1 #include <string.h>

2 #include "segm.h"

3 #include "desc.h"

4

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

6 #define BIT_ACCESSED (1 << 8)

7 #define BIT_WR (1 << 9)

8 #define BIT_RD (1 << 9)

9 #define BIT_EXP_DOWN (1 << 10)

10 #define BIT_CONFORMING (1 << 10)

11 #define BIT_CODE (1 << 11)

12 #define BIT_NOSYS (1 << 12)

13 #define BIT_PRESENT (1 << 15)

14 /* TSS busy bit */

15 #define BIT_BUSY (1 << 9)

16

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

18 #define BIT_BIG (1 << 6)

19 #define BIT_DEFAULT (1 << 6)

20 #define BIT_GRAN (1 << 7)

21

22 enum {TYPE_DATA, TYPE_CODE};

23

24 #define TSS_TYPE_BITS 0x900

25 #define TSS_BUSY BIT_BUSY

26

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

28 static void task_desc(desc_t *desc, uint32_t base, uint32_t limit, int dpl, unsigned int busy);

29

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

31 void setup_selectors(uint16_t code, uint16_t data);

32 void set_gdt(uint32_t addr, uint16_t limit);

33

34

35 /* our global descriptor table */

36 static desc_t gdt[6];

37

38

39 void init_segm(void)

40 {

41 memset(gdt, 0, sizeof gdt);

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

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

44 segm_desc(gdt + SEGM_UCODE, 0, 0xffffffff, 3, TYPE_CODE);

45 segm_desc(gdt + SEGM_UDATA, 0, 0xffffffff, 3, TYPE_DATA);

46 task_desc(gdt + SEGM_TASK, 0, 0xffffffff, 3, TSS_BUSY);

47

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

49

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

51 }

52

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

54 uint16_t selector(int idx, int rpl)

55 {

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

57 }

58

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

60 {

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

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

63

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

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

66 * starting from bit 8

67 */

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

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

70

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

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

73 */

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

75 }

76

77 static void task_desc(desc_t *desc, uint32_t base, uint32_t limit, int dpl, unsigned int busy)

78 {

79 desc->d[0] = limit & 0xffff;

80 desc->d[1] = base & 0xffff;

81

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

83 TSS_TYPE_BITS | busy;

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

85 }