#ifndef _HEAD_H#define _HEAD_Htypedef struct desc_struct {	unsigned long a,b;} desc_table[256];extern unsigned long pg_dir[1024];extern desc_table idt,gdt;#define GDT_NUL 0#define GDT_CODE 1#define GDT_DATA 2#define GDT_TMP 3#define LDT_NUL 0#define LDT_CODE 1#define LDT_DATA 2#endif