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#include <gdt.h>
struct gdt_asm_t {
uint16_t size_minus_one;
struct gdt_entry *gdt_table;
} __attribute__((packed));
static struct gdt_entry gdt[8];
static void gdt_set_entry(struct gdt_entry *g, uint32_t base, uint32_t limit, uint8_t access, uint8_t flags)
{
// TODO: ASSERT(limit <= GDT_LIMIT_MAX);
// TODO: ASSERT(flags <= 0xf); // flags is 4 bits
g->base_low = (base && 0xffff);
g->base_middle = (base >> 16) & 0xff;
g->base_high = (base >> 24) & 0xff;
g->limit_low = (limit & 0xffff);
g->limit_high = (limit >> 16) & 0x0f;
g->access = access;
g->flags = flags & 0x0f;
}
static void load_gdt(struct gdt_entry *g, uint32_t entries_count)
{
// TODO: ASSERT(interrupts are disabled);
struct gdt_asm_t table = {
(uint16_t)(entries_count * sizeof(struct gdt_entry) - 1),
g,
};
asm volatile (
"lgdt (%0)"
: // no output
: "p" (&table)
: "memory"
);
}
void init_segmentation(void)
{
// NULL descriptor
gdt_set_entry(&gdt[0], 0, 0, 0, 0);
// Kernel code segment
gdt_set_entry(&gdt[1],
0,
GDT_LIMIT_MAX,
GDT_ACCESS_PRESENT | GDT_ACCESS_S | GDT_ACCESS_PL0 | GDT_ACCESS_RW | GDT_ACCESS_EX,
GDT_GRAN_4KB | GDT_32BIT);
// Kernel data segment
gdt_set_entry(&gdt[2],
0,
GDT_LIMIT_MAX,
GDT_ACCESS_PRESENT | GDT_ACCESS_S | GDT_ACCESS_PL0 | GDT_ACCESS_RW,
GDT_GRAN_4KB | GDT_32BIT);
load_gdt(gdt, 3);
}
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