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#pragma once
#include <stdbool.h>
#include <stdint.h>
enum Ring { Ring0 = 0x0, Ring1 = 0x1, Ring2 = 0x2, Ring3 = 0x3 };
struct __attribute__((packed)) Access {
bool accessed : 1; // if 0, is set by processor when accessed
bool readwrite : 1; // code seg: read toggle; data seg: write toggle
bool direction : 1; // code seg: conforming bit; data seg: direction bit
bool executable : 1; // executable bit
bool segment : 1; // true for code/data; false for gates/tss
enum Ring privilege : 2; // descriptor privilege level
bool present : 1; // true for every active segment
};
_Static_assert(sizeof(struct Access) == 1, "access byte size");
static const struct Access null_access = {false, false, false, false, false, Ring0, false};
static const struct Access ktext_access = {.readwrite = true, .executable = true, .segment = true, .present = true};
static const struct Access kdata_access = {.readwrite = true, .segment = true, .present = true};
// 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
uint8_t 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),
};
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