Initialize GDT

src/Makefile

@@ -3,7 +3,7 @@ CCPREFIX = i686-elf-
 AS = $(CCPREFIX)as
 CC = $(CCPREFIX)gcc
 
-OBJS = boot.s.o main.c.o logger.c.o console.c.o
+OBJS = boot.s.o main.c.o logger.c.o console.c.o gdt.c.o gdt.asm.o
 
 all: kernel
 
@@ -18,3 +18,6 @@ kernel: $(OBJS)
 
 %.s.o: %.s
 	$(AS) $< -o $@
+
+%.asm.o: %.asm
+	nasm -felf -o $@ $<

src/gdt.asm

@@ -0,0 +1,15 @@
+[GLOBAL gdt_flush]
+
+gdt_flush:
+    mov eax, [esp+4]
+    lgdt [eax]
+
+    mov ax, 0x10
+    mov ds, ax
+    mov es, ax
+    mov fs, ax
+    mov gs, ax
+    mov ss, ax
+    jmp 0x08:.flush
+.flush:
+    ret

src/gdt.c

@@ -0,0 +1,37 @@
+#include "gdt.h"
+
+static struct GdtPointer gdt_pointer;
+
+static struct GdtEntry gdt_entries[5];
+
+static void gdt_set_gate(int32_t num, uint32_t base, uint32_t limit, uint8_t access, uint8_t gran);
+
+void gdt_flush(uint32_t pointer);
+
+void gdt_initialize()
+{
+    gdt_set_gate(0, 0, 0, 0, 0);                // Null segment
+    gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF); // Code segment
+    gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF); // Data segment
+    gdt_set_gate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF); // User mode code segment
+    gdt_set_gate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF); // User mode data segment
+
+    gdt_pointer.limit = sizeof(struct GdtEntry) * 5 - 1;
+    gdt_pointer.base  = (uint32_t)&gdt_entries;
+
+    gdt_flush((uint32_t)&gdt_pointer);
+}
+
+void gdt_set_gate(int32_t num, uint32_t base, uint32_t limit, uint8_t access, uint8_t gran)
+{
+    gdt_entries[num].base_low    = (base & 0xFFFF);
+    gdt_entries[num].base_middle = (base >> 16) & 0xFF;
+    gdt_entries[num].base_high   = (base >> 24) & 0xFF;
+
+    gdt_entries[num].limit_low   = (limit & 0xFFFF);
+    gdt_entries[num].granularity = (limit >> 16) & 0x0F;
+
+    gdt_entries[num].granularity |= gran & 0xF0;
+
+    gdt_entries[num].access = access;
+} 

src/gdt.h

@@ -0,0 +1,24 @@
+#ifndef TAILIX_KERNEL_INCLUDED_GDT
+#define TAILIX_KERNEL_INCLUDED_GDT 1
+
+#include <stdint.h>
+
+struct GdtPointer {
+    uint16_t limit;
+    uint32_t base;
+}
+__attribute__((packed));
+
+struct GdtEntry {
+    uint16_t limit_low;
+    uint16_t base_low;
+    uint8_t  base_middle;
+    uint8_t  access;
+    uint8_t  granularity;
+    uint8_t  base_high;
+}
+__attribute__((packed));
+
+void gdt_initialize();
+
+#endif

src/main.c

@@ -1,9 +1,15 @@
 #include "logger.h"
+#include "gdt.h"
 
 void main()
 {
     logger_initialize();
     logger_info("Kernel initialization started.");
+
+    logger_info("GDT initialization started.");
+    gdt_initialize();
+    logger_info("GDT initialization finished.");
+
     logger_warn("Nothing to do.");
     logger_fail("Halt.");
 }