memory_segments.c

#include "memory_segments.h"

#define SEGMENT_DESCRIPTOR_COUNT 3

#define SEGMENT_BASE 0
#define SEGMENT_LIMIT 0xFFFFF

#define SEGMENT_CODE_TYPE 0x9A
#define SEGMENT_DATA_TYPE 0x92

/*
 * Flags part of `limit_and_flags`.
 * 1100
 * 0 - Available for system use
 * 0 - Long mode
 * 1 - Size (0 for 16-bit, 1 for 32)
 * 1 - Granularity (0 for 1B - 1MB, 1 for 4KB - 4GB)
*/
#define SEGMENT_FLAGS_PART 0x0C

static struct GDTDescriptor gdt_descriptors[SEGMENT_DESCRIPTOR_COUNT];

void segments_init_descriptor(int index, unsigned int base_address, unsigned int limit, unsigned char access_byte, unsigned char flags)
{
	gdt_descriptors[index].base_low = base_address & 0xFFFF;
	gdt_descriptors[index].base_middle = (base_address >> 16) & 0xFF;
	gdt_descriptors[index].base_high = (base_address >> 24) & 0xFF;

	gdt_descriptors[index].limit_low = limit & 0xFFFF;
	gdt_descriptors[index].limit_and_flags = (limit >> 16) & 0xF;
	gdt_descriptors[index].limit_and_flags |= (flags << 4) & 0xF0;

	gdt_descriptors[index].access_byte = access_byte;
}

void segments_install_gdt()
{
	gdt_descriptors[0].base_low = 0;
	gdt_descriptors[0].base_middle = 0;
	gdt_descriptors[0].base_high = 0;
	gdt_descriptors[0].limit_low = 0;
	gdt_descriptors[0].access_byte = 0;
	gdt_descriptors[0].limit_and_flags = 0;

	// The null descriptor which is never referenced by the processor. 
        // Certain emulators, like Bochs, will complain about limit exceptions if you do not have one present. 
        // Some use this descriptor to store a pointer to the GDT itself (to use with the LGDT instruction).
        // The null descriptor is 8 bytes wide and the pointer is 6 bytes wide so it might just be the perfect place for this.
        // From:  http://wiki.osdev.org/GDT_Tutorial
	struct GDT* gdt_ptr = (struct GDT*)gdt_descriptors;
	gdt_ptr->address = (unsigned int)gdt_descriptors;
	gdt_ptr->size = (sizeof(struct GDTDescriptor) * SEGMENT_DESCRIPTOR_COUNT) - 1;

	// See http://wiki.osdev.org/GDT_Tutorial
	segments_init_descriptor(1, SEGMENT_BASE, SEGMENT_LIMIT, SEGMENT_CODE_TYPE, SEGMENT_FLAGS_PART);
	segments_init_descriptor(2, SEGMENT_BASE, SEGMENT_LIMIT, SEGMENT_DATA_TYPE, SEGMENT_FLAGS_PART);

	segments_load_gdt(*gdt_ptr);
	segments_load_registers();
}