text: /* Descriptor table such as GDT/IDT */
  
  #include "bootpack.h"
  
  void init_gdtidt(void) {
    struct SEGMENT_DESCRIPTOR *gdt = (struct SEGMENT_DESCRIPTOR *)ADR_GDT;
    struct GATE_DESCRIPTOR    *idt = (struct GATE_DESCRIPTOR    *)ADR_IDT;
    int i;
  
    /* Initialize GDT */
    for (i = 0; i <= LIMIT_GDT / 8; i++) {
      set_segmdesc(gdt + i, 0, 0, 0);
    }
    set_segmdesc(gdt + 1,   0xffffffff, 0x00000000, AR_DATA32_RW);
    set_segmdesc(gdt + 2, LIMIT_BOTPAK, ADR_BOTPAK, AR_CODE32_ER);
    load_gdtr(LIMIT_GDT, ADR_GDT);
  
    /* Initialize IDT */
    for (i = 0; i <= LIMIT_IDT / 8; i++) {
      set_gatedesc(idt + i, 0, 0, 0);
    }
    load_idtr(LIMIT_IDT, ADR_IDT);
  
    return;
  }
  
  void set_segmdesc(
    struct SEGMENT_DESCRIPTOR *sd,
    unsigned int limit, int base, int ar
  ) {
    if (limit > 0xfffff) {
      ar |= 0x8000; /* G_bit = 1 */
      limit /= 0x1000;
    }
  
    sd->limit_low    = limit & 0xffff;
    sd->base_low     = base & 0xffff;
    sd->base_mid     = (base >> 16) & 0xff;
    sd->access_right = ar & 0xff;
    sd->limit_high   = ((limit >> 16) & 0x0f) | ((ar >> 8) & 0xf0);
    sd->base_high    = (base >> 24) & 0xff;
  
    return;
  }
  
  void set_gatedesc(
    struct GATE_DESCRIPTOR *gd,
    int offset, int selector, int ar
  ) {
    gd->offset_low   = offset & 0xffff;
    gd->selector     = selector;
    gd->dw_count     = (ar >> 8) & 0xff;
    gd->access_right = ar & 0xff;
    gd->offset_high  = (offset >> 16) & 0xffff;
  
    return;
  }
  