#include "console.h"
#include "schedule.h"

extern void setGdt(unsigned long long *, unsigned);

// Each define here is for a specific flag in the descriptor.
// Refer to the intel documentation for a description of what each one does.
#define SEG_TYPE(x)  ((x) << 0x04) // Descriptor type (0 for system, 1 for code/data)
#define SEG_PRES(x)  ((x) << 0x07) // Present
#define SEG_SAVL(x)  ((x) << 0x0C) // Available for system use
#define SEG_LONG(x)  ((x) << 0x0D) // Long mode
#define SEG_SIZE(x)  ((x) << 0x0E) // Size (0 for 16-bit, 1 for 32)
#define SEG_GRAN(x)  ((x) << 0x0F) // Granularity (0 for 1B - 1MB, 1 for 4KB - 4GB)
#define SEG_PRIV(x) (((x) &  0x03) << 0x05)   // Set privilege level (0 - 3)
 
#define SEG_DATA_RD        0x00 // Read-Only
#define SEG_DATA_RDA       0x01 // Read-Only, accessed
#define SEG_DATA_RDWR      0x02 // Read/Write
#define SEG_DATA_RDWRA     0x03 // Read/Write, accessed
#define SEG_DATA_RDEXPD    0x04 // Read-Only, expand-down
#define SEG_DATA_RDEXPDA   0x05 // Read-Only, expand-down, accessed
#define SEG_DATA_RDWREXPD  0x06 // Read/Write, expand-down
#define SEG_DATA_RDWREXPDA 0x07 // Read/Write, expand-down, accessed
#define SEG_CODE_EX        0x08 // Execute-Only
#define SEG_CODE_EXA       0x09 // Execute-Only, accessed
#define SEG_CODE_EXRD      0x0A // Execute/Read
#define SEG_CODE_EXRDA     0x0B // Execute/Read, accessed
#define SEG_CODE_EXC       0x0C // Execute-Only, conforming
#define SEG_CODE_EXCA      0x0D // Execute-Only, conforming, accessed
#define SEG_CODE_EXRDC     0x0E // Execute/Read, conforming
#define SEG_CODE_EXRDCA    0x0F // Execute/Read, conforming, accessed
 
#define GDT_CODE_PL0 SEG_TYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
                     SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
                     SEG_PRIV(0) | SEG_CODE_EXRD
 
#define GDT_DATA_PL0 SEG_TYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
                     SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
                     SEG_PRIV(0) | SEG_DATA_RDWR
 
#define GDT_CODE_PL3 SEG_TYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
                     SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
                     SEG_PRIV(3) | SEG_CODE_EXRD
 
#define GDT_DATA_PL3 SEG_TYPE(1) | SEG_PRES(1) | SEG_SAVL(0) | \
                     SEG_LONG(0) | SEG_SIZE(1) | SEG_GRAN(1) | \
                     SEG_PRIV(3) | SEG_DATA_RDWR

#define GDT_TSS SEG_GRAN(1) | SEG_PRES(1) | SEG_PRIV(0) | SEG_SIZE(1) | SEG_CODE_EXA
                     
unsigned long long gdt[6];

unsigned long long
create_descriptor(unsigned long base, unsigned long limit, unsigned short flag)
{
    unsigned long long descriptor;
 
    // Create the high 32 bit segment
    descriptor  =  limit       & 0x000F0000;
    descriptor |= (flag <<  8) & 0x00F0FF00;
    descriptor |= (base >> 16) & 0x000000FF;
    descriptor |=  base        & 0xFF000000; 
    // Shift by 32 to allow for low part of segment
    descriptor <<= 32;
 
    // Create the low 32 bit segment
    descriptor |= base  << 16;
    descriptor |= limit  & 0x0000FFFF;
 
    return descriptor;
}

void init_tss(struct tss_t *tss) {
  gdt[5] = create_descriptor((unsigned long)tss, sizeof(struct tss_t), (GDT_TSS));
  asm __volatile__ ( "ltr %%ax" : : "a" ( 0x28 ) );
}

void init_gdt(void)
{
	
    gdt[0] = create_descriptor(0, 0, 0); // Null descriptor
    gdt[1] = create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL0)); // kernel code segment
    gdt[2] = create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL0)); // kernel data segment
    gdt[3] = create_descriptor(0, 0x000FFFFF, (GDT_CODE_PL3)); // user code segment
    gdt[4] = create_descriptor(0, 0x000FFFFF, (GDT_DATA_PL3)); // user data segment
    
    setGdt(gdt, 48);
}