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|
; $Id: bs3-cmn-SwitchTo16Bit.asm $
;; @file
; BS3Kit - Bs3SwitchTo16Bit
;
;
; Copyright (C) 2007-2019 Oracle Corporation
;
; This file is part of VirtualBox Open Source Edition (OSE), as
; available from http://www.virtualbox.org. This file is free software;
; you can redistribute it and/or modify it under the terms of the GNU
; General Public License (GPL) as published by the Free Software
; Foundation, in version 2 as it comes in the "COPYING" file of the
; VirtualBox OSE distribution. VirtualBox OSE is distributed in the
; hope that it will be useful, but WITHOUT ANY WARRANTY of any kind.
;
; The contents of this file may alternatively be used under the terms
; of the Common Development and Distribution License Version 1.0
; (CDDL) only, as it comes in the "COPYING.CDDL" file of the
; VirtualBox OSE distribution, in which case the provisions of the
; CDDL are applicable instead of those of the GPL.
;
; You may elect to license modified versions of this file under the
; terms and conditions of either the GPL or the CDDL or both.
;
%include "bs3kit-template-header.mac"
BS3_EXTERN_DATA16 g_bBs3CurrentMode
%if TMPL_BITS == 16
BS3_EXTERN_CMN Bs3Syscall
%endif
TMPL_BEGIN_TEXT
;;
; @cproto BS3_DECL(void) Bs3SwitchTo16Bit(void);
; @remarks Does not require 20h of parameter scratch space in 64-bit mode.
;
BS3_PROC_BEGIN_CMN Bs3SwitchTo16Bit, BS3_PBC_NEAR
%if TMPL_BITS == 16
push ax
push ds
; Check g_bBs3CurrentMode whether we're in v8086 mode or not.
mov al, [BS3_DATA16_WRT(g_bBs3CurrentMode)]
test al, BS3_MODE_CODE_V86
jz .ret_16bit
; Switch to ring-0 if v8086 mode.
mov ax, BS3_SYSCALL_TO_RING0
call Bs3Syscall
.ret_16bit:
pop ds
pop ax
ret
%else
push xAX
push xBX
xPUSHF
cli
; Calc new CS.
mov ax, cs
and xAX, 3
shl xAX, BS3_SEL_RING_SHIFT ; ring addend.
add xAX, BS3_SEL_R0_CS16
; Construct a far return for switching to 16-bit code.
push xAX
push .sixteen_bit
xRETF
BS3_BEGIN_TEXT16
BS3_GLOBAL_LOCAL_LABEL .sixteen_bit
; Load 16-bit segment registers.
add ax, BS3_SEL_R0_SS16 - BS3_SEL_R0_CS16
mov ss, ax
add ax, BS3_SEL_R0_DS16 - BS3_SEL_R0_SS16
mov ds, ax
mov es, ax
; Thunk the stack if necessary.
mov ebx, esp
shr ebx, 16
jz .stack_ok
int3 ; This is for later, just remove this int3 once needed.
test ax, X86_SEL_RPL
jnz .stack_rpl_must_be_0_for_custom_stacks
shl bx, X86_SEL_SHIFT
add bx, BS3_SEL_TILED
mov ss, bx
movzx esp, sp
.stack_ok:
; Update globals.
and byte [BS3_DATA16_WRT(g_bBs3CurrentMode)], ~BS3_MODE_CODE_MASK
or byte [BS3_DATA16_WRT(g_bBs3CurrentMode)], BS3_MODE_CODE_16
popfd
TONLY64 pop ebx
pop ebx
TONLY64 pop eax
pop eax
TONLY64 add sp, 4
ret (TMPL_BITS - 16) / 8 ; Return and pop 2 or 6 bytes of "parameters" (unused return value)
.stack_rpl_must_be_0_for_custom_stacks:
int3
jmp .stack_rpl_must_be_0_for_custom_stacks
TMPL_BEGIN_TEXT
%endif
BS3_PROC_END_CMN Bs3SwitchTo16Bit
;; @todo far 16-bit variant.
|
