text: //
  // descriptor_tables.c - Initialises the GDT and IDT, and defines the 
  //                       default ISR and IRQ handler.
  //                       Based on code from Bran's kernel development tutorials.
  //                       Rewritten for JamesM's kernel development tutorials.
  //
  
  #include "common.h"
  #include "descriptor_tables.h"
  #include "isr.h"
  #include <string.h>
  
  // Lets us access our ASM functions from our C code.
  extern void gdt_flush(u32int);
  extern void idt_flush(u32int);
  
  // Internal function prototypes.
  static void init_gdt();
  static void init_idt();
  static void gdt_set_gate(s32int,u32int,u32int,u8int,u8int);
  static void idt_set_gate(u8int,u32int,u16int,u8int);
  
  gdt_entry_t gdt_entries[5];
  gdt_ptr_t   gdt_ptr;
  idt_entry_t idt_entries[256];
  idt_ptr_t   idt_ptr;
  
  // Extern the ISR handler array so we can nullify them on startup.
  extern isr_t interrupt_handlers[];
  
  // Initialisation routine - zeroes all the interrupt service routines,
  // initialises the GDT and IDT.
  void init_descriptor_tables()
  {
  
      // Initialise the global descriptor table.
      init_gdt();
      // Initialise the interrupt descriptor table.
      init_idt();
      // Nullify all the interrupt handlers.
      memset(&interrupt_handlers, 0, sizeof(isr_t)*256);
  }
  
  static void init_gdt()
  {
      gdt_ptr.limit = (sizeof(gdt_entry_t) * 5) - 1;
      gdt_ptr.base  = (u32int)&gdt_entries;
  
      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_flush((u32int)&gdt_ptr);
  }
  
  // Set the value of one GDT entry.
  static void gdt_set_gate(s32int num, u32int base, u32int limit, u8int access, u8int 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;
  }
  
  static void init_idt()
  {
      idt_ptr.limit = sizeof(idt_entry_t) * 256 -1;
      idt_ptr.base  = (u32int)&idt_entries;
  
      memset(&idt_entries, 0, sizeof(idt_entry_t)*256);
  
      // Remap the irq table.
      outb(0x20, 0x11);
      outb(0xA0, 0x11);
      outb(0x21, 0x20);
      outb(0xA1, 0x28);
      outb(0x21, 0x04);
      outb(0xA1, 0x02);
      outb(0x21, 0x01);
      outb(0xA1, 0x01);
      outb(0x21, 0x0);
      outb(0xA1, 0x0);
  
      idt_set_gate( 0, (u32int)isr0 , 0x08, 0x8E);
      idt_set_gate( 1, (u32int)isr1 , 0x08, 0x8E);
      idt_set_gate( 2, (u32int)isr2 , 0x08, 0x8E);
      idt_set_gate( 3, (u32int)isr3 , 0x08, 0x8E);
      idt_set_gate( 4, (u32int)isr4 , 0x08, 0x8E);
      idt_set_gate( 5, (u32int)isr5 , 0x08, 0x8E);
      idt_set_gate( 6, (u32int)isr6 , 0x08, 0x8E);
      idt_set_gate( 7, (u32int)isr7 , 0x08, 0x8E);
      idt_set_gate( 8, (u32int)isr8 , 0x08, 0x8E);
      idt_set_gate( 9, (u32int)isr9 , 0x08, 0x8E);
      idt_set_gate(10, (u32int)isr10, 0x08, 0x8E);
      idt_set_gate(11, (u32int)isr11, 0x08, 0x8E);
      idt_set_gate(12, (u32int)isr12, 0x08, 0x8E);
      idt_set_gate(13, (u32int)isr13, 0x08, 0x8E);
      idt_set_gate(14, (u32int)isr14, 0x08, 0x8E);
      idt_set_gate(15, (u32int)isr15, 0x08, 0x8E);
      idt_set_gate(16, (u32int)isr16, 0x08, 0x8E);
      idt_set_gate(17, (u32int)isr17, 0x08, 0x8E);
      idt_set_gate(18, (u32int)isr18, 0x08, 0x8E);
      idt_set_gate(19, (u32int)isr19, 0x08, 0x8E);
      idt_set_gate(20, (u32int)isr20, 0x08, 0x8E);
      idt_set_gate(21, (u32int)isr21, 0x08, 0x8E);
      idt_set_gate(22, (u32int)isr22, 0x08, 0x8E);
      idt_set_gate(23, (u32int)isr23, 0x08, 0x8E);
      idt_set_gate(24, (u32int)isr24, 0x08, 0x8E);
      idt_set_gate(25, (u32int)isr25, 0x08, 0x8E);
      idt_set_gate(26, (u32int)isr26, 0x08, 0x8E);
      idt_set_gate(27, (u32int)isr27, 0x08, 0x8E);
      idt_set_gate(28, (u32int)isr28, 0x08, 0x8E);
      idt_set_gate(29, (u32int)isr29, 0x08, 0x8E);
      idt_set_gate(30, (u32int)isr30, 0x08, 0x8E);
      idt_set_gate(31, (u32int)isr31, 0x08, 0x8E);
      idt_set_gate(32, (u32int)irq0, 0x08, 0x8E);
      idt_set_gate(33, (u32int)irq1, 0x08, 0x8E);
      idt_set_gate(34, (u32int)irq2, 0x08, 0x8E);
      idt_set_gate(35, (u32int)irq3, 0x08, 0x8E);
      idt_set_gate(36, (u32int)irq4, 0x08, 0x8E);
      idt_set_gate(37, (u32int)irq5, 0x08, 0x8E);
      idt_set_gate(38, (u32int)irq6, 0x08, 0x8E);
      idt_set_gate(39, (u32int)irq7, 0x08, 0x8E);
      idt_set_gate(40, (u32int)irq8, 0x08, 0x8E);
      idt_set_gate(41, (u32int)irq9, 0x08, 0x8E);
      idt_set_gate(42, (u32int)irq10, 0x08, 0x8E);
      idt_set_gate(43, (u32int)irq11, 0x08, 0x8E);
      idt_set_gate(44, (u32int)irq12, 0x08, 0x8E);
      idt_set_gate(45, (u32int)irq13, 0x08, 0x8E);
      idt_set_gate(46, (u32int)irq14, 0x08, 0x8E);
      idt_set_gate(47, (u32int)irq15, 0x08, 0x8E);
  
      idt_flush((u32int)&idt_ptr);
  }
  
  static void idt_set_gate(u8int num, u32int base, u16int sel, u8int flags)
  {
      idt_entries[num].base_lo = base & 0xFFFF;
      idt_entries[num].base_hi = (base >> 16) & 0xFFFF;
  
      idt_entries[num].sel     = sel;
      idt_entries[num].always0 = 0;
      // We must uncomment the OR below when we get to using user-mode.
      // It sets the interrupt gate's privilege level to 3.
      idt_entries[num].flags   = flags /* | 0x60 */;
  }
  