| 1 | ; $Id: bs3-cmn-SwitchTo16Bit.asm 60000 2016-03-11 19:12:05Z vboxsync $
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| 2 | ;; @file
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| 3 | ; BS3Kit - Bs3SwitchTo16Bit
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| 4 | ;
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| 5 |
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| 6 | ;
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| 7 | ; Copyright (C) 2007-2015 Oracle Corporation
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| 8 | ;
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| 9 | ; This file is part of VirtualBox Open Source Edition (OSE), as
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| 10 | ; available from http://www.virtualbox.org. This file is free software;
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| 11 | ; you can redistribute it and/or modify it under the terms of the GNU
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| 12 | ; General Public License (GPL) as published by the Free Software
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| 13 | ; Foundation, in version 2 as it comes in the "COPYING" file of the
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| 14 | ; VirtualBox OSE distribution. VirtualBox OSE is distributed in the
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| 15 | ; hope that it will be useful, but WITHOUT ANY WARRANTY of any kind.
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| 16 | ;
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| 17 | ; The contents of this file may alternatively be used under the terms
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| 18 | ; of the Common Development and Distribution License Version 1.0
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| 19 | ; (CDDL) only, as it comes in the "COPYING.CDDL" file of the
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| 20 | ; VirtualBox OSE distribution, in which case the provisions of the
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| 21 | ; CDDL are applicable instead of those of the GPL.
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| 22 | ;
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| 23 | ; You may elect to license modified versions of this file under the
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| 24 | ; terms and conditions of either the GPL or the CDDL or both.
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| 25 | ;
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| 26 |
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| 27 | %include "bs3kit-template-header.mac"
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| 28 |
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| 29 | BS3_EXTERN_DATA16 g_bBs3CurrentMode
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| 30 | %if TMPL_BITS == 16
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| 31 | BS3_EXTERN_CMN Bs3Syscall
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| 32 | %endif
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| 33 | TMPL_BEGIN_TEXT
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| 34 |
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| 35 |
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| 36 | ;;
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| 37 | ; @cproto BS3_DECL(void) Bs3SwitchTo16Bit(void);
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| 38 | ; @remarks Does not require 20h of parameter scratch space in 64-bit mode.
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| 39 | ;
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| 40 | BS3_PROC_BEGIN_CMN Bs3SwitchTo16Bit
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| 41 | %if TMPL_BITS == 16
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| 42 | push ax
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| 43 | push ds
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| 44 |
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| 45 | ; Check g_bBs3CurrentMode whether we're in v8086 mode or not.
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| 46 | mov al, [BS3_DATA16_WRT(g_bBs3CurrentMode)]
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| 47 | and al, BS3_MODE_CODE_MASK
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| 48 | cmp al, BS3_MODE_CODE_V86
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| 49 | jne .ret_16bit
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| 50 |
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| 51 | ; Switch to ring-0 if v8086 mode.
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| 52 | mov ax, BS3_SYSCALL_TO_RING0
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| 53 | call Bs3Syscall
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| 54 |
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| 55 | .ret_16bit:
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| 56 | pop ds
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| 57 | pop ax
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| 58 | ret
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| 59 |
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| 60 | %else
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| 61 | push xAX
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| 62 | push xBX
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| 63 | xPUSHF
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| 64 | cli
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| 65 |
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| 66 | ; Calc new CS.
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| 67 | mov ax, cs
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| 68 | and xAX, 3
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| 69 | shl xAX, BS3_SEL_RING_SHIFT ; ring addend.
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| 70 | add xAX, BS3_SEL_R0_CS16
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| 71 |
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| 72 | ; Construct a far return for switching to 16-bit code.
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| 73 | push xAX
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| 74 | push .sixteen_bit
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| 75 | xRETF
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| 76 |
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| 77 | BS3_BEGIN_TEXT16
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| 78 | .sixteen_bit:
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| 79 |
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| 80 | ; Load 16-bit segment registers.
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| 81 | add ax, BS3_SEL_R0_SS16 - BS3_SEL_R0_CS16
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| 82 | mov ss, ax
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| 83 |
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| 84 | add ax, BS3_SEL_R0_DS16 - BS3_SEL_R0_SS16
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| 85 | mov ds, ax
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| 86 | mov es, ax
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| 87 |
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| 88 | ; Thunk the stack if necessary.
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| 89 | mov ebx, esp
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| 90 | shr ebx, 16
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| 91 | jz .stack_ok
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| 92 | int3 ; This is for later, just remove this int3 once needed.
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| 93 | test ax, X86_SEL_RPL
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| 94 | jnz .stack_rpl_must_be_0_for_custom_stacks
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| 95 | shl bx, X86_SEL_SHIFT
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| 96 | add bx, BS3_SEL_TILED
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| 97 | mov ss, bx
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| 98 | movzx esp, sp
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| 99 | .stack_ok:
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| 100 |
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| 101 | ; Update globals.
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| 102 | and byte [BS3_DATA16_WRT(g_bBs3CurrentMode)], ~BS3_MODE_CODE_MASK
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| 103 | or byte [BS3_DATA16_WRT(g_bBs3CurrentMode)], BS3_MODE_CODE_16
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| 104 |
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| 105 | popfd
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| 106 | TMPL_ONLY_64BIT_STMT pop ebx
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| 107 | pop ebx
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| 108 | TMPL_ONLY_64BIT_STMT pop eax
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| 109 | pop eax
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| 110 | TMPL_ONLY_64BIT_STMT add sp, 4
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| 111 | ret (TMPL_BITS - 16) / 8 ; Return and pop 2 or 6 bytes of "parameters" (unused return value)
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| 112 |
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| 113 | .stack_rpl_must_be_0_for_custom_stacks:
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| 114 | int3
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| 115 | jmp .stack_rpl_must_be_0_for_custom_stacks
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| 116 | TMPL_BEGIN_TEXT
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| 117 | %endif
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| 118 | BS3_PROC_END_CMN Bs3SwitchTo16Bit
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| 119 |
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