src/descriptor_tables.c

#include "descriptor_tables.h"
#include "vga.h"
#include "mem.h"

extern void gdt_flush(uint gdt);
extern void idt_flush(uint idt);

static void gdt_set_gate(uint i, uint base, uint limit, uchar access, uchar gran);
static void idt_set_gate(uchar num, uint base, ushort selector, uchar flags);

struct gdt_entry gdt_entries[5];
struct gdt_pointer gdt_pointer;

struct idt_entry idt_entries[256];
struct idt_pointer idt_pointer;

static void (* isrs[32])() =
{
	isr0,
	isr1,
	isr2,
	isr3,
	isr4,
	isr5,
	isr6,
	isr7,
	isr8,
	isr9,
	isr10,
	isr11,
	isr12,
	isr13,
	isr14,
	isr15,
	isr16,
	isr17,
	isr18,
	isr19,
	isr20,
	isr21,
	isr22,
	isr23,
	isr24,
	isr25,
	isr26,
	isr27,
	isr28,
	isr29,
	isr30,
	isr31
};

void init_gdt()
{
	vga_write("Initializing GDT...\n");
	gdt_pointer.limit = sizeof(struct gdt_entry) * 5 - 1;
	gdt_pointer.base = (uint)&gdt_entries;

	gdt_set_gate(0, 0, 0, 0, 0);          // Null segment
	gdt_set_gate(1, 0, ~0, 0x9a, 0xcf);   // Code segment
	gdt_set_gate(2, 0, ~0, 0x92, 0xcf);   // Data segment
	gdt_set_gate(3, 0, ~0, 0xfa, 0xcf);   // User mode code segment
	gdt_set_gate(4, 0, ~0, 0xf2, 0xcf);   // User mode data segment

	for (volatile uint i = 0; i < 0x1000; i++)
	{} // waste some time, for some reason this helps
	
	gdt_flush((uint) &gdt_pointer);

	vga_write("GDT Initialized\n");
}

static void gdt_set_gate(uint i, uint base, uint limit, uchar access, uchar gran)
{
	struct gdt_entry *e = &gdt_entries[i];

	e->base_low = base & 0xffff;
	e->base_middle = (base >> 16) & 0xff;
	e->base_high = (base >> 24) & 0xff;

	e->limit_low = limit & 0xffff;
	e->granularity = ((limit >> 16) & 0x0f) | (gran & 0xf0);

	e->access = access;
}

void init_idt()
{
	idt_pointer.limit = sizeof(struct idt_entry) * 256 - 1;
	idt_pointer.base = (uint)&idt_entries;

	memset(&idt_entries, 0, sizeof(idt_entries));

	vga_write("sizeof(idt_entries) = ");
	vga_putx(sizeof(idt_entries));
	vga_put('\n');

	vga_write("isr0 = ");
	vga_putx((uint)isrs[0]);
	vga_put('\n');

	for (int i = 0; i < 32; i++)
	{
		idt_set_gate(i, (uint)isrs[i], 0x08, 0x8e);
	}

	idt_flush((uint)&idt_pointer);

	vga_write("IDT Initialized!\n");
}

static void idt_set_gate(uchar num, uint base, ushort selector, uchar flags)
{
	struct idt_entry *e = &idt_entries[num];

	e->base_low = base & 0xffff;
	e->base_high = (base >> 16) & 0xffff;

	e->selector = selector;
	e->zero = 0;
	e->flags = flags /* | 0x60 */;
}

void init_descriptor_tables()
{
	init_gdt();
	init_idt();
}