; $Id: bs3-mode-SwitchToPE16_V86.asm $
;; @file
; BS3Kit - Bs3SwitchToPE16_V86
;

;
; Copyright (C) 2007-2025 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>.
;
; The contents of this file may alternatively be used under the terms
; of the Common Development and Distribution License Version 1.0
; (CDDL), a copy of it is provided in the "COPYING.CDDL" file included
; in the VirtualBox 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.
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; SPDX-License-Identifier: GPL-3.0-only OR CDDL-1.0
;

%include "bs3kit-template-header.mac"


;;
; Switch to 16-bit unpaged protected mode with 16-bit sys+tss from any other mode.
;
; @cproto   BS3_DECL(void) Bs3SwitchToPE16(void);
;
; @uses     Nothing (except high 32-bit register parts).
;
; @remarks  Obviously returns to 16-bit v8086 mode, even if the caller was
;           in 16-bit, 32-bit or 64-bit mode.
;
; @remarks  Does not require 20h of parameter scratch space in 64-bit mode.
;
%if TMPL_BITS == 16
BS3_GLOBAL_NAME_EX TMPL_NM(Bs3SwitchToPE16_V86_Safe), function , 0
%endif
BS3_PROC_BEGIN_MODE Bs3SwitchToPE16_V86, BS3_PBC_NEAR
%ifdef TMPL_PE16_V86
        ret

%else
        ;
        ; Convert the return address and jump to the 16-bit code segment.
        ;
 %if TMPL_BITS != 16
        shl     xPRE [xSP], TMPL_BITS - 16
        add     xSP, (TMPL_BITS - 16) / 8
        jmp     .sixteen_bit_segment
BS3_BEGIN_TEXT16
        BS3_SET_BITS TMPL_BITS
BS3_GLOBAL_LOCAL_LABEL .sixteen_bit_segment
 %endif

        ;
        ; Switch to 16-bit PE16 and from there to V8086.
        ;
        extern  TMPL_NM(Bs3SwitchToPE16)
        call    TMPL_NM(Bs3SwitchToPE16)
        BS3_SET_BITS 16

        ;
        ; Switch to v8086 mode (return address is already 16-bit).
        ;
        extern  _Bs3SwitchTo16BitV86_c16
        jmp     _Bs3SwitchTo16BitV86_c16
%endif
BS3_PROC_END_MODE   Bs3SwitchToPE16_V86


%if TMPL_BITS == 16
;;
; Custom far stub.
BS3_BEGIN_TEXT16_FARSTUBS
BS3_PROC_BEGIN_MODE Bs3SwitchToPE16_V86, BS3_PBC_FAR
        inc         bp
        push        bp
        mov         bp, sp

        ; Call the real thing.
        call        TMPL_NM(Bs3SwitchToPE16_V86)

 %if !BS3_MODE_IS_RM_OR_V86(TMPL_MODE)
        ; Jmp to common code for the tedious conversion.
        BS3_EXTERN_CMN Bs3SwitchHlpConvProtModeRetfPopBpDecBpAndReturn
        jmp         Bs3SwitchHlpConvProtModeRetfPopBpDecBpAndReturn
 %else
        pop         bp
        dec         bp
        retf
 %endif
BS3_PROC_END_MODE   Bs3SwitchToPE16_V86

%else
;;
; Safe far return to non-BS3TEXT16 code.
BS3_EXTERN_CMN Bs3SwitchHlpConvFlatRetToRetfProtMode
BS3_BEGIN_TEXT16
BS3_SET_BITS TMPL_BITS
BS3_PROC_BEGIN_MODE Bs3SwitchToPE16_V86_Safe, BS3_PBC_NEAR
        call        Bs3SwitchHlpConvFlatRetToRetfProtMode ; Special internal function.  Uses nothing, but modifies the stack.
        call        TMPL_NM(Bs3SwitchToPE16_V86)
        BS3_SET_BITS 16
        retf
BS3_PROC_END_MODE   Bs3SwitchToPE16_V86_Safe
%endif

