gdt.c

#include "gdt.h"

/* Three entries: null, kernel code (0x08), kernel data (0x10) */
typedef struct __attribute__((packed)) {
    uint16_t limit_lo;
    uint16_t base_lo;
    uint8_t  base_mid;
    uint8_t  access;
    uint8_t  granularity;   /* high 4 bits = flags, low 4 = limit_hi */
    uint8_t  base_hi;
} gdt_entry_t;

typedef struct __attribute__((packed)) {
    uint16_t limit;
    uint32_t base;
} gdt_ptr_t;

static gdt_entry_t gdt[3];
static gdt_ptr_t   gdt_ptr;

static void set_entry(int i,
                      uint32_t base, uint32_t limit,
                      uint8_t access, uint8_t gran)
{
    gdt[i].base_lo    = (uint16_t)(base & 0xFFFF);
    gdt[i].base_mid   = (uint8_t)((base >> 16) & 0xFF);
    gdt[i].base_hi    = (uint8_t)((base >> 24) & 0xFF);
    gdt[i].limit_lo   = (uint16_t)(limit & 0xFFFF);
    gdt[i].granularity = (uint8_t)(((limit >> 16) & 0x0F) | (gran & 0xF0));
    gdt[i].access     = access;
}

/* Defined in gdt.asm — flushes segment registers after lgdt */
extern void gdt_flush(uint32_t gdt_ptr_addr);

void gdt_init(void)
{
    gdt_ptr.limit = (uint16_t)(sizeof(gdt) - 1);
    gdt_ptr.base  = (uint32_t)&gdt;

    set_entry(0, 0, 0,          0x00, 0x00);  /* null descriptor */
    set_entry(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);  /* kernel code: ring0, exec/read */
    set_entry(2, 0, 0xFFFFFFFF, 0x92, 0xCF);  /* kernel data: ring0, read/write */

    gdt_flush((uint32_t)&gdt_ptr);
}