/* 
 * Copyright 2010 Stefan Lankes, Chair for Operating Systems,
 *                               RWTH Aachen University
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * This file is part of MetalSVM.
 */

#include <metalsvm/string.h>

/* Defines a GDT entry */
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;
	unsigned int base;
} __attribute__ ((packed));

#define GDT_ENTRIES	3

struct gdt_entry gdt[GDT_ENTRIES];
struct gdt_ptr gp;

/* 
 * This is in start.asm. We use this to properly reload
 * the new segment registers
 */
extern void gdt_flush(void);

/* Setup a descriptor in the Global Descriptor Table */
static void gdt_set_gate(int num, unsigned long base, unsigned long limit,
			  unsigned char access, unsigned char gran)
{

	/* Setup the descriptor base address */
	gdt[num].base_low = (base & 0xFFFF);
	gdt[num].base_middle = (base >> 16) & 0xFF;
	gdt[num].base_high = (base >> 24) & 0xFF;

	/* Setup the descriptor limits */
	gdt[num].limit_low = (limit & 0xFFFF);
	gdt[num].granularity = ((limit >> 16) & 0x0F);

	/* Finally, set up the granularity and access flags */
	gdt[num].granularity |= (gran & 0xF0);
	gdt[num].access = access;
}

/* 
 * This will setup the special GDT
 * pointer, set up the entries in our GDT, and then
 * finally call gdt_flush() in our assembler file in order
 * to tell the processor where the new GDT is and update the
 * new segment registers 
 */
void gdt_install(void)
{
	/* Setup the GDT pointer and limit */
	gp.limit = (sizeof(struct gdt_entry) * GDT_ENTRIES) - 1;
	gp.base = (unsigned int) &gdt;

	/* Our NULL descriptor */
	gdt_set_gate(0, 0, 0, 0, 0);

	/* 
	 * The second entry is our Code Segment. The base address
	 * is 0, the limit is 4 GByte, it uses 4KByte granularity,
	 * uses 32-bit opcodes, and is a Code Segment descriptor.
	 */
	gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);

	/* 
	 * The third entry is our Data Segment. It's EXACTLY the
	 * same as our code segment, but the descriptor type in
	 * this entry's access byte says it's a Data Segment 
	 */
	gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);
		
	/* Flush out the old GDT and install the new changes! */
	gdt_flush();
}