Added gdt first draft

kernel/Makefile

@@ -30,6 +30,7 @@ LIBS:=$(LIBS) $(KERNEL_ARCH_LIBS)
 KERNEL_OBJS=\
 $(KERNEL_ARCH_OBJS) \
 kernel/kernel.o \
+gdt/gdt.o
 
 OBJS=\
 $(ARCHDIR)/crti.o \

kernel/arch/i386/boot.S

@@ -26,6 +26,7 @@ stack_top:
 _start:
     movl $stack_top, %esp
     call _init
+    call gdt_init
     call kernel_main
     cli
 1: hlt

kernel/gdt/gdt.c

@@ -0,0 +1,42 @@
+#include<string.h>
+#include "gdt.h"
+#include<stdio.h>
+struct gdtdesc kgdt[GDTSIZE];
+struct gdtr kgdtr;
+
+
+void read_gdtr(uint32_t* tgdtr) {
+    asm("sgdt %0" : "=m" (*tgdtr));
+}
+void gdt_init(void) {
+
+	create_gdt_descriptor(0x0, 0x0, 0x0, 0x0, &kgdt[0]);
+	create_gdt_descriptor(0x0, 0xFFFFF, 0x9A, 0x0C, &kgdt[1]);
+	create_gdt_descriptor(0x0, 0xFFFFF, 0x92, 0x0C, &kgdt[2]);
+	create_gdt_descriptor(0x0, 0xFFFFF, 0xFA, 0x0C, &kgdt[3]);
+	create_gdt_descriptor(0x0, 0xFFFFF, 0xF2, 0x0C, &kgdt[4]);   
+//    	create_gdt_descriptor(0x0, 0xFFFFF, 0xF2, 0x0D, &kgdt[5]);   
+	//create_gdt_descriptor(0x0, 0x0, 0xF7, 0x0D, &kgdt[6]); 
+	kgdtr.limit = GDTSIZE*8;
+	kgdtr.base = GDT_BASE;
+	memcpy((char*) kgdtr.base, (char *) kgdt, kgdtr.limit);
+	asm("lgdtl (kgdtr)");
+	asm("movw $0x10, %ax \n \
+		movw %ax, %ds \n \
+		movw %ax, %es \n \
+		movw %ax, %fs \n \
+		movw %ax, %gs \n \
+		ljmp $0x08, $next \n \
+		next: \n	");
+}
+
+void create_gdt_descriptor(uint32_t base, uint32_t limit, uint8_t access, uint8_t flags, struct gdtdesc *desc) {
+	desc->limit0_15 = (limit & 0xffff);
+	desc->base0_15 = (base & 0xffff);
+	desc->base16_23 = (base & 0xff0000) >> 16;
+	desc->access_bytes = access;
+	desc->limit16_19 = (limit & 0xf0000) >> 16;
+	desc->flags = (flags & 0xf);
+	desc->base24_31 = (base & 0xff000000) >> 24;
+	return;
+}

kernel/gdt/gdt.h

@@ -0,0 +1,37 @@
+#ifndef _GDT_H
+#define _GDT_H
+
+#include<stddef.h>
+#include<stdint.h>
+
+struct gdtr {
+	uint16_t limit;
+	uint32_t base;
+} __attribute__ ((packed));
+
+struct gdtdesc {
+	uint16_t limit0_15;
+	uint16_t base0_15;
+	uint8_t base16_23;
+	uint8_t access_bytes;
+	uint8_t flags:4;
+	uint8_t limit16_19;
+	uint8_t base24_31;
+} __attribute__ ((packed));
+
+
+#define GDTSIZE 0xFF
+#define GDT_BASE 0x00000
+#define DESC_NULL 0x0000
+#define DESC_KERNEL_CODE 0x0008
+#define DESC_KERNEL_DATA 0x0010
+#define DESC_USER_CODE 0x0018
+#define DESC_USER_DATA 0x0020
+
+
+void create_gdt_descriptor(uint32_t base, uint32_t limit, uint8_t access, uint8_t flags, struct gdtdesc *);
+
+void init_gdt(void);
+
+
+#endif