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1 /* -*-Asm-*- */
2 /*
3  *  GRUB  --  GRand Unified Bootloader
4  *  Copyright (C) 1999,2000,2001,2002,2004   Free Software Foundation, Inc.
5  *
6  *  This program is free software; you can redistribute it and/or modify
7  *  it under the terms of the GNU General Public License as published by
8  *  the Free Software Foundation; either version 2 of the License, or
9  *  (at your option) any later version.
10  *
11  *  This program is distributed in the hope that it will be useful,
12  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
13  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
14  *  GNU General Public License for more details.
15  *
16  *  You should have received a copy of the GNU General Public License
17  *  along with this program; if not, write to the Free Software
18  *  Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
19  */
20 
21 #include <stage1.h>
22 
23 /*
24  *  defines for the code go here
25  */
26 
27 	/* Absolute addresses
28 	   This makes the assembler generate the address without support
29 	   from the linker. (ELF can't relocate 16-bit addresses!) */
30 #define ABS(x) (x-_start+0x7c00)
31 
32 	/* Print message string */
33 #define MSG(x)	movw $ABS(x), %si; call message
34 
35 	/* XXX:	binutils-2.9.1.0.x doesn't produce a short opcode for this. */
36 #define	MOV_MEM_TO_AL(x)	.byte 0xa0;  .word x
37 
38 	.file	"stage1.S"
39 
40 	.text
41 
42 	/* Tell GAS to generate 16-bit instructions so that this code works
43 	   in real mode. */
44 	.code16
45 
46 .globl _start; _start:
47 	/*
48 	 * _start is loaded at 0x7c00 and is jumped to with CS:IP 0:0x7c00
49 	 */
50 
51 	/*
52 	 * Beginning of the sector is compatible with the FAT/HPFS BIOS
53 	 * parameter block.
54 	 */
55 
56 	jmp	after_BPB
57 	nop	/* do I care about this ??? */
58 
59 	/*
60 	 * This space is for the BIOS parameter block!!!!  Don't change
61 	 * the first jump, nor start the code anywhere but right after
62 	 * this area.
63 	 */
64 
65 	. = _start + 4
66 
67 	/* scratch space */
68 mode:
69 	.byte	0
70 disk_address_packet:
71 sectors:
72 	.long	0
73 heads:
74 	.long	0
75 cylinders:
76 	.word	0
77 sector_start:
78 	.byte	0
79 head_start:
80 	.byte	0
81 cylinder_start:
82 	.word	0
83 	/* more space... */
84 
85 	. = _start + STAGE1_BPBEND
86 
87 	/*
88 	 * End of BIOS parameter block.
89 	 */
90 
91 stage1_version:
92 	.byte	COMPAT_VERSION_MAJOR, COMPAT_VERSION_MINOR
93 boot_drive:
94 	.byte	GRUB_INVALID_DRIVE	/* the disk to load stage2 from */
95 force_lba:
96 	.byte	0
97 stage2_address:
98 	.word	0x8000
99 stage2_sector:
100 	.long	1
101 stage2_segment:
102 	.word	0x800
103 
104 after_BPB:
105 
106 /* general setup */
107 	cli		/* we're not safe here! */
108 
109 	/*
110 	 * This is a workaround for buggy BIOSes which don't pass boot
111 	 * drive correctly. If GRUB is installed into a HDD, check if
112 	 * DL is masked correctly. If not, assume that the BIOS passed
113 	 * a bogus value and set DL to 0x80, since this is the only
114 	 * possible boot drive. If GRUB is installed into a floppy,
115 	 * this does nothing (only jump).
116 	 */
117 boot_drive_check:
118 	jmp	1f
119 	testb	$0x80, %dl
120 	jnz	1f
121 	movb	$0x80, %dl
122 1:
123 
124 	/*
125 	 * ljmp to the next instruction because some bogus BIOSes
126 	 * jump to 07C0:0000 instead of 0000:7C00.
127 	 */
128 	ljmp	$0, $ABS(real_start)
129 
130 real_start:
131 
132 	/* set up %ds and %ss as offset from 0 */
133 	xorw	%ax, %ax
134 	movw	%ax, %ds
135 	movw	%ax, %ss
136 
137 	/* set up the REAL stack */
138 	movw	$STAGE1_STACKSEG, %sp
139 
140 	sti		/* we're safe again */
141 
142 	/*
143 	 *  Check if we have a forced disk reference here
144 	 */
145 	MOV_MEM_TO_AL(ABS(boot_drive))	/* movb	ABS(boot_drive), %al */
146 	cmpb	$GRUB_INVALID_DRIVE, %al
147 	je	1f
148 	movb	%al, %dl
149 1:
150 	/* save drive reference first thing! */
151 	pushw	%dx
152 
153 	/* print a notification message on the screen */
154 	MSG(notification_string)
155 
156 	/* do not probe LBA if the drive is a floppy */
157 	testb	$STAGE1_BIOS_HD_FLAG, %dl
158 	jz	chs_mode
159 
160 	/* check if LBA is supported */
161 	movb	$0x41, %ah
162 	movw	$0x55aa, %bx
163 	int	$0x13
164 
165 	/*
166 	 *  %dl may have been clobbered by INT 13, AH=41H.
167 	 *  This happens, for example, with AST BIOS 1.04.
168 	 */
169 	popw	%dx
170 	pushw	%dx
171 
172 	/* use CHS if fails */
173 	jc	chs_mode
174 	cmpw	$0xaa55, %bx
175 	jne	chs_mode
176 
177 	/* check if AH=0x42 is supported if FORCE_LBA is zero */
178 	MOV_MEM_TO_AL(ABS(force_lba))	/* movb	ABS(force_lba), %al */
179 	testb	%al, %al
180 	jnz	lba_mode
181 	andw	$1, %cx
182 	jz	chs_mode
183 
184 lba_mode:
185 	/* save the total number of sectors */
186 	movl	0x10(%si), %ecx
187 
188 	/* set %si to the disk address packet */
189 	movw	$ABS(disk_address_packet), %si
190 
191 	/* set the mode to non-zero */
192 	movb	$1, -1(%si)
193 
194 	movl	ABS(stage2_sector), %ebx
195 
196 	/* the size and the reserved byte */
197 	movw	$0x0010, (%si)
198 
199 	/* the blocks */
200 	movw	$1, 2(%si)
201 
202 	/* the absolute address (low 32 bits) */
203 	movl	%ebx, 8(%si)
204 
205 	/* the segment of buffer address */
206 	movw	$STAGE1_BUFFERSEG, 6(%si)
207 
208 	xorl	%eax, %eax
209 	movw	%ax, 4(%si)
210 	movl	%eax, 12(%si)
211 
212 /*
213  * BIOS call "INT 0x13 Function 0x42" to read sectors from disk into memory
214  *	Call with	%ah = 0x42
215  *			%dl = drive number
216  *			%ds:%si = segment:offset of disk address packet
217  *	Return:
218  *			%al = 0x0 on success; err code on failure
219  */
220 
221 	movb	$0x42, %ah
222 	int	$0x13
223 
224 	/* LBA read is not supported, so fallback to CHS.  */
225 	jc	chs_mode
226 
227 	movw	$STAGE1_BUFFERSEG, %bx
228 	jmp	copy_buffer
229 
230 chs_mode:
231 	/*
232 	 *  Determine the hard disk geometry from the BIOS!
233 	 *  We do this first, so that LS-120 IDE floppies work correctly.
234 	 */
235 	movb	$8, %ah
236 	int	$0x13
237 	jnc	final_init
238 
239 	/*
240 	 *  The call failed, so maybe use the floppy probe instead.
241 	 */
242 	testb	$STAGE1_BIOS_HD_FLAG, %dl
243 	jz	floppy_probe
244 
245 	/* Nope, we definitely have a hard disk, and we're screwed. */
246 	jmp	hd_probe_error
247 
248 final_init:
249 
250 	movw	$ABS(sectors), %si
251 
252 	/* set the mode to zero */
253 	movb	$0, -1(%si)
254 
255 	/* save number of heads */
256 	xorl	%eax, %eax
257 	movb	%dh, %al
258 	incw	%ax
259 	movl	%eax, 4(%si)
260 
261 	xorw	%dx, %dx
262 	movb	%cl, %dl
263 	shlw	$2, %dx
264 	movb	%ch, %al
265 	movb	%dh, %ah
266 
267 	/* save number of cylinders */
268 	incw	%ax
269 	movw	%ax, 8(%si)
270 
271 	xorw	%ax, %ax
272 	movb	%dl, %al
273 	shrb	$2, %al
274 
275 	/* save number of sectors */
276 	movl	%eax, (%si)
277 
278 setup_sectors:
279 	/* load logical sector start (bottom half) */
280 	movl	ABS(stage2_sector), %eax
281 
282 	/* zero %edx */
283 	xorl	%edx, %edx
284 
285 	/* divide by number of sectors */
286 	divl	(%si)
287 
288 	/* save sector start */
289 	movb	%dl, 10(%si)
290 
291 	xorl	%edx, %edx	/* zero %edx */
292 	divl	4(%si)		/* divide by number of heads */
293 
294 	/* save head start */
295 	movb	%dl, 11(%si)
296 
297 	/* save cylinder start */
298 	movw	%ax, 12(%si)
299 
300 	/* do we need too many cylinders? */
301 	cmpw	8(%si), %ax
302 	jge	geometry_error
303 
304 /*
305  *  This is the loop for taking care of BIOS geometry translation (ugh!)
306  */
307 
308 	/* get high bits of cylinder */
309 	movb	13(%si), %dl
310 
311 	shlb	$6, %dl		/* shift left by 6 bits */
312 	movb	10(%si), %cl	/* get sector */
313 
314 	incb	%cl		/* normalize sector (sectors go
315 					from 1-N, not 0-(N-1) ) */
316 	orb	%dl, %cl	/* composite together */
317 	movb	12(%si), %ch	/* sector+hcyl in cl, cylinder in ch */
318 
319 	/* restore %dx */
320 	popw	%dx
321 
322 	/* head number */
323 	movb	11(%si), %dh
324 
325 /*
326  * BIOS call "INT 0x13 Function 0x2" to read sectors from disk into memory
327  *	Call with	%ah = 0x2
328  *			%al = number of sectors
329  *			%ch = cylinder
330  *			%cl = sector (bits 6-7 are high bits of "cylinder")
331  *			%dh = head
332  *			%dl = drive (0x80 for hard disk, 0x0 for floppy disk)
333  *			%es:%bx = segment:offset of buffer
334  *	Return:
335  *			%al = 0x0 on success; err code on failure
336  */
337 
338 	movw	$STAGE1_BUFFERSEG, %bx
339 	movw	%bx, %es	/* load %es segment with disk buffer */
340 
341 	xorw	%bx, %bx	/* %bx = 0, put it at 0 in the segment */
342 	movw	$0x0201, %ax	/* function 2 */
343 	int	$0x13
344 
345 	jc	read_error
346 
347 	movw	%es, %bx
348 
349 copy_buffer:
350 	movw	ABS(stage2_segment), %es
351 
352 	/*
353 	 * We need to save %cx and %si because the startup code in
354 	 * stage2 uses them without initializing them.
355 	 */
356 	pusha
357 	pushw	%ds
358 
359 	movw	$0x100, %cx
360 	movw	%bx, %ds
361 	xorw	%si, %si
362 	xorw	%di, %di
363 
364 	cld
365 
366 	rep
367 	movsw
368 
369 	popw	%ds
370 	popa
371 
372 	/* boot stage2 */
373 	jmp	*(stage2_address)
374 
375 /* END OF MAIN LOOP */
376 
377 /*
378  * BIOS Geometry translation error (past the end of the disk geometry!).
379  */
380 geometry_error:
381 	MSG(geometry_error_string)
382 	jmp	general_error
383 
384 /*
385  * Disk probe failure.
386  */
387 hd_probe_error:
388 	MSG(hd_probe_error_string)
389 	jmp	general_error
390 
391 /*
392  * Read error on the disk.
393  */
394 read_error:
395 	MSG(read_error_string)
396 
397 general_error:
398 	MSG(general_error_string)
399 
400 /* go here when you need to stop the machine hard after an error condition */
401 stop:	jmp	stop
402 
403 notification_string:	.string "GRUB "
404 geometry_error_string:	.string "Geom"
405 hd_probe_error_string:	.string "Hard Disk"
406 read_error_string:	.string "Read"
407 general_error_string:	.string " Error"
408 
409 /*
410  * message: write the string pointed to by %si
411  *
412  *   WARNING: trashes %si, %ax, and %bx
413  */
414 
415 	/*
416 	 * Use BIOS "int 10H Function 0Eh" to write character in teletype mode
417 	 *	%ah = 0xe	%al = character
418 	 *	%bh = page	%bl = foreground color (graphics modes)
419 	 */
420 1:
421 	movw	$0x0001, %bx
422 	movb	$0xe, %ah
423 	int	$0x10		/* display a byte */
424 message:
425 	lodsb
426 	cmpb	$0, %al
427 	jne	1b	/* if not end of string, jmp to display */
428 	ret
429 
430 	/*
431 	 *  Windows NT breaks compatibility by embedding a magic
432 	 *  number here.
433 	 */
434 
435 	. = _start + STAGE1_WINDOWS_NT_MAGIC
436 nt_magic:
437 	.long 0
438 	.word 0
439 
440 	/*
441 	 *  This is where an MBR would go if on a hard disk.  The code
442 	 *  here isn't even referenced unless we're on a floppy.  Kinda
443 	 *  sneaky, huh?
444 	 */
445 
446 part_start:
447 	. = _start + STAGE1_PARTSTART
448 
449 probe_values:
450 	.byte	36, 18, 15, 9, 0
451 
452 floppy_probe:
453 /*
454  *  Perform floppy probe.
455  */
456 
457 	movw	$ABS(probe_values-1), %si
458 
459 probe_loop:
460 	/* reset floppy controller INT 13h AH=0 */
461 	xorw	%ax, %ax
462 	int	$0x13
463 
464 	incw	%si
465 	movb	(%si), %cl
466 
467 	/* if number of sectors is 0, display error and die */
468 	cmpb	$0, %cl
469 	jne	1f
470 
471 /*
472  * Floppy disk probe failure.
473  */
474 	MSG(fd_probe_error_string)
475 	jmp	general_error
476 
477 fd_probe_error_string:	.string "Floppy"
478 
479 1:
480 	/* perform read */
481 	movw	$STAGE1_BUFFERSEG, %bx
482 	movw	$0x201, %ax
483 	movb	$0, %ch
484 	movb	$0, %dh
485 	int	$0x13
486 
487 	/* if error, jump to "probe_loop" */
488 	jc	probe_loop
489 
490 	/* %cl is already the correct value! */
491 	movb	$1, %dh
492 	movb	$79, %ch
493 
494 	jmp	final_init
495 
496 	. = _start + STAGE1_PARTEND
497 
498 /* the last 2 bytes in the sector 0 contain the signature */
499 	.word	STAGE1_SIGNATURE
500