#include "gdt.h"
#include <stdint.h>

/* GDT Pointer Structure */
struct gdt_ptr gp;
/* GDT entries */
struct gdt_entry gdt[5];

extern void gdt_flush(uint32_t);

/* Setup a descriptor in the Global Descriptor Table */
void gdt_set_gate(int32_t num, uint32_t base, uint32_t limit, uint8_t access, uint8_t gran)
{
    /* Setup the descriptor base address */
    gdt[num].base_low    = (base & 0xFFFF);
    gdt[num].base_middle = (base >> 16) & 0xFF;
    gdt[num].base_high   = (base >> 24) & 0xFF;

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

    /* Finally, set up the granularity and access flags */
    gdt[num].granularity |= (gran & 0xF0);
    gdt[num].access      = access;
}

/* Should be called by main. This will setup the special GDT
*  pointer, set up the first 3 entries in our GDT, and then
*  call gdt_flush() in our assembler file in order to tell
*  the processor where the new GDT is and update the
*  internal registers */
void init_gdt()
{
    /* Setup the GDT pointer and limit */
    gp.limit = (sizeof(struct gdt_entry) * 5) - 1;
    gp.base  = (uint32_t)&gdt;
    
    /* Our NULL descriptor */
    gdt_set_gate(0, 0, 0, 0, 0);

    /* The second entry is our Code Segment. The base address
    *  is 0, the limit is 4GBytes, it uses 4KByte granularity,
    *  uses 32-bit opcodes, and is a Code Segment descriptor.
    *  Please check the table above in the tutorial in order
    *  to see exactly what each value means */
    /* 0x9A = 1001 1010
       P=1, DPL=00, S=1 (Code/Data), Type=1010 (Code, Exec/Read) */
    /* 0xCF = 1100 1111 
       G=1 (4k), DB=1 (32bit), L=0, AVL=0, LimitHigh=0xF */
    gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);

    /* The third entry is our Data Segment. It's EXACTLY the
    *  same as our code segment, but the descriptor type in
    *  this entry's access byte says it's a Data Segment */
    /* 0x92 = 1001 0010
       P=1, DPL=00, S=1, Type=0010 (Data, Read/Write) */
    gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);

    /* User mode code segment */
    gdt_set_gate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF);

    /* User mode data segment */
    gdt_set_gate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF);

    /* Flush out the old GDT and install the new changes! */
    gdt_flush((uint32_t)&gp);
}