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 (BIT_ACCESSED | BIT_CODE)

25

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

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

28

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

30 void setup_selectors(uint16_t code, uint16_t data);

31 void set_gdt(uint32_t addr, uint16_t limit);

32 void set_task_reg(uint16_t tss_selector);

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

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

48

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

50 }

51

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

53 uint16_t selector(int idx, int rpl)

54 {

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

56 }

57

58 void set_tss(uint32_t addr)

59 {

60 task_desc(gdt + SEGM_TASK, 0, sizeof(struct tss) - 1, 3, TSS_BUSY);

61 set_task_reg(selector(SEGM_TASK, 0));

62 }

63

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

65 {

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

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

68

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

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

71 * starting from bit 8

72 */

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

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

75

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

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

78 */

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

80 }

81

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

83 {

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

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

86

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

88 TSS_TYPE_BITS | busy;

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

90 }