| 1 | ;; @file
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| 2 | ;; Enter and exit a minimal protected-mode environment.
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| 3 | ;;
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| 4 |
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| 5 | ;;
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| 6 | ;; Copyright (C) 2004-2015 Oracle Corporation
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| 7 | ;;
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| 8 | ;; This file is part of VirtualBox Open Source Edition (OSE), as
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| 9 | ;; available from http://www.virtualbox.org. This file is free software;
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| 10 | ;; you can redistribute it and/or modify it under the terms of the GNU
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| 11 | ;; General Public License (GPL) as published by the Free Software
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| 12 | ;; Foundation, in version 2 as it comes in the "COPYING" file of the
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| 13 | ;; VirtualBox OSE distribution. VirtualBox OSE is distributed in the
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| 14 | ;; hope that it will be useful, but WITHOUT ANY WARRANTY of any kind.
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| 15 | ;;
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| 16 |
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| 17 | ;; Caveats: May only be called from the F000 segment (16-bit). Does not
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| 18 | ;; switch stacks. Must be run with disabled interrupts(!), any exceptions
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| 19 | ;; will cause a crash. On return from pmode_enter, DS contains a selector
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| 20 | ;; which can address the entire 4GB address space.
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| 21 |
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| 22 | public pmode_enter
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| 23 | public pmode_exit
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| 24 | public pmbios_gdt_desc
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| 25 | public pmbios_gdt
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| 26 |
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| 27 | pmode_enter proc near
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| 28 |
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| 29 | push cs
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| 30 | pop ds
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| 31 | .386p
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| 32 | lgdt fword ptr [pmbios_gdt_desc]
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| 33 | mov eax, cr0
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| 34 | or al, 1
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| 35 | mov cr0, eax
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| 36 | ; jmp far ptr 20h:really_enter_pm
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| 37 | db 0EAh
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| 38 | dw really_enter_pm
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| 39 | dw 20h
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| 40 | really_enter_pm:
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| 41 | mov ax, 18h
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| 42 | mov ds, ax
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| 43 | ret
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| 44 |
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| 45 | pmode_enter endp
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| 46 |
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| 47 |
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| 48 | pmode_exit proc near
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| 49 |
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| 50 | mov ax, 40h ; Ensure RM limit/attributes
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| 51 | mov ds, ax ; (where base = selector << 4)
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| 52 |
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| 53 | mov eax, cr0
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| 54 | and al, 0FEh
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| 55 | mov cr0, eax
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| 56 | .286
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| 57 | jmp far ptr really_exit_pm
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| 58 | really_exit_pm:
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| 59 | ret
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| 60 |
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| 61 | pmode_exit endp
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| 62 |
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| 63 |
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| 64 |
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| 65 | pmbios_gdt_desc:
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| 66 | dw 40h + 7 ; last selector plus 8 - 1
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| 67 | dw pmbios_gdt
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| 68 | dw 000Fh
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| 69 |
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| 70 | pmbios_gdt:
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| 71 | dw 0, 0, 0, 0
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| 72 | dw 0, 0, 0, 0
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| 73 | dw 0ffffh, 0, 9b00h, 00cfh ; 32-bit code (0x10)
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| 74 | dw 0ffffh, 0, 9300h, 00cfh ; 32-bit data (0x18)
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| 75 | dw 0ffffh, 0, 9b0fh, 0000h ; 16-bit code, base=0xf0000
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| 76 | dw 0ffffh, 0, 9300h, 0000h ; 16-bit data, base=0x0
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| 77 | dw 0, 0, 0, 0
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| 78 | dw 0, 0, 0, 0
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| 79 | dw 0ffffh, 400h, 9300h, 0000h ; 16-bit data, base=0x40
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