add gdt setup

Makefile

@@ -1,5 +1,5 @@
 # Define the object files
-OBJS = bin/boot.o bin/kernel.o bin/idt.o bin/isr.o bin/screen.o bin/utils.o bin/string.o bin/stdio.o
+OBJS = bin/boot.o bin/kernel.o bin/idt.o bin/isr.o bin/screen.o bin/utils.o bin/string.o bin/stdio.o bin/gdt.o
 
 # Define the compiler and assembler
 CC = gcc -D__is_katauos
@@ -45,6 +45,9 @@ bin/string.o: src/string.c
 bin/stdio.o: src/stdio.c
 	$(CC) $(CFLAGS) -c $< -o $@
 
+bin/gdt.o: src/gdt.c
+	$(CC) $(CFLAGS) -c $< -o $@
+
 clean:
 	rm -rf bin/
 	rm -f $(OUTPUT)

include/gdt.h

@@ -0,0 +1,6 @@
+#ifndef GDT_H
+#define GDT_H
+
+void gdt_install();
+
+#endif

src/gdt.c

@@ -0,0 +1,84 @@
+#include "../include/gdt.h"
+#include "../include/stdio.h"
+#include <stdint.h>
+
+struct gdt_entry {
+    uint64_t descriptor;
+};
+
+struct gdt_entry gdt[5];
+
+// Each define here is for a specific flag in the descriptor.
+// Refer to the intel documentation for a description of what each one does.
+#define SEG_DESCTYPE(x)  ((x) << 0x04) // Descriptor type (0 for system, 1 for code/data)
+#define SEG_PRES(x)      ((x) << 0x07) // Present
+#define SEG_SAVL(x)      ((x) << 0x0C) // Available for system use
+#define SEG_LONG(x)      ((x) << 0x0D) // Long mode
+#define SEG_SIZE(x)      ((x) << 0x0E) // Size (0 for 16-bit, 1 for 32)
+#define SEG_GRAN(x)      ((x) << 0x0F) // Granularity (0 for 1B - 1MB, 1 for 4KB - 4GB)
+#define SEG_PRIV(x)     (((x) &  0x03) << 0x05)   // Set privilege level (0 - 3)
+ 
+#define SEG_DATA_RD        0x00 // Read-Only
+#define SEG_DATA_RDA       0x01 // Read-Only, accessed
+#define SEG_DATA_RDWR      0x02 // Read/Write
+#define SEG_DATA_RDWRA     0x03 // Read/Write, accessed
+#define SEG_DATA_RDEXPD    0x04 // Read-Only, expand-down
+#define SEG_DATA_RDEXPDA   0x05 // Read-Only, expand-down, accessed
+#define SEG_DATA_RDWREXPD  0x06 // Read/Write, expand-down
+#define SEG_DATA_RDWREXPDA 0x07 // Read/Write, expand-down, accessed
+#define SEG_CODE_EX        0x08 // Execute-Only
+#define SEG_CODE_EXA       0x09 // Execute-Only, accessed
+#define SEG_CODE_EXRD      0x0A // Execute/Read
+#define SEG_CODE_EXRDA     0x0B // Execute/Read, accessed
+#define SEG_CODE_EXC       0x0C // Execute-Only, conforming
+#define SEG_CODE_EXCA      0x0D // Execute-Only, conforming, accessed
+#define SEG_CODE_EXRDC     0x0E // Execute/Read, conforming
+#define SEG_CODE_EXRDCA    0x0F // Execute/Read, conforming, accessed
+ 
+#define GDT_CODE_PL0 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
+                     SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
+                     SEG_PRIV(0)     | SEG_CODE_EXRD
+ 
+#define GDT_DATA_PL0 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
+                     SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
+                     SEG_PRIV(0)     | SEG_DATA_RDWR
+ 
+#define GDT_CODE_PL3 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
+                     SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
+                     SEG_PRIV(3)     | SEG_CODE_EXRD
+ 
+#define GDT_DATA_PL3 SEG_DESCTYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
+                     SEG_LONG(0)     | SEG_SIZE(1) | SEG_GRAN(1) | \
+                     SEG_PRIV(3)     | SEG_DATA_RDWR
+
+void create_descriptor(uint32_t base, uint32_t limit, uint16_t flag)
+{
+    uint64_t descriptor;
+ 
+    // Create the high 32 bit segment
+    descriptor  =  limit       & 0x000F0000;         // set limit bits 19:16
+    descriptor |= (flag <<  8) & 0x00F0FF00;         // set type, p, dpl, s, g, d/b, l and avl fields
+    descriptor |= (base >> 16) & 0x000000FF;         // set base bits 23:16
+    descriptor |=  base        & 0xFF000000;         // set base bits 31:24
+ 
+    // Shift by 32 to allow for low part of segment
+    descriptor <<= 32;
+ 
+    // Create the low 32 bit segment
+    descriptor |= base  << 16;                       // set base bits 15:0
+    descriptor |= limit  & 0x0000FFFF;               // set limit bits 15:0
+ 
+    printf("0x%.16llX\n", descriptor);
+}
+
+void gdt_install()
+{
+	create_descriptor(0, 0, 0);
+	create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL0));
+	create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL0));
+	create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL3));
+	create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL3));
+
+	__asm__ __volatile__("lgdt (%0)" : : "r" (gdt));
+	printf("gdt_install done\n");
+}

src/kernel.c

@@ -1,4 +1,5 @@
 #include "../include/isr.h"
+#include "../include/gdt.h"
 #include "../include/screen.h"
 #include "../include/stdio.h"
 #include "../include/utils.h"
@@ -18,6 +19,7 @@ void kmain()
 {
 	terminal_initialize();
 	printf("KatauOS booting up\n");
+	gdt_install();
 	isr_install();
 	printf("Kernel init sequence completed\n");