gdt.cpp

#include "gdt.h"

GlobalDescriptorTable::GlobalDescriptorTable()
: nullSegmentSelector(0, 0, 0),
unusedSegmentSelector(0, 0, 0),
codeSegmentSelector(0, 64*1024*1024, 0x9A),
dataSegmentSelector(0, 64*1024*1024, 0x92) {

	uint32_t i[2];
	i[0] = (uint32_t)this;
	i[1] = sizeof(GlobalDescriptorTable) << 16;

    // Tell the processor to use this table now.
	asm volatile("lgdt (%0)": :"p" (((uint8_t *) i) + 2));
}

GlobalDescriptorTable::~GlobalDescriptorTable() { }

uint16_t GlobalDescriptorTable::DataSegmentSelector() {
	return (uint8_t*)&dataSegmentSelector - (uint8_t*)this;
}

uint16_t GlobalDescriptorTable::CodeSegmentSelector() {
	return (uint8_t*)&codeSegmentSelector - (uint8_t*)this;
}

GlobalDescriptorTable::SegmentDescriptor::SegmentDescriptor(uint32_t base, uint32_t limit, uint8_t flags) {
	uint8_t* target = (uint8_t*)this;

    // Is a 16 bit limit
	if(limit <= 65536) {
		// Configs flags
		target[6] = 0x40;
	} else {
		// Get the legal value for the limit
		if(!(limit & 0xFFF) != 0xFFF) {
			limit = (limit >> 12) - 1; 
		} else {
			limit = (limit >> 12);
		}

        // Configs flags
		target[6] = 0xC0;
	}

    // Assigning ptr values in entry
	target[0] = limit & 0xFF;
	target[1] = (limit >> 8) & 0xFF;
	target[6] |= (limit >> 16) & 0xF;

	// Encoding the entry
	target[2] = base & 0xFF;
	target[3] = (base >> 8) & 0xFF;
	target[4] = (base >> 16) & 0xFF;
	target[7] = (base >> 24) & 0xFF;

    // Access rights
	target[5] = flags;
}

uint32_t GlobalDescriptorTable::SegmentDescriptor::Base() {
    uint8_t* target = (uint8_t*)this;
    uint32_t result = target[7];
    result = (result << 8) + target[4];
    result = (result << 8) + target[3];
    result = (result << 8) + target[2];

    return result;
}

uint32_t GlobalDescriptorTable::SegmentDescriptor::Limit() {
    uint8_t* target = (uint8_t*)this;
    uint32_t result = target[6] & 0xF;
    result = (result << 8) + target[1];
    result = (result << 8) + target[0];

    if((target[6] & 0xC0) == 0xC0) {
    	result = (result << 12) | 0xFFF;
    }
}