linux-2.6/include/asm-x86/desc_defs.h

/* Written 2000 by Andi Kleen */
#ifndef __ARCH_DESC_DEFS_H
#define __ARCH_DESC_DEFS_H

/*
 * Segment descriptor structure definitions, usable from both x86_64 and i386
 * archs.
 */

#ifndef __ASSEMBLY__

#include <linux/types.h>

/*
 * FIXME: Acessing the desc_struct through its fields is more elegant,
 * and should be the one valid thing to do. However, a lot of open code
 * still touches the a and b acessors, and doing this allow us to do it
 * incrementally. We keep the signature as a struct, rather than an union,
 * so we can get rid of it transparently in the future -- glommer
 */
// 8 byte segment descriptor
struct desc_struct {
	union {
		struct { unsigned int a, b; };
		struct {
			u16 limit0;
			u16 base0;
			unsigned base1: 8, type: 4, s: 1, dpl: 2, p: 1;
			unsigned limit: 4, avl: 1, l: 1, d: 1, g: 1, base2: 8;
		};

	};
} __attribute__((packed));

enum {
	GATE_INTERRUPT = 0xE,
	GATE_TRAP = 0xF,
	GATE_CALL = 0xC,
};

// 16byte gate
struct gate_struct64 {
	u16 offset_low;
	u16 segment;
	unsigned ist : 3, zero0 : 5, type : 5, dpl : 2, p : 1;
	u16 offset_middle;
	u32 offset_high;
	u32 zero1;
} __attribute__((packed));

#define PTR_LOW(x) ((unsigned long)(x) & 0xFFFF)
#define PTR_MIDDLE(x) (((unsigned long)(x) >> 16) & 0xFFFF)
#define PTR_HIGH(x) ((unsigned long)(x) >> 32)

enum {
	DESC_TSS = 0x9,
	DESC_LDT = 0x2,
};

// LDT or TSS descriptor in the GDT. 16 bytes.
struct ldttss_desc {
	u16 limit0;
	u16 base0;
	unsigned base1 : 8, type : 5, dpl : 2, p : 1;
	unsigned limit1 : 4, zero0 : 3, g : 1, base2 : 8;
	u32 base3;
	u32 zero1;
} __attribute__((packed));

#ifdef CONFIG_X86_64
typedef struct gate_struct64 gate_desc;
#else
typedef struct desc_struct gate_desc;
#endif

struct desc_ptr {
	unsigned short size;
	unsigned long address;
} __attribute__((packed)) ;


#endif /* !__ASSEMBLY__ */

#endif