#include "../include/gdt.h"
#include "../include/string.h"
#include "../include/paging.h"

struct tss_entry_struct tss;

void bss_init()
{
	memset((void *)((int)_edata), 0, KERNEL_BSS_SIZE);
}

uint32_t get_last_boot_addr()
{
	return ((uint32_t)_end & PAGE_MASK) + PAGE_SIZE;
}

void load_gdt(uint32_t gdt_ptr) {
    asm volatile (
        "lgdt (%0)\n\t"
        "movw %1, %%ax\n\t"
        "movw %%ax, %%ds\n\t"
        "movw %%ax, %%es\n\t"
        "movw %%ax, %%fs\n\t"
        "movw %%ax, %%gs\n\t"
        "movw %%ax, %%ss\n\t"
        "ljmp %2, $1f\n\t"
        "1:\n\t"
        :
        : "r" (gdt_ptr), "i" (KERNEL_DS), "i" (KERNEL_CS)
        : "eax"
    );
}

void flush_tss(void) {
    asm volatile (
        "mov %0, %%ax\n\t"
        "ltr %%ax\n\t"
        :
        : "i" (0x28)
        : "ax"
    );
}

static void gdt_set_entry(int num, uint32_t base_addr, uint32_t limit, char loflags, char hiflags)
{
	num /= sizeof(struct seg_desc);
	gdt[num].sd_lolimit = limit & 0xFFFF;
	gdt[num].sd_lobase = base_addr & 0xFFFFFF;
	gdt[num].sd_loflags = loflags;
	gdt[num].sd_hilimit = (limit >> 16) & 0x0F;
	gdt[num].sd_hiflags = hiflags;
	gdt[num].sd_hibase = (base_addr >> 24) & 0xFF;
}

void gdt_init(void)
{
	uint8_t loflags;

	memset(&tss, 0, sizeof(tss));
    tss.ss0 = KERNEL_DS;
	tss.iomap_base = 0xFFFF; // Disable I/O bitmap
	tss.esp0 = 0xC0010000; /* kernel stack address (firstly defined in boot.asm)*/
	tss.cs = KERNEL_CS;
	tss.ds = KERNEL_DS;
	tss.es = KERNEL_DS;
	tss.fs = KERNEL_DS;
	tss.gs = KERNEL_DS;
	tss.ss = KERNEL_DS;

	gdt_set_entry(0, 0, 0, 0, 0);	/* null descriptor */

	loflags = SD_CODE | SD_CD | SD_DPL0 | SD_PRESENT;
	gdt_set_entry(KERNEL_CS, 0, 0xFFFFFFFF, loflags, SD_OPSIZE32 | SD_PAGE4KB);
	loflags = SD_DATA | SD_CD | SD_DPL0 | SD_PRESENT;
	gdt_set_entry(KERNEL_DS, 0, 0xFFFFFFFF, loflags, SD_OPSIZE32 | SD_PAGE4KB);

	loflags = SD_CODE | SD_CD | SD_DPL3 | SD_PRESENT;
	gdt_set_entry(USER_CS, 0, 0xFFFFFFFF, loflags, SD_OPSIZE32 | SD_PAGE4KB);
	loflags = SD_DATA | SD_CD | SD_DPL3 | SD_PRESENT;
	gdt_set_entry(USER_DS, 0, 0xFFFFFFFF, loflags, SD_OPSIZE32 | SD_PAGE4KB);

	loflags = 0x89; // P=1, DPL=0, S=0 (system), Type=0x9 (32-bit TSS)
    gdt_set_entry(TSS, (uint32_t)&tss, sizeof(struct tss_entry_struct) - 1, SD_PRESENT | SD_DPL0 | SD_TSSPRESENT, 0);

	load_gdt((uint32_t)&gdtr);
	flush_tss();
}