1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
|
;------------------------------------------------------------------------------
;
; 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
;
;------------------------------------------------------------------------------
.686p
.model flat,C
EXTERNDEF C m16Start:BYTE
EXTERNDEF C m16Size:WORD
EXTERNDEF C mThunk16Attr:WORD
EXTERNDEF C m16Gdt:WORD
EXTERNDEF C m16GdtrBase:WORD
EXTERNDEF C mTransition:WORD
;THUNK_ATTRIBUTE_BIG_REAL_MODE EQU 1
;THUNK_ATTRIBUTE_DISABLE_A20_MASK_INT_15 EQU 2
;THUNK_ATTRIBUTE_DISABLE_A20_MASK_KBD_CTRL EQU 4
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 ?
_EFLAGS DD ?
_EIP DD ?
_CS DW ?
_SS DW ?
IA32_REGS ENDS
.const
m16Size DW offset InternalAsmThunk16 - offset m16Start
mThunk16Attr DW offset _ThunkAttr - offset m16Start
m16Gdt DW offset _NullSegDesc - offset m16Start
m16GdtrBase DW offset _16GdtrBase - offset m16Start
mTransition DW offset _EntryPoint - offset m16Start
.code
m16Start LABEL BYTE
SavedGdt LABEL FWORD
DW ?
DD ?
_BackFromUserCode PROC
push ss
push cs
DB 66h
call @Base ; push eip
@Base:
pushf ; pushfd actually
cli ; disable interrupts
push gs
push fs
push es
push ds
pushaw ; pushad actually
DB 66h, 0bah ; mov edx, imm32
_ThunkAttr DD ?
test dl, THUNK_ATTRIBUTE_DISABLE_A20_MASK_INT_15
jz @1
mov eax, 15cd2401h ; mov ax, 2401h & int 15h
cli ; disable interrupts
jnc @2
@1:
test dl, THUNK_ATTRIBUTE_DISABLE_A20_MASK_KBD_CTRL
jz @2
in al, 92h
or al, 2
out 92h, al ; deactivate A20M#
@2:
mov eax, ss
DB 67h
lea bp, [esp + sizeof (IA32_REGS)]
mov word ptr (IA32_REGS ptr [esi - sizeof (IA32_REGS)])._ESP, bp
mov ebx, (IA32_REGS ptr [esi - sizeof (IA32_REGS)])._EIP
shl ax, 4 ; shl eax, 4
add bp, ax ; add ebp, eax
DB 66h, 0b8h ; mov eax, imm32
SavedCr4 DD ?
mov cr4, eax
DB 66h
lgdt fword ptr cs:[edi + (offset SavedGdt - offset @Base)]
DB 66h, 0b8h ; mov eax, imm32
SavedCr0 DD ?
mov cr0, eax
DB 0b8h ; mov ax, imm16
SavedSs DW ?
mov ss, eax
DB 66h, 0bch ; mov esp, imm32
SavedEsp DD ?
DB 66h
retf ; return to protected mode
_BackFromUserCode ENDP
_EntryPoint DD offset _ToUserCode - offset m16Start
DW 8h
_16Idtr FWORD (1 SHL 10) - 1
_16Gdtr LABEL FWORD
DW offset GdtEnd - offset _NullSegDesc - 1
_16GdtrBase DD offset _NullSegDesc
_ToUserCode PROC
mov edx, ss
mov ss, ecx ; set new segment selectors
mov ds, ecx
mov es, ecx
mov fs, ecx
mov gs, ecx
mov cr0, eax
mov cr4, ebp ; real mode starts at next instruction
mov ss, esi ; set up 16-bit stack segment
xchg sp, bx ; set up 16-bit stack pointer
DB 66h
call @Base ; push eip
@Base:
pop bp ; ebp <- offset @Base
DB 67h ; address size override
push [esp + sizeof (IA32_REGS) + 2]
lea eax, [esi + (offset @RealMode - offset @Base)]
push eax
retf
@RealMode:
mov cs:[esi + (offset SavedSs - offset @Base)], edx
mov cs:[esi + (offset SavedEsp - offset @Base)], bx
DB 66h
lidt fword ptr cs:[esi + (offset _16Idtr - offset @Base)]
popaw ; popad actually
pop ds
pop es
pop fs
pop gs
popf ; popfd
DB 66h
retf ; transfer control to user code
_ToUserCode ENDP
_NullSegDesc DQ 0
_16CsDesc LABEL QWORD
DW -1
DW 0
DB 0
DB 9bh
DB 8fh ; 16-bit segment, 4GB limit
DB 0
_16DsDesc LABEL QWORD
DW -1
DW 0
DB 0
DB 93h
DB 8fh ; 16-bit segment, 4GB limit
DB 0
GdtEnd LABEL QWORD
;
; @param RegSet Pointer to a IA32_DWORD_REGS structure
; @param Transition Pointer to the transition code
; @return The address of the 16-bit stack after returning from user code
;
InternalAsmThunk16 PROC USES ebp ebx esi edi ds es fs gs
mov esi, [esp + 36] ; esi <- RegSet
movzx edx, (IA32_REGS ptr [esi])._SS
mov edi, (IA32_REGS ptr [esi])._ESP
add edi, - (sizeof (IA32_REGS) + 4) ; reserve stack space
mov ebx, edi ; ebx <- stack offset
imul eax, edx, 16 ; eax <- edx * 16
push sizeof (IA32_REGS) / 4
add edi, eax ; edi <- linear address of 16-bit stack
pop ecx
rep movsd ; copy RegSet
mov eax, [esp + 40] ; eax <- address of transition code
mov esi, edx ; esi <- 16-bit stack segment
lea edx, [eax + (offset SavedCr0 - offset m16Start)]
mov ecx, eax
and ecx, 0fh
shl eax, 12
lea ecx, [ecx + (offset _BackFromUserCode - offset m16Start)]
mov ax, cx
stosd ; [edi] <- return address of user code
sgdt fword ptr [edx + (offset SavedGdt - offset SavedCr0)]
sidt fword ptr [esp + 36] ; save IDT stack in argument space
mov eax, cr0
mov [edx], eax ; save CR0 in SavedCr0
and eax, 7ffffffeh ; clear PE, PG bits
mov ebp, cr4
mov [edx + (offset SavedCr4 - offset SavedCr0)], ebp
and ebp, 300h ; clear all but PCE and OSFXSR bits
push 10h
pop ecx ; ecx <- selector for data segments
lgdt fword ptr [edx + (offset _16Gdtr - offset SavedCr0)]
pushfd
call fword ptr [edx + (offset _EntryPoint - offset SavedCr0)]
popfd
lidt fword ptr [esp + 36] ; restore protected mode IDTR
lea eax, [ebp - sizeof (IA32_REGS)]
ret
InternalAsmThunk16 ENDP
END
|
