#ifndef GDT_H
#define GDT_H

#include <stdint.h>

extern char _end[], _edata[];

#define KERNEL_CS       0x08    /* kernel code segment */
#define KERNEL_DS       0x10    /* kernel data segment */
#define USER_CS         0x18    /* user code segment */
#define USER_DS         0x20    /* user data segment */
#define TSS             0x28    /* TSS segment */

#define KERNEL_BSS_SIZE		((int)_end - (int)_edata)

#define NR_GDT_ENTRIES	6	/* entries in GDT descriptor */
#define NR_IDT_ENTRIES	256	/* entries in IDT descriptor */

/* low flags of Segment Descriptors */
#define SD_CODE		0x0A	/* CODE Exec/Read */
#define SD_DATA		0x02	/* DATA Read/Write */

#define SD_32INTRGATE	0x0E	/* 32-bit Interrupt Gate (0D110) */
#define SD_32TRAPGATE	0x0F	/* 32-bit Trap Gate (0D111) */

#define SD_CD 		0x10	/* 0 = system / 1 = code/data */
#define SD_DPL0		0x00	/* priority level 0 (kernel) */
#define SD_DPL3		0x60	/* priority level 3 (user) */
#define SD_PRESENT	0x80	/* segment present or valid */

/* high flags Segment Descriptors */
#define SD_OPSIZE32	0x04	/* 32-bit code and data segments */
#define SD_PAGE4KB	0x08	/* page granularity (4KB) */

/* low flags of the TSS Descriptors */
#define SD_TSSPRESENT	0x89	/* TSS present and not busy flag */

/* EFLAGS */
#define EF_IOPL		12	/* IOPL bit */

struct desc_r {
	uint16_t limit;
	uint32_t base_addr;
} __attribute__((packed));

struct seg_desc {
	unsigned sd_lolimit : 16;	/* segment limit 0-15 bits */
	unsigned sd_lobase  : 24;	/* base address 0-23 bits */
	unsigned sd_loflags :  8;	/* flags (P, DPL, S and TYPE) */
	unsigned sd_hilimit :  4;	/* segment limit 16-19 bits */
	unsigned sd_hiflags :  4;	/* flags (G, DB, 0 and AVL) */
	unsigned sd_hibase  :  8;	/* base address 24-31 bits */
} __attribute__((packed));

struct gate_desc {
	unsigned gd_looffset: 16;	/* offset 0-15 bits */
	unsigned gd_selector: 16;	/* segment selector */
	unsigned gd_flags   : 16;	/* flags (P, DPL, TYPE, 0 and NULL) */
	unsigned gd_hioffset: 16;	/* offset 16-31 bits */
} __attribute__((packed));

struct tss_entry_struct {
	uint32_t prev_tss; // The previous TSS - with hardware task switching these form a kind of backward linked list.
	uint32_t esp0;     // The stack pointer to load when changing to kernel mode.
	uint32_t ss0;      // The stack segment to load when changing to kernel mode.
	// Everything below here is unused.
	uint32_t esp1; // esp and ss 1 and 2 would be used when switching to rings 1 or 2.
	uint32_t ss1;
	uint32_t esp2;
	uint32_t ss2;
	uint32_t cr3;
	uint32_t eip;
	uint32_t eflags;
	uint32_t eax;
	uint32_t ecx;
	uint32_t edx;
	uint32_t ebx;
	uint32_t esp;
	uint32_t ebp;
	uint32_t esi;
	uint32_t edi;
	uint32_t es;
	uint32_t cs;
	uint32_t ss;
	uint32_t ds;
	uint32_t fs;
	uint32_t gs;
	uint32_t ldt;
	uint16_t trap;
	uint16_t iomap_base;
} __attribute__((packed));

extern struct tss_entry_struct tss;

#endif