Sync/kernel/gdt.c

#include <kernel/utils.h>
#include <kernel/serial.h>
#include <stddef.h>
#include <stdint.h>


struct gdt_item {
	uint16_t low_limit;
	uint16_t low_base;
	uint8_t middle_base;
	uint8_t access;
	uint8_t granular;
	uint8_t high_base;
} __attribute__((packed)); //Prevent compiler optimisation by packing

struct gdt_ptr {
	uint16_t limit;
	unsigned int base;
} __attribute__((packed));

struct gdt_item gdt[3]; //3-entry gdt
struct gdt_ptr gp;

extern void load_gdt(uint32_t);

void gdt_set_gate(int num, uint32_t base,
				uint32_t limit, uint8_t access,
				uint8_t gran) {
		//Implementation taken from osdever.net
		gdt[num].low_base = (base & 0xFFFF);
		gdt[num].middle_base = (base >> 16) & 0xFF;
		gdt[num].high_base = (base >> 24) & 0xFF;
		
		gdt[num].low_limit = (limit & 0xFFFF);
		gdt[num].granular = ((limit >> 16) & 0x0F);
		
		gdt[num].granular = (gran & 0xF0);
		gdt[num].access = access;
}

void gdt_install() {
	gp.limit = (sizeof(struct gdt_item) * 3) - 1;
	gp.base = (unsigned int)&gdt;
	
	
	gdt_set_gate(0, 0, 0, 0, 0); //NULL item
	
	//Code Segment - base 0, 4KiB granularity, 4GiB limit
	gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
	
	//Data Segment - ditto
	gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
	
	//Apply changes
	load_gdt((uint32_t) &gp);
}