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
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
|
/*
* GRUB -- GRand Unified Bootloader
* Copyright (C) 1999,2000,2001,2002,2006,2007,2009,2010 Free Software Foundation, Inc.
*
* GRUB is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* GRUB is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with GRUB. If not, see <http://www.gnu.org/licenses/>.
*/
#include <grub/symbol.h>
#include <grub/machine/boot.h>
/*
* defines for the code go here
*/
#define MSG(x) movw $x, %si; call LOCAL(message)
.file "diskboot.S"
.text
/* Tell GAS to generate 16-bit instructions so that this code works
in real mode. */
.code16
.globl start, _start
start:
_start:
/*
* _start is loaded at 0x8000 and is jumped to with
* CS:IP 0:0x8000 in kernel.
*/
/*
* we continue to use the stack for boot.img and assume that
* some registers are set to correct values. See boot.S
* for more information.
*/
/* save drive reference first thing! */
pushw %dx
/* print a notification message on the screen */
pushw %si
MSG(notification_string)
popw %si
/* this sets up for the first run through "bootloop" */
movw $LOCAL(firstlist), %di
/* save the sector number of the second sector in %ebp */
movl (%di), %ebp
/* this is the loop for reading the rest of the kernel in */
LOCAL(bootloop):
/* check the number of sectors to read */
cmpw $0, 8(%di)
/* if zero, go to the start function */
je LOCAL(bootit)
LOCAL(setup_sectors):
/* check if we use LBA or CHS */
cmpb $0, -1(%si)
/* use CHS if zero, LBA otherwise */
je LOCAL(chs_mode)
/* load logical sector start */
movl (%di), %ebx
movl 4(%di), %ecx
/* the maximum is limited to 0x7f because of Phoenix EDD */
xorl %eax, %eax
movb $0x7f, %al
/* how many do we really want to read? */
cmpw %ax, 8(%di) /* compare against total number of sectors */
/* which is greater? */
jg 1f
/* if less than, set to total */
movw 8(%di), %ax
1:
/* subtract from total */
subw %ax, 8(%di)
/* add into logical sector start */
addl %eax, (%di)
adcl $0, 4(%di)
/* set up disk address packet */
/* the size and the reserved byte */
movw $0x0010, (%si)
/* the number of sectors */
movw %ax, 2(%si)
/* the absolute address */
movl %ebx, 8(%si)
movl %ecx, 12(%si)
/* the segment of buffer address */
movw $GRUB_BOOT_MACHINE_BUFFER_SEG, 6(%si)
/* save %ax from destruction! */
pushw %ax
/* the offset of buffer address */
movw $0, 4(%si)
/*
* BIOS call "INT 0x13 Function 0x42" to read sectors from disk into memory
* Call with %ah = 0x42
* %dl = drive number
* %ds:%si = segment:offset of disk address packet
* Return:
* %al = 0x0 on success; err code on failure
*/
movb $0x42, %ah
int $0x13
jc LOCAL(read_error)
movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
jmp LOCAL(copy_buffer)
LOCAL(chs_mode):
/* load logical sector start (top half) */
movl 4(%di), %eax
orl %eax, %eax
jnz LOCAL(geometry_error)
/* load logical sector start (bottom half) */
movl (%di), %eax
/* zero %edx */
xorl %edx, %edx
/* divide by number of sectors */
divl (%si)
/* save sector start */
movb %dl, 10(%si)
xorl %edx, %edx /* zero %edx */
divl 4(%si) /* divide by number of heads */
/* save head start */
movb %dl, 11(%si)
/* save cylinder start */
movw %ax, 12(%si)
/* do we need too many cylinders? */
cmpw 8(%si), %ax
jge LOCAL(geometry_error)
/* determine the maximum sector length of this read */
movw (%si), %ax /* get number of sectors per track/head */
/* subtract sector start */
subb 10(%si), %al
/* how many do we really want to read? */
cmpw %ax, 8(%di) /* compare against total number of sectors */
/* which is greater? */
jg 2f
/* if less than, set to total */
movw 8(%di), %ax
2:
/* subtract from total */
subw %ax, 8(%di)
/* add into logical sector start */
addl %eax, (%di)
adcl $0, 4(%di)
/*
* This is the loop for taking care of BIOS geometry translation (ugh!)
*/
/* get high bits of cylinder */
movb 13(%si), %dl
shlb $6, %dl /* shift left by 6 bits */
movb 10(%si), %cl /* get sector */
incb %cl /* normalize sector (sectors go
from 1-N, not 0-(N-1) ) */
orb %dl, %cl /* composite together */
movb 12(%si), %ch /* sector+hcyl in cl, cylinder in ch */
/* restore %dx */
popw %dx
pushw %dx
/* head number */
movb 11(%si), %dh
pushw %ax /* save %ax from destruction! */
/*
* BIOS call "INT 0x13 Function 0x2" to read sectors from disk into memory
* Call with %ah = 0x2
* %al = number of sectors
* %ch = cylinder
* %cl = sector (bits 6-7 are high bits of "cylinder")
* %dh = head
* %dl = drive (0x80 for hard disk, 0x0 for floppy disk)
* %es:%bx = segment:offset of buffer
* Return:
* %al = 0x0 on success; err code on failure
*/
movw $GRUB_BOOT_MACHINE_BUFFER_SEG, %bx
movw %bx, %es /* load %es segment with disk buffer */
xorw %bx, %bx /* %bx = 0, put it at 0 in the segment */
movb $0x2, %ah /* function 2 */
int $0x13
jc LOCAL(read_error)
/* save source segment */
movw %es, %bx
LOCAL(copy_buffer):
/* load addresses for copy from disk buffer to destination */
movw 10(%di), %es /* load destination segment */
/* restore %ax */
popw %ax
/* determine the next possible destination address (presuming
512 byte sectors!) */
shlw $5, %ax /* shift %ax five bits to the left */
addw %ax, 10(%di) /* add the corrected value to the destination
address for next time */
/* save addressing regs */
pusha
pushw %ds
/* get the copy length */
shlw $3, %ax
movw %ax, %cx
xorw %di, %di /* zero offset of destination addresses */
xorw %si, %si /* zero offset of source addresses */
movw %bx, %ds /* restore the source segment */
cld /* sets the copy direction to forward */
/* perform copy */
rep /* sets a repeat */
movsw /* this runs the actual copy */
/* restore addressing regs and print a dot with correct DS
(MSG modifies SI, which is saved, and unused AX and BX) */
popw %ds
MSG(notification_step)
popa
/* check if finished with this dataset */
cmpw $0, 8(%di)
jne LOCAL(setup_sectors)
/* update position to load from */
subw $GRUB_BOOT_MACHINE_LIST_SIZE, %di
/* jump to bootloop */
jmp LOCAL(bootloop)
/* END OF MAIN LOOP */
LOCAL(bootit):
/* print a newline */
MSG(notification_done)
popw %dx /* this makes sure %dl is our "boot" drive */
ljmp $0, $(GRUB_BOOT_MACHINE_KERNEL_ADDR + 0x200)
/*
* BIOS Geometry translation error (past the end of the disk geometry!).
*/
LOCAL(geometry_error):
MSG(geometry_error_string)
jmp LOCAL(general_error)
/*
* Read error on the disk.
*/
LOCAL(read_error):
MSG(read_error_string)
LOCAL(general_error):
MSG(general_error_string)
/* go here when you need to stop the machine hard after an error condition */
LOCAL(stop): jmp LOCAL(stop)
notification_string: .asciz "loading"
notification_step: .asciz "."
notification_done: .asciz "\r\n"
geometry_error_string: .asciz "Geom"
read_error_string: .asciz "Read"
general_error_string: .asciz " Error"
/*
* message: write the string pointed to by %si
*
* WARNING: trashes %si, %ax, and %bx
*/
/*
* Use BIOS "int 10H Function 0Eh" to write character in teletype mode
* %ah = 0xe %al = character
* %bh = page %bl = foreground color (graphics modes)
*/
1:
movw $0x0001, %bx
movb $0xe, %ah
int $0x10 /* display a byte */
incw %si
LOCAL(message):
movb (%si), %al
cmpb $0, %al
jne 1b /* if not end of string, jmp to display */
ret
/*
* This area is an empty space between the main body of code below which
* grows up (fixed after compilation, but between releases it may change
* in size easily), and the lists of sectors to read, which grows down
* from a fixed top location.
*/
.word 0
.word 0
.org 0x200 - GRUB_BOOT_MACHINE_LIST_SIZE
LOCAL(firstlist): /* this label has to be before the first list entry!!! */
/* fill the first data listing with the default */
blocklist_default_start:
/* this is the sector start parameter, in logical sectors from
the start of the disk, sector 0 */
.long 2, 0
blocklist_default_len:
/* this is the number of sectors to read. grub-mkimage
will fill this up */
.word 0
blocklist_default_seg:
/* this is the segment of the starting address to load the data into */
.word (GRUB_BOOT_MACHINE_KERNEL_SEG + 0x20)
|
