1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
|
#pragma once
#include <stdbool.h>
#include <stdint.h>
enum Ring { Ring0 = 0x0, Ring1 = 0x1, Ring2 = 0x2, Ring3 = 0x3 };
struct __attribute__((packed)) Access {
unsigned accessed : 1; /* if 0, is set by processor when accessed */
unsigned readwrite : 1; /* code seg: read toggle; data seg: write toggle */
unsigned direction : 1; /* code seg: conforming bit; data seg: direction bit */
unsigned executable : 1; /* executable bit */
unsigned segment : 1; /* true for code/data; false for gates/tss */
unsigned privilege : 2; /* descriptor privilege level */
unsigned present : 1; /* true for every active segment */
};
/* _Static_assert(sizeof(struct Access) == 1, "access byte size"); */
static const struct Access null_access = {0, 0, 0, 0, 0, Ring0, 0};
static const struct Access ktext_access = {0, 1, 0, 1, 1, Ring0, 1};
static const struct Access kdata_access = {0, 1, 0, 0, 1, Ring0, 1};
/* Segment Descriptor
* A memory structure (part of a table) that tells the CPU the attributes of a given segment
* |31| | | | | | |24|23|22|21|20|19| | |16|15| | | | | | | 8| 7| | | | | | | 0|
* | base_31_24 | G|DB| | A| lim_19_16 | access | base_23_16 |
* | base_15_0 | limit_15_0 |
* |31| | | | | | | | | | | | | | |16|15| | | | | | | | | | | | | | | 0|
* limit size of segment - 1, either in bytes or in 4KiB chunks (check flags)
* base address of segment
* access
* flags defines the segment chunks and 16/32 bit */
struct __attribute__((packed)) SegmentDescriptor_t {
uint16_t limit_15_0; /* low bits of segment limit */
uint16_t base_15_0; /* low bits of segment base address */
uint8_t base_23_16; /* middle bits of segment base address */
uint8_t access; /* access byte */
unsigned limit_19_16 : 4; /* high bits of segment limit */
/* flags */
bool a : 1; /* unused, available for software use */
bool rsv : 1; /* reserved */
bool db : 1; /* false => 16-bit seg; true => 32-bit seg */
bool granularity : 1; /* limit scaled by 4k when set */
uint8_t base_31_24; /* high bits of segment address */
};
/* _Static_assert(sizeof(struct SegmentDescriptor_t) == 8, "segment descriptor size"); */
void SegmentDescriptor(struct SegmentDescriptor_t *self, unsigned base, unsigned limit, uint8_t access);
void gdt_install();
enum SegmentIndex {
ktextDescriptor = 2 * sizeof(struct SegmentDescriptor_t),
kdataDescriptor = 3 * sizeof(struct SegmentDescriptor_t)
};
|
