1;------------------------------------------------------------------------------
2;*
3;*   Copyright (c) 2006 - 2007, Intel Corporation. All rights reserved.<BR>
4;*   This program and the accompanying materials
5;*   are licensed and made available under the terms and conditions of the BSD License
6;*   which accompanies this distribution.  The full text of the license may be found at
7;*   http://opensource.org/licenses/bsd-license.php
8;*
9;*   THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
10;*   WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
11;*
12;*    Mbr.asm
13;*
14;*   Abstract:
15;*
16;------------------------------------------------------------------------------
17
18    .model  small
19;   .dosseg
20    .stack
21    .486p
22    .code
23
24BLOCK_SIZE                EQU     0200h
25BLOCK_MASK                EQU     01ffh
26BLOCK_SHIFT               EQU     9
27
28; ****************************************************************************
29; Code loaded by BIOS at 0x0000:0x7C00
30; ****************************************************************************
31
32        org 0h
33Start:
34
35; ****************************************************************************
36; Start Print
37; ****************************************************************************
38
39        mov  ax,0b800h
40        mov  es,ax
41        mov  ax, 07c0h
42        mov  ds, ax
43        lea  si, cs:[StartString]
44        mov  cx, 10
45        mov  di, 160
46        rep  movsw
47
48; ****************************************************************************
49; Print over
50; ****************************************************************************
51
52; ****************************************************************************
53; Initialize segment registers and copy code at 0x0000:0x7c00 to 0x0000:0x0600
54; ****************************************************************************
55        xor   ax, ax                              ; AX = 0x0000
56        mov   bx, 07c00h                          ; BX = 0x7C00
57        mov   bp, 0600h                           ; BP = 0x0600
58        mov   si, OFFSET RelocatedStart           ; SI = Offset(RelocatedStart)
59        mov   cx, 0200h                           ; CX = 0x0200
60        sub   cx, si                              ; CS = 0x0200 - Offset(RelocatedStart)
61        lea   di, [bp+si]                         ; DI = 0x0600 + Offset(RelocatedStart)
62        lea   si, [bx+si]                         ; BX = 0x7C00 + Offset(RelocatedStart)
63        mov   ss, ax                              ; SS = 0x0000
64        mov   sp, bx                              ; SP = 0x7C00
65        mov   es,ax                               ; ES = 0x0000
66        mov   ds,ax                               ; DS = 0x0000
67        push  ax                                  ; PUSH 0x0000
68        push  di                                  ; PUSH 0x0600 + Offset(RelocatedStart)
69        cld                                       ; Clear the direction flag
70        rep   movsb                               ; Copy 0x0200 bytes from 0x7C00 to 0x0600
71        retf                                      ; JMP 0x0000:0x0600 + Offset(RelocatedStart)
72
73; ****************************************************************************
74; Code relocated to 0x0000:0x0600
75; ****************************************************************************
76
77RelocatedStart:
78; ****************************************************************************
79; Get Driver Parameters to 0x0000:0x7BFC
80; ****************************************************************************
81
82        xor   ax,ax                               ; AX = 0
83        mov   ss,ax                               ; SS = 0
84        add   ax,1000h
85        mov   ds,ax
86
87        mov   sp,07c00h                           ; SP = 0x7c00
88        mov   bp,sp                               ; BP = 0x7c00
89
90        mov   ah,8                                ; AH = 8 - Get Drive Parameters Function
91        mov   byte ptr [bp+PhysicalDrive],dl      ; BBS defines that BIOS would pass the booting driver number to the loader through DL
92        int   13h                                 ; Get Drive Parameters
93        xor   ax,ax                               ; AX = 0
94        mov   al,dh                               ; AL = DH
95        inc   al                                  ; MaxHead = AL + 1
96        push  ax                                  ; 0000:7bfe = MaxHead
97        mov   al,cl                               ; AL = CL
98        and   al,03fh                             ; MaxSector = AL & 0x3f
99        push  ax                                  ; 0000:7bfc = MaxSector
100
101; ****************************************************************************
102; Read Target DBR from hard disk to 0x0000:0x7C00
103; ****************************************************************************
104
105        xor   ax, ax
106        mov   al, byte ptr [bp+MbrPartitionIndicator]  ; AX = MbrPartitionIndex
107        cmp   al, 0ffh                                 ; 0xFF means do legacy MBR boot
108        jnz   EfiDbr
109LegacyMbr:
110        mov   eax, 00000600h                      ; Assume LegacyMBR is backuped in Sector 6
111        jmp   StartReadTo7C00                     ; EAX = Header/Sector/Tracker/Zero
112
113EfiDbr:
114        cmp   al, 4                               ; MbrPartitionIndex should < 4
115        jae   BadDbr
116        shl   ax, 4                               ; AX  = MBREntrySize * Index
117        add   ax, 1beh                            ; AX  = MBREntryOffset
118        mov   di, ax                              ; DI  = MBREntryOffset
119
120        ; Here we don't use the C/H/S information provided by Partition table
121        ;  but calculate C/H/S from LBA ourselves
122        ;       Ci: Cylinder number
123        ;       Hi: Header number
124        ;       Si: Sector number
125        mov   eax, dword ptr es:[bp + di + 8]     ; Start LBA
126        mov   edx, eax
127        shr   edx, 16                             ; DX:AX = Start LBA
128                                                  ;       = Ci * (H * S) + Hi * S + (Si - 1)
129
130        ; Calculate C/H/S according to LBA
131        mov   bp, 7bfah
132        div   word ptr [bp+2]                     ; AX = Hi + H*Ci
133                                                  ; DX = Si - 1
134        inc   dx                                  ; DX = Si
135        push  dx                                  ; 0000:7bfa = Si  <----
136        xor   dx, dx                              ; DX:AX = Hi + H*Ci
137        div   word ptr [bp+4]                     ; AX = Ci         <----
138                                                  ; DX = Hi         <----
139
140StartReadTo7C00:
141
142        mov   cl, byte ptr [bp]                   ; Si
143        mov   ch, al                              ; Ci[0-7]
144        or    cl, ah                              ; Ci[8,9]
145        mov   bx, 7c00h                           ; ES:BX = 0000:7C00h
146        mov   ah, 2h                              ; Function 02h
147        mov   al, 1                               ; 1 Sector
148        mov   dh, dl                              ; Hi
149        mov   bp, 0600h
150        mov   dl, byte ptr [bp + PhysicalDrive]   ; Drive number
151        int   13h
152        jc    BadDbr
153
154
155
156; ****************************************************************************
157; Transfer control to BootSector - Jump to 0x0000:0x7C00
158; ****************************************************************************
159        xor   ax, ax
160        push  ax                                  ; PUSH 0x0000 - Segment
161        mov   di, 07c00h
162        push  di                                  ; PUSH 0x7C00 - Offset
163        retf                                      ; JMP 0x0000:0x7C00
164
165; ****************************************************************************
166; ERROR Condition:
167; ****************************************************************************
168
169BadDbr:
170    push ax
171    mov  ax, 0b800h
172    mov  es, ax
173    mov  ax, 060h
174    mov  ds, ax
175    lea  si, cs:[ErrorString]
176    mov  di, 320
177    pop  ax
178    call A2C
179    mov  [si+16], ah
180    mov  [si+18], al
181    mov  cx, 10
182    rep  movsw
183Halt:
184    jmp   Halt
185
186StartString:
187    db 'M', 0ch, 'B', 0ch, 'R', 0ch, ' ', 0ch, 'S', 0ch, 't', 0ch, 'a', 0ch, 'r', 0ch, 't', 0ch, '!', 0ch
188ErrorString:
189    db 'M', 0ch, 'B', 0ch, 'R', 0ch, ' ', 0ch, 'E', 0ch, 'r', 0ch, 'r', 0ch, ':', 0ch, '?', 0ch, '?', 0ch
190
191; ****************************************************************************
192; A2C - convert Ascii code stored in AH to character stored in AX
193; ****************************************************************************
194A2C:
195    mov  al, ah
196    shr  ah, 4
197    and  al, 0Fh
198    add  ah, '0'
199    add  al, '0'
200
201    cmp  ah, '9'
202    jle  @f
203    add  ah, 7
204@@:
205
206    cmp al, '9'
207    jle @f
208    add al, 7
209@@:
210    ret
211
212
213; ****************************************************************************
214; PhysicalDrive - Used to indicate which disk to be boot
215;                 Can be patched by tool
216; ****************************************************************************
217    org   01B6h
218PhysicalDrive         db  80h
219
220; ****************************************************************************
221; MbrPartitionIndicator - Used to indicate which MBR partition to be boot
222;                         Can be patched by tool
223;                         OxFF means boot to legacy MBR. (LBA OFFSET 6)
224; ****************************************************************************
225    org   01B7h
226MbrPartitionIndicator db 0
227
228; ****************************************************************************
229; Unique MBR signature
230; ****************************************************************************
231    org   01B8h
232    db 'DUET'
233
234; ****************************************************************************
235; Unknown
236; ****************************************************************************
237    org   01BCh
238    dw 0
239
240; ****************************************************************************
241; MBR Entry - To be patched
242; ****************************************************************************
243    org   01BEh
244    dd 0, 0, 0, 0
245    org   01CEh
246    dd 0, 0, 0, 0
247    org   01DEh
248    dd 0, 0, 0, 0
249    org   01EEh
250    dd 0, 0, 0, 0
251
252; ****************************************************************************
253; Sector Signature
254; ****************************************************************************
255
256  org 01FEh
257SectorSignature:
258  dw        0aa55h      ; Boot Sector Signature
259
260  end
261
262