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
|
/* Segment constants */
#define BOOT_SEG 0x7C0 /* Segment we reside on startup */
#define BOOT_RELOC_SEG 0x7A0 /* Relocation segment */
/* Constants */
#define RDAR_MAGIC ">RDAR!"
#define RDAR_MAGIC_LEN 6
#define BOOT_PATH "boot/biosboot.bin"
#define BOOT_PATH_LEN 17
.code16
_start:
/* Normalize CS + other selectors */
ljmp $BOOT_SEG,$1f
1: xor %ax, %ax
mov %ax, %ss
mov %ax, %gs
mov %ax, %fs
/*
* The second stage bootloader expects to be loaded to 0x7C00 but since we are
* already executing at this address we'll need to relocate ourselves first before
* we clobber it.
*/
mov $BOOT_SEG, %ax
mov %ax, %ds
mov $BOOT_RELOC_SEG, %ax
mov %ax, %es
xor %si, %si
xor %di, %di
mov $0x200, %cx
cld
rep movsb
/* Jump to our relocated self */
ljmp $BOOT_RELOC_SEG,$reloc
reloc:
/* Load %DS with our new segment */
mov $BOOT_RELOC_SEG, %ax
mov %ax, %ds
/* Save the drive number */
mov %dl, drive_num
/* Set our stack */
mov $0x7C00, %sp
/* Write out our boot string */
mov $boot_msg, %si
call putstr
/* Load two sectors */
movw $2, dap_blkcnt
movl $1, dap_lba_low
call disk_read_lba
/* Get stage2 */
call find_stage2
1: cli
hlt
jmp 1b
/*
* Locate stage2 on the disk
*/
find_stage2:
push %ds
push %es
/* Verify that magic is correct */
mov $BOOT_SEG, %ax
mov %ax, %ds
xor %bx, %bx /* Tracks %SI */
xor %si, %si
mov $BOOT_RELOC_SEG, %ax
mov %ax, %es
mov $rdar_magic, %di
mov $RDAR_MAGIC_LEN, %cx
rep cmpsb
jne .bad_magic
/* Iterate every data descriptor */
clc /* Carry set if found */
1: movw %ds:6, %cx /* Add data descriptor size */
mov %bx, %si
mov %si, %dx
add %cx, %bx
mov $rdar_magic, %di
mov $RDAR_MAGIC_LEN, %cx
rep cmpsb
jne .not_found
/* Does the path match? */
mov %dx, %si
add $8, %si
mov $stage2_path, %di
mov $BOOT_PATH_LEN, %cx
rep cmpsb
jne 1b
stc
pop %es
pop %ds
jnc .not_found
/* TODO: On found, return the offset to the data in %CX */
ret
.not_found:
pop %ds
mov $not_found_msg, %si
call putstr
1: cli
hlt
jmp 1b
.bad_magic:
/* Restore %DS */
mov $BOOT_RELOC_SEG, %ax
mov %ax, %ds
mov $bad_magic_msg, %si
call putstr
1: cli
hlt
jmp 1b
/*
* Read sectors from the disk
*/
disk_read_lba:
pusha
mov $dap, %si
mov $0x42, %ah
mov drive_num, %dl
int $0x13
jc .error
popa
ret
.error:
mov $disk_err_msg, %si
call putstr
1: cli
hlt
jmp 1b
/*
* Write a string to the console
*
* %si: String to write
*
* XXX: Clobbers %AL
*/
putstr:
lodsb
or %al, %al
jz 1f
call putchr
jmp putstr
1: ret
/*
* Write a single character to the console
*
* %al: Character to write
*/
putchr:
mov $0x0E, %ah
xor %bh, %bh
int $0x10
ret
dap:
.byte 0x10
.byte 0x00
dap_blkcnt:
.word 0
dap_destptr:
.word 0x7C00
dap_destseg:
.word 0x0000
dap_lba_low:
.long 0
dap_lba_high:
.long 0
drive_num: .byte 0x00
boot_msg: .ascii "booting\r\n\0"
bad_magic_msg: .ascii "bad RDAR magic\r\n\0"
not_found_msg: .ascii "stage 2 not found\r\n\0"
disk_err_msg: .ascii "\r\n!!!disk error!!!\r\n\0"
rdar_magic: .ascii RDAR_MAGIC
stage2_path: .ascii BOOT_PATH
.org 510
.byte 0x55
.byte 0xAA
|
