#include <kernel/system.h>

struct gdt_entry {
    unsigned short limit_low;
    unsigned short base_low;
    unsigned char base_middle;
    unsigned char access;
    unsigned char granularity;
    unsigned char base_high;
} __attribute__((packed));

struct gdt_ptr {
    unsigned short limit;
    void *base;
} __attribute__((packed));

struct gdt_entry gdt[3];
struct gdt_ptr gp;

extern void gdt_flush();

void gdt_set_gate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran) {
    // Set descriptor base address
    gdt[num].base_low = (base & 0xFFFF);
    gdt[num].base_middle = (base >> 16) & 0xFF;
    gdt[num].base_high = (base >> 24) & 0xFF;

    // Set descriptor limits
    gdt[num].limit_low = (limit & 0xFFFF);
    gdt[num].granularity = ((limit >> 16) & 0x0F);

    // Set granularity and access flags
    gdt[num].granularity |= (gran & 0xF0);
    gdt[num].access = access;
}

void gdt_install() {
    // Set GDT pointer and limit
    gp.limit = (sizeof(struct gdt_entry) * 3) - 1;
    gp.base = &gdt;

    // NULL descriptor
    gdt_set_gate(0, 0, 0, 0, 0);

    // Code segment
    gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);

    // Data segment
    gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);

    // Remove old GDT and install the new changes!
    gdt_flush();

    vga_log("Installed Global Descriptor Table", 2);
}