src/arch/x86/gdt.c

@@ -51,6 +51,43 @@ static void gdt_flush()
     );
 }
 
+/*
+ * get_set_entry - Sets a GDT entry
+ * @num:    Number of the entry (index in GDT)
+ * @flags:  Flags (Granularity, Size, Long mode)
+ * @access: Access byte (contains Descriptor Privilege Level)
+ *
+ * This function fills a GDT entry with the specified @flags
+ * and @access byte. The base and limit fields are left to zero
+ * because we don't use segmentation for memory management.
+ */
+
+static void gdt_set_entry(int num, uint8_t flags, uint8_t access)
+{
+    uint64_t gdt_entry = 0;
+
+    gdt_entry |= (access << 8);
+    gdt_entry |= (flags << 20);
+    
+    // Rest (base, limit) is always zero
+    gdt_entries[num] = gdt_entry << 32;
+}
+
+/*
+ * gdt_set_tss - Setup the TSS entry in the GDT
+ * @num: Number of the entry (index in GDT)
+ *
+ * This function sets up a Task State Segment entry
+ * in the Global Descriptor Table.
+ *
+ * The entry is 128-bit long, so it actually takes
+ * two 64-bit GDT entries.
+ */
+static void gdt_set_tss(int num)
+{
+
+}
+
 /*
  * gdt_init - Global Descriptor Table initialization 
  *
@@ -62,38 +99,11 @@ static void gdt_flush()
  */
 void gdt_init()
 {
-    // Null descriptor (required)
+    gdt_set_entry(0, 0, 0); // Null descriptor  (0x0)
-    gdt_entries[0] = 0;
+    gdt_set_entry(1, 0xA, 0x9B); // Kernel code (0x8)
-
+    gdt_set_entry(2, 0xC, 0x93); // Kernel data (0x10)
-    // Kernel code segment
+    gdt_set_entry(3, 0xA, 0xFB); // User code   (0x18)
-    uint64_t kernel_code = 0;
+    gdt_set_entry(4, 0xC, 0xF3); // User data   (0x20)
-    kernel_code |= 0b1101 << 8; // Selector type: accessed, read-enable, no conforming
-    kernel_code |= 1 << 12; // not a system descriptor
-    kernel_code |= 0 << 13; // DPL field = 0
-    kernel_code |= 1 << 15; // Present
-    kernel_code |= 1 << 21; // Long mode
-
-    // Left shift 32 bits so we place our stuff in the upper 32 bits of the descriptor.
-    // The lower 32 bits contain limit and part of base and therefore are ignored in Long Mode
-    // (because we'll use paging; segmentation is used only for legacy)
-    gdt_entries[1] = kernel_code << 32;
-
-    uint64_t kernel_data = 0;
-    kernel_data |= 0b0011 << 8;
-    kernel_data |= 1 << 12;
-    kernel_data |= 0 << 13;
-    kernel_data |= 1 << 15;
-    kernel_data |= 1 << 21;
-
-    gdt_entries[2] = kernel_data << 32;
-
-    // We re-use the kernel descriptors here, and just update their DPL fields
-    // (Descriptor privilege level) from ring 0 -> to ring 3 (userspace)
-    uint64_t user_code = kernel_code | (3 << 13);
-    gdt_entries[3] = user_code;
-
-    uint64_t user_data = kernel_data | (3 << 13);
-    gdt_entries[4] = user_data;
 
     // The -1 subtraction is some wizardry explained in the OSDev wiki -> GDT
     gdtr.limit = NUM_GDT_ENTRIES * sizeof(uint64_t) - 1;
@@ -103,5 +113,7 @@ void gdt_init()
     gdt_load();
     gdt_flush();
 
+    // Here we should load the task register with TSS selector
+
     DEBUG("GDT initialized");
 }