Espresso/drivers/gdt.c

#include <stdio.h>

#include <gdt.h>
#include <drivers/gdt_ec.h>

#define GDT_ENTRIES 5
uint64_t gdt[GDT_ENTRIES];

struct {
    uint16_t limit;
    uint32_t base;
} __attribute__((packed)) gp;


extern void gdt_flush(uint32_t);

void gdt_install(bool prnt_gdt)
{
  if (prnt_gdt)
  {
    printf("<===== GDT =====>\n");
  }
  
  create_descriptor(0, 0, 0, 0, prnt_gdt); /* Null */
  create_descriptor(1, 0, 0x000FFFFF, GDT_CODE_PL0, prnt_gdt); /* Kernel code */
  create_descriptor(2, 0, 0x000FFFFF, GDT_DATA_PL0, prnt_gdt); /* Kernel data */
  create_descriptor(3, 0, 0x000FFFFF, GDT_CODE_PL3, prnt_gdt); /* User code */
  create_descriptor(4, 0, 0x000FFFFF, GDT_DATA_PL3, prnt_gdt); /* User data */
  
  if (prnt_gdt)
  {
    printf("<===== GDT =====>\n");
  }

  gp.limit = sizeof(gdt) - 1;
  gp.base = (uint32_t)&gdt;

  gdt_flush((uint32_t)&gp);
}

void create_descriptor(int index, uint32_t base, uint32_t limit, uint16_t flag, bool prnt_gdt)
{
  uint64_t descriptor;

  descriptor  =  limit       & 0x000F0000; /* limit bits 19:16 */
  descriptor |= (flag <<  8) & 0x00F0FF00; /* flags and access */
  descriptor |= (base >> 16) & 0x000000FF; /* base bits 23:16 */
  descriptor |=  base        & 0xFF000000; /* base bits 31:24 */

  descriptor <<= 32;

  descriptor |= ((uint64_t)base  << 16);   /* base bits 15:0 */
  descriptor |= (limit  & 0x0000FFFF);     /* limit bits 15:0 */

  gdt[index] = descriptor;

  if (prnt_gdt)
  {
    printf("GDT[%d] = 0x%llX\n", index, descriptor);
  }
}