.code64

.globl real.regs
real.regs:
reax:
	.int 0x0
rebx:
	.int 0x0
recx:
	.int 0x0
redx:
	.int 0x0
redi:
	.int 0x0
resi:
	.int 0x0


# :real_call
# This function is intended to be called from long mode
# and calls another function in real mode. Note that precautions have
# to be made. Recall that real mode can only access below 0xFFFFF.
# So the target function, and objects of arguments passed to it cannot
# be located above that. A reasonable scheme is to put the result
# in a buffer and then, once back in long mode, copy its content to higher
# above.
# The procedure to go from Long Mode back to Real Mode is explained in much details
# in the AMD64 Programmer's Manual Vol. 2, in particular, the figure 1-6. in section 1.3.
.globl real.call
real.call:
	# :Push all the segments registers
	push %rbx
	push %r12
	push %r13
	push %r14
	push %r15
	push %di

	cli


	lgdt gdtr32
	pushq $(gdt32_code - gdt32)
	pushq $real_call_to_pmode_down
	retfq
.code32
real_call_to_pmode_down:
	# :Here, we are in compatibility mode (32 bits)

	# :Disable paging
	mov %cr0, %eax
	and $~(1 << 31), %eax
	mov %eax, %cr0

	# :Disabling long mode in msr
	mov $0xc0000080, %ecx
	rdmsr
	and $~(1 << 8), %eax
	wrmsr

	# :Here we are in true protected mode (32 bits)
	# Let's continue our descent to real mode
	# by switching our gdt to 16 bits
	lgdt gdtr16
	ljmp $(gdt16_code - gdt16), $real_call_to_16bits_pmode_down
.code16
real_call_to_16bits_pmode_down:

	# :Disable protected mode
	mov %cr0, %eax
	and $~1, %eax
	mov %eax, %cr0

	ljmp $0, $real_call_to_16bits_rmode_down
real_call_to_16bits_rmode_down:
	mov $0, %ax
	mov %ax, %ds
	mov %ax, %es
	mov %ax, %ss
	mov %ax, %gs
	mov %ax, %fs

	# :real mode, yay
	
	lidt bios_idtr

	sti

	mov rebx, %ebx
	mov recx, %ecx
	mov redx, %edx
	mov redi, %edi
	mov resi, %esi

	pop %ax	
	call *%ax
		
	mov %eax, reax
	mov %ebx, rebx
	mov %ecx, recx
	mov %edx, redx
	mov %edi, redi
	mov %esi, resi

	cli
	
	# :Restore protected mode
	mov %cr0, %eax
	or $1, %eax
	mov %eax, %cr0

	lgdt gdtr32
	ljmp $(gdt32_code - gdt32), $real_call_to_pmode_up
.code32
real_call_to_pmode_up:
	# :Restore PAE (probably unneeded)
	mov %cr4, %eax
	or $(1 << 5), %eax
	mov %eax, %cr4

	# :Restore long mode
	mov $0xc0000080, %ecx
	rdmsr
	or $(1 << 8), %eax
	wrmsr

	# :Restore paging
	mov %cr0, %eax
	or $1 << 31, %eax
	mov %eax, %cr0

	lgdt gdtr64
	ljmp $(gdt64_code - gdt64), $real_call_to_longmode_up
.code64
real_call_to_longmode_up:
	mov $(gdt64_data - gdt64), %ax
	mov %ax, %fs
	mov %ax, %gs
	mov %ax, %ss
	mov %ax, %es
	mov %ax, %ds

real_call_end:
	pop %r15
	pop %r14
	pop %r13
	pop %r12
	pop %rbx
	ret

.globl testt
testt:
	mov $1, %al
	mov $0x43, %bh
	mov $0x6, %ah
	mov $0, %ch
	mov $0, %cl
	mov $5, %dh
	mov $5, %dl
	int $0x10
	ret