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
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
|
#
# Copyright (c) 1998 Robert Nordier
# All rights reserved.
#
# Redistribution and use in source and binary forms are freely
# permitted provided that the above copyright notice and this
# paragraph and the following disclaimer are duplicated in all
# such forms.
#
# This software is provided "AS IS" and without any express or
# implied warranties, including, without limitation, the implied
# warranties of merchantability and fitness for a particular
# purpose.
#
# $FreeBSD$
.set MEM_REL,0x700 # Relocation address
.set MEM_ARG,0x900 # Arguments
.set MEM_ORG,0x7c00 # Origin
.set MEM_BUF,0x8c00 # Load area
.set MEM_BTX,0x9000 # BTX start
.set MEM_JMP,0x9010 # BTX entry point
.set MEM_USR,0xa000 # Client start
.set PRT_OFF,0x1be # Partition offset
.set PRT_NUM,0x4 # Partitions
.set PRT_BSD,0xa5 # Partition type
.set SIZ_PAG,0x1000 # Page size
.set SIZ_SEC,0x200 # Sector size
.globl start
.globl xread
start: jmp main # Start recognizably
.org 0x4,0x90
# External read from disk
xread: pushl %ss # Address
popl %ds # data
xread.1: o16 # Starting
pushb $0x0 # absolute
pushl %ecx # block
pushl %eax # number
pushl %es # Address of
pushl %ebx # transfer buffer
xorl %eax,%eax # Number of
movb %dh,%al # blocks to
pushl %eax # transfer
pushb $0x10 # Size of packet
movl %esp,%ebp # Packet pointer
callwi(read) # Read from disk
leaw1r(0x10,_bp_,_sp) # Clear stack
lret # To far caller
# Bootstrap
main: cld # String ops inc
xorl %ecx,%ecx # Zero
movl %cx,%es # Address
movl %cx,%ds # data
movl %cx,%ss # Set up
movwir(start,_sp) # stack
movl %esp,%esi # Source
movwir(MEM_REL,_di) # Destination
incb %ch # Word count
rep # Copy
movsl # code
movwir(part4,_si) # Partition
cmpb $0x80,%dl # Hard drive?
jb main.4 # No
movb $0x1,%dh # Block count
callwi(nread) # Read MBR
movwir(0x1,_cx) # Two passes
main.1: movwir(MEM_BUF+PRT_OFF,_si) # Partition table
movb $0x1,%dh # Partition
main.2: cmpbi1(PRT_BSD,0x4,_si_) # Our partition type?
jne main.3 # No
jecxz main.5 # If second pass
tstbi0(0x80,_si_) # Active?
jnz main.5 # Yes
main.3: addl $0x10,%esi # Next entry
incb %dh # Partition
cmpb $0x1+PRT_NUM,%dh # In table?
jb main.2 # Yes
decl %ecx # Do two
jecxz main.1 # passes
movwir(msg_part,_si) # Message
jmp error # Error
main.4: xorl %edx,%edx # Partition:drive
main.5: movwrm(_dx,MEM_ARG) # Save args
movb $0x10,%dh # Sector count
callwi(nread) # Read disk
movwir(MEM_BTX,_bx) # BTX
movw1r(0xa,_bx_,_si) # Point past
addl %ebx,%esi # it
movwir(MEM_USR+SIZ_PAG,_di) # Client page 1
movwir(MEM_BTX+0xe*SIZ_SEC,_cx) # Byte
subl %esi,%ecx # count
rep # Relocate
movsb # client
subl %edi,%ecx # Byte count
xorb %al,%al # Zero
rep # assumed
stosb # bss
callwi(seta20) # Enable A20
jmpnwi(start+MEM_JMP-MEM_ORG) # Start BTX
# Enable A20
seta20: cli # Disable interrupts
seta20.1: inb $0x64,%al # Get status
testb $0x2,%al # Busy?
jnz seta20.1 # Yes
movb $0xd1,%al # Command: Write
outb %al,$0x64 # output port
seta20.2: inb $0x64,%al # Get status
testb $0x2,%al # Busy?
jnz seta20.2 # Yes
movb $0xdf,%al # Enable
outb %al,$0x60 # A20
sti # Enable interrupts
ret # To caller
# Local read from disk
nread: movwir(MEM_BUF,_bx) # Transfer buffer
movw1r(0x8,_si_,_ax) # Get
movw1r(0xa,_si_,_cx) # LBA
pushl %cs # Read from
callwi(xread.1) # disk
jnc return # If success
movwir(msg_read,_si) # Message
# Error exit
error: callwi(putstr) # Display message
movwir(prompt,_si) # Display
callwi(putstr) # prompt
xorb %ah,%ah # BIOS: Get
int $0x16 # keypress
int $0x19 # BIOS: Reboot
# Display string
putstr.0: movwir(0x7,_bx) # Page:attribute
movb $0xe,%ah # BIOS: Display
int $0x10 # character
putstr: lodsb # Get char
testb %al,%al # End of string?
jne putstr.0 # No
ereturn: movb $0x1,%ah # Invalid
stc # argument
return: ret # To caller
# Read from disk
read: cs_ # LBA support
tstbim(0x80,MEM_REL+flags-start)# enabled?
jz read.1 # No
movwir(0x55aa,_bx) # Magic
pushl %edx # Save
movb $0x41,%ah # BIOS: Check
int $0x13 # extensions present
popl %edx # Restore
jc read.1 # If error
cmpwir(0xaa55,_bx) # Magic?
jne read.1 # No
testb $0x1,%cl # Packet interface?
jz read.1 # No
movl %ebp,%esi # Disk packet
movb $0x42,%ah # BIOS: Extended
int $0x13 # read
ret # To caller
read.1: pushl %edx # Save
movb $0x8,%ah # BIOS: Get drive
int $0x13 # parameters
movb %dh,%ch # Max head number
popl %edx # Restore
jc return # If error
andb $0x3f,%cl # Sectors per track
jz ereturn # If zero
cli # Disable interrupts
o16 # Get
movw1r(0x8,_bp_,_ax) # LBA
pushl %edx # Save
movzbw %cl,%bx # Divide by
xorw %dx,%dx # sectors
divw %bx,%ax # per track
movb %ch,%bl # Max head number
movb %dl,%ch # Sector number
incl %ebx # Divide by
xorb %dl,%dl # number
divw %bx,%ax # of heads
movb %dl,%bh # Head number
popl %edx # Restore
o16 # Cylinder number
cmpl $0x3ff,%eax # supportable?
sti # Enable interrupts
ja ereturn # No
xchgb %al,%ah # Set up cylinder
rorb $0x2,%al # number
orb %ch,%al # Merge
incl %eax # sector
xchgl %eax,%ecx # number
movb %bh,%dh # Head number
subb %ah,%al # Sectors this track
movb1r(0x2,_bp_,_ah) # Blocks to read
cmpb %ah,%al # To read
jb read.2 # this
movb %ah,%al # track
read.2: movwir(0x5,_di) # Try count
read.3: lesw1r(0x4,_bp_,_bx) # Transfer buffer
pushl %eax # Save
movb $0x2,%ah # BIOS: Read
int $0x13 # from disk
popl %ebx # Restore
jnc read.4 # If success
decl %edi # Retry?
jz read.6 # No
xorb %ah,%ah # BIOS: Reset
int $0x13 # disk system
xchgl %ebx,%eax # Block count
jmp read.3 # Continue
read.4: movzbl %bl,%eax # Sectors read
addwr1(_ax,0x8,_bp_) # Adjust
jnc read.5 # LBA,
incw1(0xa,_bp_) # transfer
read.5: shlb %bl # buffer
addbr1(_bl,0x5,_bp_) # pointer,
subbr1(_al,0x2,_bp_) # block count
ja read.1 # If not done
read.6: ret # To caller
# Messages
msg_read: .asciz "Read"
msg_part: .asciz "Boot"
prompt: .asciz " error\r\n"
flags: .byte FLAGS # Flags
.org PRT_OFF,0x90
# Partition table
.fill 0x30,0x1,0x0
part4: .byte 0x80, 0x00, 0x01, 0x00
.byte 0xa5, 0xff, 0xff, 0xff
.byte 0x00, 0x00, 0x00, 0x00
.byte 0x50, 0xc3, 0x00, 0x00
.word 0xaa55 # Magic number
|
