kernel: Properly initialize GDT

6 files changed, 65 insertions, 12 deletions

diff --git a/kernel/CMakeLists.txt b/kernel/CMakeLists.txt
index d2a019b..94d0e15 100644
--- a/kernel/CMakeLists.txt
+++ b/kernel/CMakeLists.txt
@@ -4,6 +4,7 @@ project(kernel C ASM)
 set(KERNEL_SRC
   boot/init/boot.s
   boot/gdt.c
+  boot/gdt.s
 
   kernel/kernel.c
   kernel/halt.c
diff --git a/kernel/boot/gdt.c b/kernel/boot/gdt.c
index 34bbfe3..4644b3d 100644
--- a/kernel/boot/gdt.c
+++ b/kernel/boot/gdt.c
@@ -18,6 +18,8 @@
 
 #include <stdint.h>
 
+#include <kernel/io.h>
+
 #include <boot/gdt.h>
 
 GDT_entry GDT[] = {
@@ -28,7 +30,22 @@ GDT_entry GDT[] = {
   GDT_ENTRY(0, 0xfffff, KERNEL_CODE_SEGMENT_ACCESS_FLAGS, FLAGS),
 
   /* Kernel Mode Data Segment */
-  GDT_ENTRY(0, 0xfffff, KERNEL_DATA_SEGMENT_ACCESS_FLAGS, FLAGS)
+  GDT_ENTRY(0, 0xfffff, KERNEL_DATA_SEGMENT_ACCESS_FLAGS, FLAGS),
+
+  /* User Mode Code Segment */
+  GDT_ENTRY(0, 0xfffff, USER_CODE_SEGMENT_ACCESS_FLAGS, FLAGS),
+
+  /* User Mode Data Segment */
+  GDT_ENTRY(0, 0xfffff, USER_DATA_SEGMENT_ACCESS_FLAGS, FLAGS)
+
+  /* TODO: TSS? */
+  /* TODO: LDT? */
 };
 
-GDT_descriptor g_GDT_descriptor = { sizeof(GDT) - 1, GDT };
+GDT_descriptor g_GDT_descriptor = { sizeof(GDT_entry) * 5 - 1, &GDT[0] };
+
+void
+GDT_load(void)
+{
+  _GDT_flush(&g_GDT_descriptor);
+}
diff --git a/kernel/boot/gdt.s b/kernel/boot/gdt.s
new file mode 100644
index 0000000..ee50e4e
--- /dev/null
+++ b/kernel/boot/gdt.s
@@ -0,0 +1,26 @@
+.intel_syntax noprefix
+
+.global _GDT_flush
+.type _GDT_flush, @function
+_GDT_flush:
+  /* First Argument (Pointer to the GDT) */
+  mov eax, [esp + 4]
+  
+  /* Load GDT */
+  lgdt [eax]
+
+  /* Offset For Kernel Data Segment (16 bits) */
+  mov eax, 0x10
+
+  /* Set the Data Segment Selectors */
+  mov ds, ax
+  mov es, ax
+  mov fs, ax
+  mov gs, ax
+  mov ss, ax
+
+  /* Set the Code Segment Selector */
+  jmp 0x08:.flush
+
+.flush:
+  ret
diff --git a/kernel/boot/init/boot.s b/kernel/boot/init/boot.s
index 32ebe8f..35a89a2 100644
--- a/kernel/boot/init/boot.s
+++ b/kernel/boot/init/boot.s
@@ -96,8 +96,6 @@ _start:
   runtime support to work as well.
   */
 
-  /* TODO: Initialize global descriptor table */
-
   /* Call the global constructors. */
   call _init
 
diff --git a/kernel/include/boot/gdt.h b/kernel/include/boot/gdt.h
index b07d64d..7cf7d35 100644
--- a/kernel/include/boot/gdt.h
+++ b/kernel/include/boot/gdt.h
@@ -19,6 +19,8 @@
 #ifndef __boot_gdt_h
 #define __boot_gdt_h
 
+#include <stdint.h>
+
 /* Access Flags:
  * 7 PRESENT
  * 6 PRIVILEGE
@@ -38,14 +40,20 @@
 #define READABLE_WRITABLE(x) (x << 1)
 #define ACCESSED(x) x
 
-#define KERNEL_CODE_SEGMENT_ACCESS_FLAGS                                      \
-  PRESENT(1) | PRIVILEGE(0) | TYPE(1) | EXECUTABLE(1)                         \
+#define CODE_SEGMENT_ACCESS_FLAGS(privilege)                                  \
+  PRESENT(1) | PRIVILEGE(privilege) | TYPE(1) | EXECUTABLE(1)                 \
       | DIRECTION_CONFORMING(0) | READABLE_WRITABLE(1) | ACCESSED(0)
 
-#define KERNEL_DATA_SEGMENT_ACCESS_FLAGS                                      \
-  PRESENT(1) | PRIVILEGE(0) | TYPE(1) | EXECUTABLE(0)                         \
+#define DATA_SEGMENT_ACCESS_FLAGS(privilege)                                  \
+  PRESENT(1) | PRIVILEGE(privilege) | TYPE(1) | EXECUTABLE(0)                 \
       | DIRECTION_CONFORMING(0) | READABLE_WRITABLE(1) | ACCESSED(0)
 
+#define KERNEL_CODE_SEGMENT_ACCESS_FLAGS CODE_SEGMENT_ACCESS_FLAGS(0)
+#define KERNEL_DATA_SEGMENT_ACCESS_FLAGS DATA_SEGMENT_ACCESS_FLAGS(0)
+
+#define USER_CODE_SEGMENT_ACCESS_FLAGS CODE_SEGMENT_ACCESS_FLAGS(3)
+#define USER_DATA_SEGMENT_ACCESS_FLAGS DATA_SEGMENT_ACCESS_FLAGS(3)
+
 /* Other Flags:
  * 3 GRANULARITY
  * 2 SIZE
@@ -92,7 +100,7 @@ typedef struct {
   GDT_entry *ptr; /* Address of GDT */
 } __attribute__((packed)) GDT_descriptor;
 
-/* TODO: Implement GDT_load() */
-void __attribute((cdecl)) GDT_load(void);
+extern void _GDT_flush(GDT_descriptor *);
+void GDT_load(void);
 
 #endif
diff --git a/kernel/kernel/kernel.c b/kernel/kernel/kernel.c
index 20e5bb4..7deb6ec 100644
--- a/kernel/kernel/kernel.c
+++ b/kernel/kernel/kernel.c
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <boot/gdt.h>
 #include <kernel/halt.h>
 
 #include <libk/io.h>
@@ -26,11 +27,13 @@
 void
 kernel_main(void)
 {
-  vga_text_buffer_initialize();
-  serial_initialize();
+  GDT_load();
 
   printk("Kernel", "Started.");
 
+  vga_text_buffer_initialize();
+  serial_initialize();
+
   // exit();
   halt(); /* If exit() fails (on real hardware) */
 }