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# RCC-Project
#
# Copyright (C) 2026 Richard Qin
#
# This file is part of RCC-Project.
#
# RCC-Project 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 3 of the License, or (at your option) any
# later version.
#
# RCC-Project 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
# RCC-Project; see the file COPYING. If not see
# <http://www.gnu.org/licenses/>.
.intel_syntax noprefix
.code16
.section .text16.mode, "ax"
.extern post_main
.global enter_protected_mode
.type enter_protected_mode, @function
enter_protected_mode:
cli
# Enable A20 through the fast A20 gate.
in al, 0x92
or al, 0x02
and al, 0xFE
out 0x92, al
# Make data references address the ROM code segment.
push cs
pop ds
call 1f
1:
pop bx
add bx, gdt_descriptor - 1b
lgdt [bx]
# Enable protected mode
mov eax, cr0
or eax, 0x00000001
mov cr0, eax
# Reload CS with the 32-bit code selector.
.byte 0x66
.byte 0xEA
.long protected_mode_entry
.word 0x0008
.align 8
gdt_start:
.quad 0x0000000000000000
gdt_code:
.word 0xFFFF
.word 0x0000
.byte 0x00
.byte 0x9A
.byte 0xCF
.byte 0x00
gdt_data:
.word 0xFFFF
.word 0x0000
.byte 0x00
.byte 0x92
.byte 0xCF
.byte 0x00
gdt_code16:
.word 0xFFFF
.word 0x0000
.byte 0x0F
.byte 0x9A
.byte 0x0F
.byte 0x00
gdt_end:
gdt_descriptor:
.word gdt_end - gdt_start - 1
.long gdt_start
.code32
.section .text32, "ax"
.global protected_mode_entry
.type protected_mode_entry, @function
protected_mode_entry:
mov ax, 0x0010
mov ds, ax
mov es, ax
mov ss, ax
mov fs, ax
mov gs, ax
mov esp, 0x00007000
call post_main
call leave_protected_mode
protected_mode_halt:
hlt
jmp protected_mode_halt
.global leave_protected_mode
.type leave_protected_mode, @function
.extern __rom_start
.extern protected16_return
.extern real_mode_return
leave_protected_mode:
# Keep the original 16-bit return address at 0x7FFE intact.
sub esp, 12
# Stack[0]: protected-mode 16-bit code offset
# Stack[2]: protected-mode 16-bit code selector
mov eax, OFFSET FLAT:protected16_return
mov ebx, OFFSET FLAT:__rom_start
sub eax, ebx
mov word ptr [esp], ax
mov word ptr [esp + 2], 0x0018
# Stack[4]: real-mode offset
# Stack[6]: real-mode segment
mov eax, OFFSET FLAT:real_mode_return
mov ebx, OFFSET FLAT:__rom_start
sub eax, ebx
mov word ptr [esp + 4], ax
shr ebx, 4
mov word ptr [esp + 6], bx
# Enter the 16-bit protected-mode code segment.
.byte 0x66
.byte 0xFF, 0x2C, 0x24
.code16
.section .text16.mode, "ax"
.global protected16_return
.type protected16_return, @function
protected16_return:
mov bx, sp
add bx, 4
# Leave protected mode while already using a 16-bit code segment.
mov eax, cr0
and eax, 0xFFFFFFFE
mov cr0, eax
# Jump to the prepared real-mode CS:IP pointer.
.byte 0xFF, 0x2F
.global real_mode_return
.type real_mode_return, @function
real_mode_return:
xor ax, ax
mov ds, ax
mov es, ax
mov ss, ax
mov fs, ax
mov gs, ax
mov sp, 0x7FFE
ret
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