1 files changed, 91 insertions, 0 deletions
diff --git a/src/VBox/Devices/PC/BIOS/pmode.inc b/src/VBox/Devices/PC/BIOS/pmode.inc
new file mode 100644
index 00000000..a72cb4ae
--- /dev/null
+++ b/src/VBox/Devices/PC/BIOS/pmode.inc
@@ -0,0 +1,91 @@
+; $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
+