gdt.c

#include "gdt.h"

extern void load_gdt(struct gdt_ptr*);

struct gdt_entry gdt_entries[5];
struct gdt_ptr gdt_ptr;

static void gdt_set_gate(int32_t entry, uint32_t base, uint32_t limit, uint8_t access, uint8_t gran) {
    gdt_entries[entry].base_low = (base & 0xFFFF);
    gdt_entries[entry].base_middle = (base >> 16) & 0xFF;
    gdt_entries[entry].base_high = (base >> 24) & 0xFF;

    gdt_entries[entry].limit_low = (limit & 0xFFFF);
    gdt_entries[entry].granularity = (limit >> 16) & 0x0F;

    gdt_entries[entry].granularity |= gran & 0xF0;
    gdt_entries[entry].access = access;
}

void gdt_init() {
  gdt_ptr.limit = (sizeof(struct gdt_entry) * 5) - 1;
  gdt_ptr.base = (uint32_t)&gdt_entries;

  gdt_set_gate(0, 0, 0, 0, 0);                // Null segment
  gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF); // Code segment
  gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF); // Data segment
  gdt_set_gate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF); // User mode code segment
  gdt_set_gate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF); // User mode data segment

  load_gdt(&gdt_ptr);
}