#include "gdt.h"

static struct gdt gdt = { 0 };

static void create_kernel_ds(void)
{
    struct gdt_entry *entry = gdt.entries + KERNEL_DS_INDEX;

    uint32_t limit = 0xfffff;
    uint32_t base = 0x0;

    entry->granularity = 1;
    entry->db = 1;
    entry->l = 0;
    entry->available = 0;

    entry->present = 1;
    entry->desc_priv = 0;
    entry->desc_type = 1;
    entry->ex = 0;
    entry->dc = 0;
    entry->rw = 1;
    entry->accessed = 0;

    entry->limit_1 = limit & 0xffff;
    entry->limit_2 = (limit >> 16) & 0xf;

    entry->base_1 = base & 0xffff;
    entry->base_2 = (base >> 16) & 0xff;
    entry->base_3 = (base >> 24) & 0xff;
}

static void create_kernel_cs(void)
{
    struct gdt_entry *entry = gdt.entries + KERNEL_CS_INDEX;

    uint32_t limit = 0xfffff;
    uint32_t base = 0x0;

    entry->granularity = 1;
    entry->db = 1;
    entry->l = 0;
    entry->available = 0;

    entry->present = 1;
    entry->desc_priv = 0;
    entry->desc_type = 1;
    entry->ex = 1;
    entry->dc = 0;
    entry->rw = 1;
    entry->accessed = 0;

    entry->limit_1 = limit & 0xffff;
    entry->limit_2 = (limit >> 16) & 0xf;

    entry->base_1 = base & 0xffff;
    entry->base_2 = (base >> 16) & 0xff;
    entry->base_3 = (base >> 24) & 0xff;
}

static void create_gdt(void)
{
    create_kernel_ds();
    create_kernel_cs();
}

static void load_gdt(void)
{
    struct gdt_r gdtr;
    gdtr.addr = (uint32_t)&gdt;
    gdtr.limit = GDT_SIZE - 1;

    asm volatile("lgdt %0\n"
            :
            : "m" (gdtr)
            : "memory");
}

static void load_ds(void)
{
    struct segment_selector selector = KERNEL_DS_SEGMENT_SELECTOR;

    asm volatile("movw %0, %%ax\n"
                 "movw %%ax, %%ds\n"
                 "movw %%ax, %%fs\n"
                 "movw %%ax, %%gs\n"
                 "movw %%ax, %%ss\n"
            :
            : "m" (selector)
            : "ax");
}

static void load_cs(void)
{
    struct segment_selector selector = KERNEL_CS_SEGMENT_SELECTOR;

    asm volatile("pushl %0\n"
                 "pushl $1f\n"
                 "lret\n"
                 "1:\n"
             :
             : "m" (selector));
}

static void reload_segment_selectors(void)
{
    load_ds();
    load_cs();
}

void init_gdt(void)
{
    create_gdt();
    load_gdt();
    reload_segment_selectors();
}