/*
    Copyright (C) 2012, Aaron Lindsay <aaron@aclindsay.com>

    This file is part of Aedrix.

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License along
    with this program; if not, write to the Free Software Foundation, Inc.,
    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
 */

#include <arch/segmentation.h>

#define KERNEL_START_VIRTUAL 0xc0000000
#define KERNEL_START_PAGE (KERNEL_START_VIRTUAL >> 22)
#define virt_to_phys(addr) (addr - KERNEL_START_VIRTUAL)

#define STACK_SIZE 0x4000 /* 16k */

/* setup multiboot header */
#define ALIGN     (1<<0)                   /* align loaded modules on page boundaries */
#define MEMINFO   (1<<1)                   /* provide memory map */
#define FLAGS     (ALIGN | MEMINFO)        /* this is the Multiboot 'flag' field */
#define MAGIC     0x1BADB002               /* 'magic number' lets bootloader find the header */
#define CHECKSUM  (-(MAGIC + FLAGS))       /* checksum required */

.align 4
.long MAGIC
.long FLAGS
.long CHECKSUM

/*
 * Kernel entry in assembly for i386 architecture. This handles saving off
 * multiboot information, setting up initial page tables to map the kernel to
 * where it is linked against, ensures segmentation is setup right (large segments
 * over entire address range), and finally jumps to the kernel's main() function
 * in C.
 */
.global start
start:
	movl  %eax, virt_to_phys(magic)		/* Save off multiboot magic number */
	movl  %ebx, virt_to_phys(mbd)		/* Save off multiboot data structure */

	movl $virt_to_phys(init_page_dir), %eax	/* load page directory base register */
	movl %eax, %cr3

	movl %cr4, %eax
	orl $0x00000010, %eax			/* set PSE bit for 4mb pages */
	movl %eax, %cr4

	movl %cr0, %eax
	orl $0x80000000, %eax			/* set PG bit to enable paging */
	movl %eax, %cr0

	lea paging_enabled, %eax
	jmp *%eax
paging_enabled:
	movl $0, init_page_dir			/* unmap identity mapping and invalidate */
	invlpg 0

	movl $(stack + STACK_SIZE), %esp	/* set up the stack */

	/* setup two large kernel segments over entire 4gb address range */
	pushl $initial_gdt			/* setup the pointer to the GDT */
	pushw $(INITIAL_GDT_SIZE-1)
	lgdt (%esp)
	addl $6, %esp
	ljmp $SEGMENT_SELECTOR_KERNEL_CS, $gdt_reload	/* long jump to set %cs */
gdt_reload:

	/* reset remaining segment registers */
	movl $SEGMENT_SELECTOR_KERNEL_DS, %eax
	movl %eax, %ds
	movl %eax, %es
	movl %eax, %fs
	movl %eax, %gs
	movl %eax, %ss

	call main				/* call kernel proper */

	cli
hang:
	hlt                                 /* halt machine should kernel return */
	jmp   hang

.section .init

/* Note: if the offsets into the GDT change, they must be updated in arch/segmentation.h */
.set INITIAL_GDT_SIZE, 8*6
initial_gdt:
	.long 0x00000000 /* 0x00 intentionally empty */
	.long 0x00000000
	.long 0x00000000 /* 0x08 TODO TSS */
	.long 0x00000000
	.long 0x0000ffff /* 0x10 kernel code segment (%cs) */
	.long 0x00cf9a00
	.long 0x0000ffff /* 0x18 kernel data segment (%ds) */
	.long 0x00cf9200
	.long 0x0000ffff /* 0x20 userspace code segment (%cs) */
	.long 0x00cffa00
	.long 0x0000ffff /* 0x28 userspace data segment (%ds) */
	.long 0x00cff200

.align 0x1000
init_page_dir:
	/* This page directory entry identity-maps the first 4MB of the 32-bit physical address space.
	All bits are clear except the following:
	bit 7: PS The kernel page is 4MB.
	bit 1: RW The kernel page is read/write.
	bit 0: P  The kernel page is present.
	This entry must be here -- otherwise the kernel will crash immediately after paging is
	enabled because it can't fetch the next instruction! It's ok to unmap this page later. */
	.long 0x00000083
	.rept (KERNEL_START_PAGE - 1)
		.long 0                 /* Pages before kernel space. */
	.endr
	/* This page directory entry defines a 4MB page containing the kernel. */
	.long 0x00000083
	.rept (1024 - KERNEL_START_PAGE - 1)
		.long 0  /* Pages after the kernel image. */
	.endr

/* reserve initial kernel stack space */
.align 32
.lcomm stack, STACK_SIZE		/* reserve stack space on a doubleword boundary */

/* place to store multiboot header information */
.global mbd
.comm  mbd, 4				/* we will use this in i386_main */
.global magic
.comm  magic, 4				/* we will use this in i386_main */

/* TODO FIXME remove this - needs fix to dependencies first */
.global atags_ptr
atags_ptr:
	.long 0