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struct gdt_entry {
unsigned short limit_low;
unsigned short base_low;
unsigned char base_middle;
unsigned char access;
unsigned char granularity;
unsigned char base_high;
} __attribute__((packed));
struct gdt_ptr {
unsigned short limit;
void *base;
} __attribute__((packed));
struct gdt_entry gdt[3];
struct gdt_ptr gp;
extern void gdt_flush();
void gdt_set_gate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran) {
// Set descriptor base address
gdt[num].base_low = (base & 0xFFFF);
gdt[num].base_middle = (base >> 16) & 0xFF;
gdt[num].base_high = (base >> 24) & 0xFF;
// Set descriptor limits
gdt[num].limit_low = (limit & 0xFFFF);
gdt[num].granularity = ((limit >> 16) & 0x0F);
// Set granularity and access flags
gdt[num].granularity |= (gran & 0xF0);
gdt[num].access = access;
}
void gdt_install() {
// Set GDT pointer and limit
gp.limit = (sizeof(struct gdt_entry) * 3) - 1;
gp.base = &gdt;
// NULL descriptor
gdt_set_gate(0, 0, 0, 0, 0);
// Code segment
gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
// Data segment
gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
// Remove old GDT and install the new changes!
gdt_flush();
}
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