1 #include <string.h>

2 #include "segm.h"

3

4 /* bits for the 3rd 16bt part of the descriptor */

5 #define BIT_ACCESSED (1 << 8)

6 #define BIT_WR (1 << 9)

7 #define BIT_RD (1 << 9)

8 #define BIT_EXP_DOWN (1 << 10)

9 #define BIT_CONFORMING (1 << 10)

10 #define BIT_CODE (1 << 11)

11 #define BIT_NOSYS (1 << 12)

12 #define BIT_PRESENT (1 << 15)

13

14 /* bits for the last 16bit part of the descriptor */

15 #define BIT_BIG (1 << 6)

16 #define BIT_DEFAULT (1 << 6)

17 #define BIT_GRAN (1 << 7)

18

19 enum {TYPE_DATA, TYPE_CODE};

20

21 static void segm_desc(desc_t *desc, uint32_t base, uint32_t limit, int dpl, int type);

22

23 /* these functions are implemented in segm-asm.S */

24 void setup_selectors(uint16_t code, uint16_t data);

25 void set_gdt(uint32_t addr, uint16_t limit);

26

27

28 /* our global descriptor table */

29 static desc_t gdt[4];

30

31

32 void init_segm(void)

33 {

34 memset(gdt, 0, sizeof gdt);

35 segm_desc(gdt + SEGM_KCODE, 0, 0xffffffff, 0, TYPE_CODE);

36 segm_desc(gdt + SEGM_KDATA, 0, 0xffffffff, 0, TYPE_DATA);

37

38 set_gdt((uint32_t)gdt, sizeof gdt - 1);

39

40 setup_selectors(selector(SEGM_KCODE, 0), selector(SEGM_KDATA, 0));

41 }

42

43 /* constructs a GDT selector based on index and priviledge level */

44 uint16_t selector(int idx, int rpl)

45 {

46 return (idx << 3) | (rpl & 3);

47 }

48

49 static void segm_desc(desc_t *desc, uint32_t base, uint32_t limit, int dpl, int type)

50 {

51 desc->d[0] = limit & 0xffff; /* low order 16bits of limit */

52 desc->d[1] = base & 0xffff; /* low order 16bits of base */

53

54 /* third 16bit part contains the last 8 bits of base, the 2 priviledge

55 * level bits starting on bit 13, present flag on bit 15, and type bits

56 * starting from bit 8

57 */

58 desc->d[2] = ((base >> 16) & 0xff) | ((dpl & 3) << 13) | BIT_PRESENT |

59 BIT_NOSYS | (type == TYPE_DATA ? BIT_WR : (BIT_RD | BIT_CODE));

60

61 /* last 16bit part contains the last nibble of limit, the last byte of

62 * base, and the granularity and deafult/big flags in bits 23 and 22 resp.

63 */

64 desc->d[3] = ((limit >> 16) & 0xf) | ((base >> 16) & 0xff00) | BIT_GRAN | BIT_BIG;

65 }