; 
; 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.

; This is the kernel's entry point. We switch to the protected mode
; and jump to our kernel.

[BITS 16]
SECTION .text
GLOBAL _start
ORG 0x90200
_start:
	jmp _realstart

	msg db "Hello from MetalSVM bootloader!!",0

	struc GDT_STR
		s0_15:   resw 1
		b0_15:   resw 1
		b16_23:  resb 1
		flags:   resb 1
		access:  resb 1
		b24_31:  resb 1
	endstruc

	gdt_start dw gdt_size
	gdt_ptr dd dummy_descriptor
	
	dummy_descriptor: istruc GDT_STR		
		at s0_15, dw 0
		at b0_15, dw 0
		at b16_23, db 0
		at flags, db 0
		at access, db 0
		at b24_31, db 0
        iend

	; 4GB 32-bit code , 9ah = 10011010b = Present, DPL 00,No System, Code Exec/Read.
	code32_descriptor: istruc GDT_STR 
		at s0_15, dw 0x0ffff
		at b0_15, dw 0
		at b16_23, db 0
		at flags, db 0x9a
		at access, db 0xcf
		at b24_31, db 0 
	iend 

	; 4GB 32-bit data,   92h = 10010010b = Present, DPL 00, No System, Data Read/Write
	data32_descriptor: istruc GDT_STR 
		at s0_15, dw 0x0ffff
		at b0_15, dw 0
		at b16_23, db 0
		at flags, db 0x92
		at access, db 0xcf
		at b24_31, db 0
	iend

	gdt_size equ $ - (dummy_descriptor) - 1

enable_A20:
	call    a20wait
	mov     al,0xAD
	out     0x64,al
 
	call    a20wait
	mov     al,0xD0
	out     0x64,al
 
	call    a20wait2
	in      al,0x60
	push    eax
 
	call    a20wait
	mov     al,0xD1
	out     0x64,al
 
	call    a20wait
	pop     eax
	or      al,2
	out     0x60,al

	call    a20wait
	mov     al,0xAE
	out     0x64,al
 
	call    a20wait
	ret
 
a20wait:
	in      al,0x64
	test    al,2	
	jnz     a20wait
	ret
 
a20wait2:
	in      al,0x64
	test    al,1
	jz      a20wait2
	ret

_realstart:
	lgdt [gdt_start]

	; call enable_A20

	; switch to protected mode by setting PE bit
	mov eax, cr0
	or al, 0x1
	mov cr0, eax

	; far jump to the 32bit code
	jmp dword 0x08 : _pmstart

[BITS 32]
_pmstart:
	; MetalSVM support Multiboot
	; => Therefore, we switch to the same machine state. Afterwards, we jump to our kernel
	xor eax, eax
	mov ax, 0x10
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax
	mov ss, ax
	mov eax, 0x2BAD002
	xor ebx, ebx ; => it exists no valid multiboot information structure
	jmp 0x08 : 0x100000

	TIMES 4096-($-$$) DB 0