c b/src/gdt/gdt.c
new file mode 100644
index 0000000..d0d0c2c
--- /dev/null
+++ b/src/gdt/gdt.c
@@ -0,0 +1,70 @@
+#include "gdt.h"
+#include "kprintf.h"
+
+extern void setGdt(void *);
+
+void encodeGdtEntry(uint8_t *target, struct gdt_entry source)
+{
+    // Check the limit to make sure that it can be encoded
+    if (source.limit > 0xFFFFF) {
+        kprintf(KERN_ERR, "GDT cannot encode limits larger than 0xFFFFF");
+    }
+    
+    // Encode the limit
+    target[0] = source.limit & 0xFF;
+    target[1] = (source.limit >> 8) & 0xFF;
+    target[6] = (source.limit >> 16) & 0x0F;
+    
+    // Encode the base
+    target[2] = source.base & 0xFF;
+    target[3] = (source.base >> 8) & 0xFF;
+    target[4] = (source.base >> 16) & 0xFF;
+    target[7] = (source.base >> 24) & 0xFF;
+    
+    // Encode the access byte
+    target[5] = source.access_byte;
+    
+    // Encode the flags
+    target[6] |= (source.flags << 4);
+}
+//                  sizeof(Segment Descriptor) * nb(Segment Descriptor)
+uint8_t *gdt_entries = (uint8_t *) 0x00000800;
+struct gdt_descriptor gdtr; 
+
+void initGdt()
+{
+    gdtr.size = 8 * 5 - 1;
+    gdtr.base = (uint32_t) &gdt_entries;
+
+    struct gdt_entry gdt_entry_null_descriptor = { .base = 0,
+                                                   .limit = 0x00000000,
+                                                   .access_byte = 0x00,
+                                                   .flags = 0x0};
+    encodeGdtEntry(gdt_entries + 0x0000, gdt_entry_null_descriptor);
+
+    struct gdt_entry gdt_entry_kernel_mode_code_segment = { .base = 0,
+                                                            .limit = 0xFFFFF,
+                                                            .access_byte = 0x9A,
+                                                            .flags = 0xC };
+    encodeGdtEntry(gdt_entries + 0x0008, gdt_entry_kernel_mode_code_segment);
+
+    struct gdt_entry gdt_entry_kernel_mode_data_segment = { .base = 0,
+                                                            .limit = 0xFFFFF,
+                                                            .access_byte = 0x92,
+                                                            .flags = 0xC };
+    encodeGdtEntry(gdt_entries + 0x0010, gdt_entry_kernel_mode_data_segment);
+
+    struct gdt_entry gdt_entry_user_mode_code_segment = { .base = 0,
+                                                          .limit = 0xFFFFF,
+                                                          .access_byte = 0xFA,
+                                                          .flags = 0xC };
+    encodeGdtEntry(gdt_entries + 0x0018, gdt_entry_user_mode_code_segment);
+
+    struct gdt_entry gdt_entry_user_mode_data_segment = { .base = 0,
+                                                          .limit = 0xFFFFF,
+                                                          .access_byte = 0xF2,
+                                                          .flags = 0xC };
+    encodeGdtEntry(gdt_entries + 0x0020, gdt_entry_user_mode_data_segment);
+
+    setGdt(&gdtr);
+}
\ No newline at end of file
diff --git a/src/gdt/sgdt.s b/src/gdt/sgdt.s
new file mode 100644
index 0000000..87829d9
--- /dev/null
+++ b/src/gdt/sgdt.s
@@ -0,0 +1,12 @@
+.globl setGdt
+
+gdtr: .word 0          # For limit storage
+      .long 0          # For base storage
+
+setGdt:
+   movw 4(%esp), %ax   # Move the word at [esp + 4] into AX
+   movw %ax, gdtr      # Move AX into gdtr
+   movl 8(%esp), %eax  # Move the double word at [esp + 8] into EAX
+   movl %eax, gdtr + 2 # Move EAX into gdtr + 2
+   lgdt gdtr           # Load the GDT
+   ret
\ No newline at end of file
diff --git a/src/kernel.c b/src/kernel.c
index 5d5cf31..800cd23 100644
--- a/src/kernel.c
+++ b/src/kernel.c
@@ -22,8 +22,9 @@ void kernel_main(void)
 	terminal_initialize();
 
 	/* Newline support is left as an exercise. */
-	for (int i = 1001; i; i--)
+	for (int i = 100; i; i--)
 		kprintf(0, "%d\n", i);
+	void initGdt();
 	kprintf(0, "mange ta mere avec ton argument a kprintf fdp\n");
 	while (true)
 		terminal_getkey();