;------------------------------------------------------------------------------ ; ; Copyright (c) 2006, Intel Corporation ; All rights reserved. This program and the accompanying materials ; are licensed and made available under the terms and conditions of the BSD License ; which accompanies this distribution. The full text of the license may be found at ; http://opensource.org/licenses/bsd-license.php ; ; THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS, ; WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED. ; ; Module Name: ; ; Thunk.asm ; ; Abstract: ; ; Real mode thunk ; ;------------------------------------------------------------------------------ .data NullSegSel DQ 0 _16CsSegSel LABEL QWORD DW -1 DW 0 DB 0 DB 9bh DB 8fh ; 16-bit segment DB 0 _16BitDsSel LABEL QWORD DW -1 DW 0 DB 0 DB 93h DB 8fh ; 16-bit segment DB 0 GdtEnd LABEL QWORD .const _16Gdtr LABEL FWORD DW offset GdtEnd - offset NullSegSel - 1 DQ offset NullSegSel _16Idtr FWORD (1 SHL 10) - 1 .code IA32_REGS STRUC 4t _EDI DD ? _ESI DD ? _EBP DD ? _ESP DD ? _EBX DD ? _EDX DD ? _ECX DD ? _EAX DD ? _DS DW ? _ES DW ? _FS DW ? _GS DW ? _RFLAGS DQ ? _EIP DD ? _CS DW ? _SS DW ? IA32_REGS ENDS InternalAsmThunk16 PROC USES rbp rbx rsi rdi r12 r13 r14 r15 mov eax, ds push rax mov eax, es push rax push fs push gs mov rsi, rcx ; rsi <- RegSet push sizeof (IA32_REGS) pop rcx movzx r8, (IA32_REGS ptr [rsi])._SS xor rdi, rdi mov edi, (IA32_REGS ptr [rsi])._ESP sub rdi, rcx ; reserve space on realmode stack push rdi ; save stack offset imul rax, r8, 16 add rdi, rax ; rdi <- linear address of 16-bit stack rep movsb ; copy RegSet mov rsi, r8 ; si <- 16-bit stack segment pop rbx ; rbx <- 16-bit stack offset mov rdi, rdx ; rdi <- realmode patch lea eax, @BackToThunk ; rax <- address to back from real mode push rax ; use in a far return mov eax, cs mov [rsp + 4], eax ; save CS lea eax, @16Return ; thus @Return must < 4GB stosd ; set ret address offset xor eax, eax stosw ; set ret CS base to 0 mov eax, esp stosd ; rsp must < 4GB mov eax, ss stosd mov rax, cr0 mov ecx, eax ; ecx <- CR0 and ecx, 7ffffffeh ; clear PE, PG bits stosd mov rax, cr4 mov ebp, eax and ebp, 300h ; clear all but PCE and OSFXSR bits stosd sidt fword ptr [rsp + 70h] ; use parameter space to save IDTR sgdt fword ptr [rdi] lea edi, _16Idtr lea eax, @16Start ; rax <- seg:offset of @16Start push rax mov dword ptr [rsp + 4], 8 push 10h pop rax ; rax <- 10h as dataseg selector lgdt _16Gdtr retf @16Start: ; 16-bit starts here mov ss, eax ; set SS to be a 16-bit segment mov cr0, rcx ; disable protected mode mov cr4, rbp DB 66h mov ecx, 0c0000080h rdmsr and ah, NOT 1 ; clear LME wrmsr mov ss, esi ; set up 16-bit stack mov sp, bx ; mov esp, ebx actually lidt fword ptr [edi] DB 66h, 61h ; popad DB 1fh ; pop ds DB 7 ; pop es pop fs pop gs add sp, 8 ; skip _RFLAGS DB 66h retf ; transfer control to 16-bit code @16Return: DB 66h push 0 ; high order 32 bits of rflags pushf ; pushfd actually push gs push fs DB 6 ; push es DB 1eh ; push ds DB 66h, 60h ; pushad DB 67h, 66h, 0c5h, 74h, 24h, 30h ; lds esi, [esp + 12*4] DB 66h mov eax, [esi + 12] mov cr4, rax ; restore CR4 DB 66h lgdt fword ptr [esi + 16] DB 66h mov ecx, 0c0000080h rdmsr or ah, 1 ; set LME wrmsr DB 66h mov eax, [esi + 8] mov cr0, rax ; restore CR0 xor ax, ax ; xor eax, eax actually mov eax, ss mov dword ptr (IA32_REGS ptr [esp])._SS, eax shl ax, 4 ; shl eax, 4 actually add ax, sp ; add eax, esp actually add sp, sizeof (IA32_REGS) ; add esp, sizeof (IA32_REGS) DB 66h mov dword ptr (IA32_REGS ptr [esp - sizeof (IA32_REGS)])._ESP, esp DB 66h lss esp, fword ptr [esi] ; restore protected mode stack DB 66h retf ; go back to protected mode @BackToThunk: lidt fword ptr [rsp + 68h] ; restore protected mode IDTR shl rax, 32 shr rax, 32 ; clear high order 32 bits of RAX pop gs pop fs pop rcx mov es, ecx pop rcx mov ds, ecx ret InternalAsmThunk16 ENDP END