gdt.h

#ifndef _CPU_GDT_H
#define _CPU_GDT_H
 
#include <stdint.h>
 
// High flags
// Note that in long mode: D=0, L=1
#define GDT_LONG_MODE  0x20
// Low flags
#define GDT_PRESENT    0x80
// a. Descriptor Privilege Level
#define GDT_RING_0     0x00
#define GDT_RING_1     0x20
#define GDT_RING_2     0x40
#define GDT_RING_3     0x60
// b. Conforming
#define GDT_CONFORMING 0x04
 
// For now, we make use of three GDT entries
#define GDT_SEGMENTS             3
// 1. "null" descriptor
#define GDT_NULL_OFFSET          0x00
// 2. kernel code descriptor
//
//    Note: In 64-bit code segments descriptors,
//          the CPU recognizes:
//              a. Present bit
//              b. DPL field
//              c. Conforming bit
//          All other fields are ignored.
#define GDT_KCODE                0x08
#define GDT_KCODE_LIMIT          0
#define GDT_KCODE_BASE           0
#define GDT_KCODE_MISC_LOW       GDT_PRESENT + GDT_RING_0
#define GDT_KCODE_MISC_HIGH      GDT_LONG_MODE
// 3. kernel data descriptor
//
//    Note: In 64-bit code segments descriptors,
//          the CPU recognizes only the Present bit.
//          All other fields are ignored.
#define GDT_KDATA                0x10
#define GDT_KDATA_LIMIT          0
#define GDT_KDATA_BASE           0
#define GDT_KDATA_MISC_LOW       GDT_PRESENT
#define GDT_KDATA_MISC_HIGH      0
 
 
#define GDT_ENTRY_CREATE(lim, base, low, high) { \
        lim & 0xffff,        /* Segment limit (low) */ \
        base  & 0xffff,      /* Base Address  (low) */ \
        (base >> 16) & 0xff, /* Base Address  (med) */ \
        low,                 /* Present/DPL/Conform */ \
        high + (lim >> 16),  /* Flags/Limit  (high) */ \
        base >> 24,          /* Base Address (high) */ \
}
 
// A GDT is necessary for our kernel
struct cpu_gdt_entry {
        uint16_t limit_low;
        uint16_t base_low;
        uint8_t  base_med;
        uint8_t  present_dpl_and_conform;
        uint8_t  flags_and_limit_high;
        uint8_t  base_high;
} __attribute__((packed));
 
// lgdt loads this data into the register
struct cpu_gdt_register {
        uint16_t limit;
        uint64_t base;
} __attribute__((packed));
 
// Load the kernel's gdt in the GDT register
void cpu_gdt_bind(void);
// Describe the currently loaded GDT.
void cpu_gdt_describe(char *buf, uint64_t bufsz);
 
#endif