1 files changed, 120 insertions, 0 deletions
diff --git a/src/VBox/ValidationKit/bootsectors/bs3kit/bs3-cmn-SwitchTo16Bit.asm b/src/VBox/ValidationKit/bootsectors/bs3kit/bs3-cmn-SwitchTo16Bit.asm
new file mode 100644
index 00000000..28c17268
--- /dev/null
+++ b/src/VBox/ValidationKit/bootsectors/bs3kit/bs3-cmn-SwitchTo16Bit.asm
@@ -0,0 +1,120 @@
+; $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.
+