Add: GDT, entry initializing for code and data segments

gdt.c

@@ -0,0 +1,48 @@
+#include "gdt.h"
+
+struct gdt_entry
+{
+	unsigned short limit_low;
+	unsigned short base_low;
+	unsigned char base_middle;
+	unsigned char access;
+	unsigned char granularity;
+	unsigned char base_high;
+} __attribute__((packed));
+
+struct gdt_ptr
+{
+	unsigned short limit;
+	unsigned int base;
+} __attribute__((packed));
+
+struct gdt_entry gdt[3];
+struct gdt_ptr gp;
+
+extern void gdt_flush();
+
+void gdt_set_gate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran)
+{
+	gdt[num].base_low = (base & 0xFFFF);
+	gdt[num].base_middle = (base >> 16) & 0xFF;
+	gdt[num].base_high = (base >> 24) & 0xFF;
+
+	gdt[num].limit_low = (limit & 0xFFFF);
+	gdt[num].granularity = ((limit >> 16) & 0x0F);
+
+	gdt[num].granularity |= (gran & 0xF0);
+	gdt[num].access = access;
+}
+
+void gdt_install()
+{
+	gp.limit = (sizeof(struct gdt_entry)*3) - 1;
+	gp.base = (unsigned int)&gdt;
+
+	gdt_set_gate(0, 0, 0, 0, 0);
+
+	gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
+	gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
+
+	gdt_flush();
+}

gdt.h

@@ -0,0 +1,8 @@
+#ifndef GDT_H
+#define GDT_H
+
+void gdt_set_gate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran);
+
+void gdt_install();
+
+#endif

kmain.c

@@ -1,14 +1,19 @@
 #include "stdio.h"
 #include "serial.h"
+#include "gdt.h"
 
 int kmain(int retvalue)
 {
   
+  gdt_install();
+
   // serial testing
 
   init_serial();
+  
   log("serial connection established", 3);
-  log("Kernel started", 2);
+  log("initialized GDT entries", 2);
+  log("kernel started", 2);
 
   clear();
 

loader.s

@@ -14,6 +14,7 @@ align 4
 extern kmain
 
 loader:
+  cli
   ; mov eax, 0xCAFEBABE
   push dword 42
   call kmain
@@ -21,6 +22,22 @@ loader:
 .loop:
   jmp .loop
 
+global gdt_flush
+extern gp
+
+gdt_flush:
+	lgdt [gp]
+	mov ax, 0x10
+	mov ds, ax
+	mov es, ax
+	mov fs, ax
+	mov gs, ax
+	mov ss, ax
+	jmp 0x08:flush2
+
+flush2:
+	ret
+
 section .bss
 align 4 
 kernel_stack:

makefile

@@ -1,4 +1,4 @@
-OBJECTS = loader.o kmain.o stdio.o io.o string.o serial.o 
+OBJECTS = loader.o kmain.o stdio.o io.o string.o serial.o gdt.o 
 CC = gcc
 CFLAGS = -m32 -nostdlib -nostdinc -fno-builtin -fno-stack-protector -nostartfiles -nodefaultlibs -Wall -Wextra -c
 LDFLAGS = -T link.ld -melf_i386