1#------------------------------------------------------------------------------
2#*
3#*   Copyright (c) 2006 - 2012, 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    .code16
19
20.equ                      BLOCK_SIZE, 0x0200
21.equ                      BLOCK_MASK, 0x01ff
22.equ                      BLOCK_SHIFT, 9
23
24# ****************************************************************************
25# Code loaded by BIOS at 0x0000:0x7C00
26# ****************************************************************************
27
28.org 0x0
29
30.global _start
31_start:
32
33# ****************************************************************************
34# Start Print
35# ****************************************************************************
36
37        movw $0xb800, %ax
38        movw %ax, %es
39        movw $0x7c0, %ax
40        movw %ax, %ds
41        leaw %cs:StartString, %si
42        movw $10, %cx
43        movw $160, %di
44        rep
45        movsw
46
47# ****************************************************************************
48# Print over
49# ****************************************************************************
50
51# ****************************************************************************
52# Initialize segment registers and copy code at 0x0000:0x7c00 to 0x0000:0x0600
53# ****************************************************************************
54        xorw  %ax, %ax                            # AX = 0x0000
55        movw  $0x7c00, %bx                        # BX = 0x7C00
56        movw  $0x600, %bp                         # BP = 0x0600
57        movw  $RelocatedStart, %si                # SI = Offset(RelocatedStart)
58        movw  $0x200, %cx                         # CX = 0x0200
59        subw  %si, %cx                            # CS = 0x0200 - Offset(RelocatedStart)
60        leaw  (%bp,%si,), %di                     # DI = 0x0600 + Offset(RelocatedStart)
61        leaw  (%bx,%si,), %si                     # BX = 0x7C00 + Offset(RelocatedStart)
62        movw  %ax, %ss                            # SS = 0x0000
63        movw  %bx, %sp                            # SP = 0x7C00
64        movw  %ax, %es                            # ES = 0x0000
65        movw  %ax, %ds                            # DS = 0x0000
66        pushw %ax                                 # PUSH 0x0000
67        pushw %di                                 # PUSH 0x0600 + Offset(RelocatedStart)
68        cld                                       # Clear the direction flag
69        rep
70        movsb                                     # Copy 0x0200 bytes from 0x7C00 to 0x0600
71        retl                                      # 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        xorw  %ax, %ax                            # AX = 0
83        movw  %ax, %ss                            # SS = 0
84        addw  $0x1000, %ax
85        movw  %ax, %ds
86
87        movw  $0x7c00, %sp                        # SP = 0x7c00
88        movw  %sp, %bp                            # BP = 0x7c00
89
90        movb  $8, %ah                             # AH = 8 - Get Drive Parameters Function
91        movb  %dl, PhysicalDrive(%bp)             # BBS defines that BIOS would pass the booting driver number to the loader through DL
92        int   $0x13                               # Get Drive Parameters
93        xorw  %ax, %ax                            # AX = 0
94        movb  %dh, %al                            # AL = DH
95        incb  %al                                 # MaxHead = AL + 1
96        pushw %ax                                 # 0000:7bfe = MaxHead
97        movb  %cl, %al                            # AL = CL
98        andb  $0x3f, %al                          # MaxSector = AL & 0x3f
99        pushw %ax                                 # 0000:7bfc = MaxSector
100
101# ****************************************************************************
102# Read Target DBR from hard disk to 0x0000:0x7C00
103# ****************************************************************************
104
105        xorw  %ax, %ax
106        movb  MbrPartitionIndicator(%bp), %al          # AX = MbrPartitionIndex
107        cmpb  $0xff, %al                               # 0xFF means do legacy MBR boot
108        jnz   EfiDbr
109LegacyMbr:
110        movl  $0x0000600, %eax                    # Assume LegacyMBR is backuped in Sector 6
111        jmp   StartReadTo7C00                     # EAX = Header/Sector/Tracker/Zero
112
113EfiDbr:
114        cmpb  $4, %al                             # MbrPartitionIndex should < 4
115        jae   BadDbr
116        shlw  $4, %ax                             # AX  = MBREntrySize * Index
117        addw  $0x1be, %ax                         # AX  = MBREntryOffset
118        movw  %ax, %di                            # 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        movl  %es:8(%bp,%di,), %eax               # Start LBA
126        movl  %eax, %edx
127        shrl  $16, %edx                           # DX:AX = Start LBA
128                                                  #       = Ci * (H * S) + Hi * S + (Si - 1)
129
130        # Calculate C/H/S according to LBA
131        movw  $0x7bfa, %bp
132        divw  2(%bp)                              # AX = Hi + H*Ci
133                                                  # DX = Si - 1
134        incw  %dx                                 # DX = Si
135        pushw %dx                                 # 0000:7bfa = Si  <----
136        xorw  %dx, %dx                            # DX:AX = Hi + H*Ci
137        divw  4(%bp)                              # AX = Ci         <----
138                                                  # DX = Hi         <----
139
140StartReadTo7C00:
141
142        movb  (%bp), %cl                          # Si
143        movb  %al, %ch                            # Ci[0-7]
144        orb   %ah, %cl                            # Ci[8,9]
145        movw  $0x7c00, %bx                        # ES:BX = 0000:7C00h
146        movb  $0x2, %ah                           # Function 02h
147        movb  $1, %al                             # 1 Sector
148        movb  %dl, %dh                            # Hi
149        movw  $0x600, %bp
150        movb  PhysicalDrive(%bp), %dl             # Drive number
151        int   $0x13
152        jc    BadDbr
153
154
155
156# ****************************************************************************
157# Transfer control to BootSector - Jump to 0x0000:0x7C00
158# ****************************************************************************
159        xorw  %ax, %ax
160        pushw %ax                                 # PUSH 0x0000 - Segment
161        movw  $0x7c00, %di
162        pushw %di                                 # PUSH 0x7C00 - Offset
163        retl                                      # JMP 0x0000:0x7C00
164
165# ****************************************************************************
166# ERROR Condition:
167# ****************************************************************************
168
169BadDbr:
170    pushw %ax
171    movw $0xb800, %ax
172    movw %ax, %es
173    movw $0x60, %ax
174    movw %ax, %ds
175    leaw %cs:ErrorString, %si
176    movw $320, %di
177    popw %ax
178    call A2C
179    movb %ah, 16(%si)
180    movb %al, 18(%si)
181    movw $10, %cx
182    rep
183    movsw
184Halt:
185    jmp   Halt
186
187StartString:
188.byte 'M', 0x0c, 'B', 0x0c, 'R', 0x0c, ' ', 0x0c, 'S', 0x0c, 't', 0x0c, 'a', 0x0c, 'r', 0x0c, 't', 0x0c, '!', 0x0c
189ErrorString:
190.byte 'M', 0x0c, 'B', 0x0c, 'R', 0x0c, ' ', 0x0c, 'E', 0x0c, 'r', 0x0c, 'r', 0x0c, ':', 0x0c, '?', 0x0c, '?', 0x0c
191
192# ****************************************************************************
193# A2C - convert Ascii code stored in AH to character stored in AX
194# ****************************************************************************
195A2C:
196    movb %ah, %al
197    shrb $4, %ah
198    andb $0xF, %al
199    addb '0', %ah
200    addb '0', %al
201
202    cmpb '9', %ah
203    jle  A2C_L1
204    addb $7, %ah
205A2C_L1:
206
207    cmpb '9', %al
208    jle A2C_L2
209    addb $7, %al
210A2C_L2:
211    ret
212
213
214# ****************************************************************************
215# PhysicalDrive - Used to indicate which disk to be boot
216#                 Can be patched by tool
217# ****************************************************************************
218.org   0x01B6
219PhysicalDrive:        .byte 0x80
220
221# ****************************************************************************
222# MbrPartitionIndicator - Used to indicate which MBR partition to be boot
223#                         Can be patched by tool
224#                         OxFF means boot to legacy MBR. (LBA OFFSET 6)
225# ****************************************************************************
226.org   0x01B7
227MbrPartitionIndicator: .byte 0
228
229# ****************************************************************************
230# Unique MBR signature
231# ****************************************************************************
232.org   0x01B8
233    .ascii "DUET"
234
235# ****************************************************************************
236# Unknown
237# ****************************************************************************
238.org   0x01BC
239    .word 0
240
241# ****************************************************************************
242# MBR Entry - To be patched
243# ****************************************************************************
244.org   0x01BE
245    .long 0,0,0,0
246.org   0x01CE
247    .long 0,0,0,0
248.org   0x01DE
249    .long 0,0,0,0
250.org   0x01EE
251    .long 0,0,0,0
252
253# ****************************************************************************
254# Sector Signature
255# ****************************************************************************
256
257.org 0x01FE
258SectorSignature:
259  .word     0xaa55      # Boot Sector Signature
260
261
262
263