k-chow/k/gdt.h

#ifndef GDT_H
#define GDT_H

#include <stdint.h>

#define GDT_NB_ENTRIES 3
#define GDT_SIZE (GDT_NB_ENTRIES * sizeof(struct gdt_entry))

#define KERNEL_CS_INDEX 1
#define KERNEL_DS_INDEX 2

#define KERNEL_CS_SEGMENT_SELECTOR {0, 0, KERNEL_CS_INDEX}
#define KERNEL_DS_SEGMENT_SELECTOR {0, 0, KERNEL_DS_INDEX}

struct gdt_entry
{
    uint16_t limit_1    : 16; // 16 lsbs of the segment limit
    uint16_t base_1     : 16; // 16 lsbs of the segment base address


    uint8_t base_2      : 8;  // 8 next bits of the segment base address

    /* Access Byte */
    uint8_t accessed    : 1;  // set to 0, is set to 1 by CPU when segment is
                              // accessed
    uint8_t rw          : 1;  // read/write bit, 1 to grant read acces on code
                              // segments or write access on data segments
    uint8_t dc          : 1;  // Direction/Conforming bit
    uint8_t ex          : 1;  // Executable bit
    uint8_t desc_type   : 1;  // Descriptor type: 1 for data or code segmentm
                              // 0 for system segments (like TSS)
    uint8_t desc_priv   : 2;  // Descriptor privilege
    uint8_t present     : 1;  // Always set to 1


    uint8_t limit_2     : 4;  // 4 msbs of the segment limit

    /* Flags */
    uint8_t available   : 1;  // Available for use by system software
    uint8_t l           : 1;  // 64-bit code segment
    uint8_t db          : 1;  // size, 0 for 16 bits, 1 for 32 bits
    uint8_t granularity : 1;  // limit unit, 0 for bytes, 1 for 4KiB (1 page)


    uint8_t base_3      : 8;  // 8 msbs of the segment base
} __attribute__ ((packed));

struct gdt
{
    struct gdt_entry entries[GDT_NB_ENTRIES];
};

struct gdt_r
{
    uint16_t limit; // total size of the GDT - 1 in bytes
    uint32_t addr;  // base address of the DGT
} __attribute__ ((packed));

struct segment_selector
{
    uint8_t rpl             : 2;  // request privilege level
    uint8_t table_indicator : 1;  // 0 for GDT, 1 for LDT
    uint16_t index          : 13; // index of the entry in the table
} __attribute__ ((packed));

void init_gdt(void);

#endif  /* !GDT_H */