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; $Id: pmode.inc $
;; @file
; Enter and exit a minimal protected-mode environment.
;
;
; Copyright (C) 2004-2023 Oracle and/or its affiliates.
;
; This file is part of VirtualBox base platform packages, as
; available from https://www.virtualbox.org.
;
; 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, in version 3 of the
; License.
;
; 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, see <https://www.gnu.org/licenses>.
;
; SPDX-License-Identifier: GPL-3.0-only
;
;; Caveats: May only be called from the F000 segment (16-bit). Does not
;; switch stacks. Must be run with disabled interrupts(!), any exceptions
;; will cause a crash. On return from pmode_enter, DS contains a selector
;; which can address the entire 4GB address space.
public pmode_enter
public pmode_exit
public pmbios_gdt_desc
public pmbios_gdt
pmode_enter proc near
push cs
pop ds
.386p
lgdt fword ptr [pmbios_gdt_desc]
mov eax, cr0
or al, 1
mov cr0, eax
; jmp far ptr 20h:really_enter_pm
db 0EAh
dw really_enter_pm
dw 20h
really_enter_pm:
mov ax, 18h
mov ds, ax
ret
pmode_enter endp
pmode_exit proc near
mov ax, 40h ; Ensure RM limit/attributes
mov ds, ax ; (where base = selector << 4)
mov eax, cr0
and al, 0FEh
mov cr0, eax
SET_DEFAULT_CPU_286
jmp far ptr really_exit_pm
really_exit_pm:
ret
pmode_exit endp
pmbios_gdt_desc:
dw 40h + 7 ; last selector plus 8 - 1
dw pmbios_gdt
dw 000Fh
pmbios_gdt:
dw 0, 0, 0, 0
dw 0, 0, 0, 0
dw 0ffffh, 0, 9b00h, 00cfh ; 32-bit code (0x10)
dw 0ffffh, 0, 9300h, 00cfh ; 32-bit data (0x18)
dw 0ffffh, 0, 9b0fh, 0000h ; 16-bit code, base=0xf0000
dw 0ffffh, 0, 9300h, 0000h ; 16-bit data, base=0x0
dw 0, 0, 0, 0
dw 0, 0, 0, 0
dw 0ffffh, 400h, 9300h, 0000h ; 16-bit data, base=0x40
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