feat(gdt): add gdt loading

Signed-off-by: Julien CLEMENT <julien.clement@epita.fr>

k/Makefile

@@ -30,7 +30,8 @@ OBJS	= \
 	  libvga.o \
 	  list.o \
 	  memory.o \
-	  serial.o
+	  serial.o \
+	  gdt.o
 
 
 DEPS = $(OBJS:.o=.d)

k/gdt.c

@@ -0,0 +1,74 @@
+#include "gdt.h"
+
+static struct gdt gdt = { 0 };
+
+static void create_kernel_ds()
+{
+}
+
+static void create_kernel_cs()
+{
+}
+
+static void create_gdt()
+{
+    create_kernel_ds();
+    create_kernel_cs();
+}
+
+static void load_gdt()
+{
+    struct gdt_r gdtr;
+    gdtr.addr = (uint32_t)&gdt;
+    gdtr.limit = GDT_SIZE - 1;
+
+    asm volatile("lgdt %0\n"
+            :
+            : "m" (gdtr)
+            : "memory");
+}
+
+static void load_ds()
+{
+    struct segment_selector selector;
+    selector.index = KERNEL_DS_INDEX;
+    selector.table_indicator = 0;
+    selector.rpl = 0;
+
+    asm volatile("movw %0, %%ax\n"
+                 "movw %%ax, %%ds\n"
+                 "movw %%ax, %%fs\n"
+                 "movw %%ax, %%gs\n"
+                 "movw %%ax, %%ss\n"
+            :
+            : "m" (selector)
+            : "ax");
+}
+
+static void load_cs()
+{
+    struct segment_selector selector;
+    selector.index = KERNEL_CS_INDEX;
+    selector.table_indicator = 0;
+    selector.rpl = 0;
+
+    asm volatile("pushl %0\n"
+                 "pushl $1f\n"
+                 "lret\n"
+                 "1:\n"
+             :
+             : "m" (selector));
+}
+
+static void reload_segment_selectors()
+{
+    load_ds();
+    load_cs();
+}
+
+void init_gdt()
+{
+    create_gdt();
+    load_gdt();
+    reload_segment_selectors();
+}

k/gdt.h

@@ -3,6 +3,12 @@
 
 #include <stdint.h>
 
+#define GDT_NB_ENTRIES 3
+#define GDT_SIZE (GDT_NB_ENTRIES * sizeof(struct gdt_entry))
+
+#define KERNEL_CS_INDEX 1
+#define KERNEL_DS_INDEX 2
+
 struct gdt_entry
 {
     uint16_t limit_1    : 16;
@@ -20,4 +26,24 @@ struct gdt_entry
     uint8_t base_3      : 8;
 } __attribute__ ((packed));
 
+struct gdt
+{
+    struct gdt_entry entries[GDT_NB_ENTRIES];
+};
+
+struct gdt_r
+{
+    uint32_t addr;
+    uint16_t limit;
+} __attribute__ ((packed));
+
+struct segment_selector
+{
+    uint8_t rpl             : 2;
+    uint8_t table_indicator : 1;
+    uint16_t index          : 13;
+} __attribute__ ((packed));
+
+void init_gdt();
+
 #endif  /* !GDT_H */

k/serial.c

@@ -24,12 +24,12 @@ void init_serial(void)
 
 int write(const char *buf, size_t count)
 {
-    int sent;
+    size_t sent;
 
     for(sent = 0; sent < count; ++sent)
     {
         u8 line_status = inb(COM1 + 5);
-        if (!line_status & EMPTY_TRANSMITTER)
+        if (!(line_status & EMPTY_TRANSMITTER))
             break;
         outb(COM1, buf[sent]);
     }