2 ; Win2k FAT32 Boot Sector
4 ; Brian Palmer <brianp@sginet.com>
8 ; The BP register is initialized to 0x7c00, the start of
9 ; the boot sector. The SP register is initialized to
10 ; 0x7bf4, leaving 12 bytes of data storage space above
13 ; The DWORD that gets stored at 0x7bf4 is 0xffffffff ??
15 ; The DWORD that gets stored at 0x7bf8 is the count of
16 ; total sectors of the volume, calculated from the BPB.
18 ; The DWORD that gets stored at 0x7bfc is the logical
19 ; sector number of the start of the data area.
37 MaxRootEntries dw 0 ;512 - Always zero for FAT32 volumes
38 TotalSectors dw 0 ;2880 - Always zero for FAT32 volumes
39 MediaDescriptor db 0f8h
40 SectorsPerFat dw 0 ;9 - Always zero for FAT32 volumes
49 RootDirStartCluster dd 0
52 Reserved1 times 12 db 0
53 ; End FAT32 Inserted Info
57 SerialNumber dd 00000000h
58 VolumeLabel db 'NO NAME '
59 FileSystem db 'FAT32 '
62 00007C5A 33C9 xor cx,cx
63 00007C5C 8ED1 mov ss,cx ; Setup the stack
64 00007C5E BCF47B mov sp,0x7bf4 ; Give us 12 bytes of space above the stack
65 00007C61 8EC1 mov es,cx
66 00007C63 8ED9 mov ds,cx
67 00007C65 BD007C mov bp,0x7c00
68 00007C68 884E02 mov [bp+0x2],cl ; Zero out the nop instruction?? (3rd byte of the boot sector)
69 00007C6B 8A5640 mov dl,[bp+BootDrive]
70 00007C6E B408 mov ah,0x8
71 00007C70 CD13 int 0x13 ; Int 13, func 8 - Get Drive Parameters
72 00007C72 7305 jnc drive_param_ok ; If no error jmp
75 00007C74 B9FFFF mov cx,0xffff ; We couldn't determine the drive parameters
76 00007C77 8AF1 mov dh,cl ; So just set the CHS to 0xff
79 00007C79 660FB6C6 movzx eax,dh ; Store the number of heads in eax
80 00007C7D 40 inc ax ; Make it one-based because the bios returns it zero-based
81 00007C7E 660FB6D1 movzx edx,cl ; Store the sectors per track in edx
82 00007C82 80E23F and dl,0x3f ; Mask off the cylinder bits
83 00007C85 F7E2 mul dx ; Multiply the sectors per track with the heads, result in dx:ax
84 00007C87 86CD xchg cl,ch ; Switch the cylinder with the sectors
85 00007C89 C0ED06 shr ch,0x6 ; Move the top two cylinder bits down where they should be
86 00007C8C 41 inc cx ; Make it one-based because the bios returns it zero-based
87 00007C8D 660FB7C9 movzx ecx,cx
88 00007C91 66F7E1 mul ecx ; Multiply the cylinders with (heads * sectors) [stored in dx:ax already]
89 00007C94 668946F8 mov [bp-0x8],eax ; This value is the number of total sectors on the disk, so save it for later
90 00007C98 837E1600 cmp word [bp+TotalSectors],byte +0x0 ; Check the old 16-bit value of TotalSectors
91 00007C9C 7538 jnz print_ntldr_error_message ; If it is non-zero then exit with an error
93 00007C9E 837E2A00 cmp word [bp+FSVersion],byte +0x0 ; Check the file system version word
94 00007CA2 7732 ja print_ntldr_error_message ; If it is not zero then exit with an error
98 ; We are now ready to load our second sector of boot code
99 ; But first, a bit of undocumented information about how
100 ; Win2k stores it's second sector of boot code.
102 ; The FAT32 filesystem was designed so that you can store
103 ; multiple sectors of boot code. The boot sector of a FAT32
104 ; volume is actually three sectors long. Microsoft extended
105 ; the BPB so much that you can't fit enough code in the
106 ; boot sector to make it work. So they extended it. Sector 0
107 ; is the traditional boot sector, sector 1 is the FSInfo sector,
108 ; and sector 2 is used to store extra boot code to make up
109 ; for the lost space the BPB takes.
111 ; Now this creates an interesting problem. Suppose for example
112 ; that the user has Win98 and Win2k installed. The Win2k
113 ; boot sector is stored at sector 0 and the Win98 boot sector is
114 ; stored as BOOTSECT.DOS on the file system. Now if Win2k were
115 ; to store it's second sector of boot code in sector 2 like
116 ; the fat spec says to do then when you try to dual boot back
117 ; to Win98 the Win98 boot sector will load Win2k's second
118 ; sector of boot code. Understand? ;-)
120 ; To get around this problem Win2k stores it's second sector
121 ; of boot code elsewhere. This sector is always stored at sector 13
122 ; on the file system. Now don't ask me what happens when you don't
123 ; have enough reserved sectors to store it, but I've never seen a
124 ; FAT32 volume that didn't have at least 32 reserved sectors.
127 00007CA4 668B461C mov eax,[bp+HiddenSectors] ; Get the count of hidden sectors
128 00007CA8 6683C00C add eax,byte +0xc ; Add 12 to that value so that we are loading the 13th sector of the volume
129 00007CAC BB0080 mov bx,0x8000 ; Read the sector to address 0x8000
130 00007CAF B90100 mov cx,0x1 ; Read just one sector
131 00007CB2 E82B00 call read_sectors ; Read it
132 00007CB5 E94803 jmp 0x8000 ; Jump to the next sector of boot code
134 print_disk_error_message:
135 00007CB8 A0FA7D mov al,[DISK_ERR_offset_from_0x7d00]
137 00007CBB B47D mov ah,0x7d
138 00007CBD 8BF0 mov si,ax
141 00007CC0 84C0 test al,al
142 00007CC2 7417 jz reboot
143 00007CC4 3CFF cmp al,0xff
144 00007CC6 7409 jz print_reboot_message
145 00007CC8 B40E mov ah,0xe
146 00007CCA BB0700 mov bx,0x7
147 00007CCD CD10 int 0x10
148 00007CCF EBEE jmp short get_another_char
149 print_reboot_message:
150 00007CD1 A0FB7D mov al,[RESTART_ERR_offset_from_0x7d00]
151 00007CD4 EBE5 jmp short putchars
152 print_ntldr_error_message:
153 00007CD6 A0F97D mov al,[NTLDR_ERR_offset_from_0x7d00]
154 00007CD9 EBE0 jmp short putchars
157 00007CDC CD16 int 0x16
158 00007CDE CD19 int 0x19
162 00007CE2 663B46F8 cmp eax,[bp-0x8]
163 00007CE6 0F824A00 jc near 0x7d34
164 00007CEA 666A00 o32 push byte +0x0
165 00007CED 6650 push eax
168 00007CF1 666810000100 push dword 0x10010
169 00007CF7 807E0200 cmp byte [bp+0x2],0x0
170 00007CFB 0F852000 jnz near 0x7d1f
171 00007CFF B441 mov ah,0x41
172 00007D01 BBAA55 mov bx,0x55aa
173 00007D04 8A5640 mov dl,[bp+BootDrive]
174 00007D07 CD13 int 0x13
175 00007D09 0F821C00 jc near 0x7d29
176 00007D0D 81FB55AA cmp bx,0xaa55
177 00007D11 0F851400 jnz near 0x7d29
178 00007D15 F6C101 test cl,0x1
179 00007D18 0F840D00 jz near 0x7d29
180 00007D1C FE4602 inc byte [bp+0x2]
181 00007D1F B442 mov ah,0x42
182 00007D21 8A5640 mov dl,[bp+BootDrive]
183 00007D24 8BF4 mov si,sp
184 00007D26 CD13 int 0x13
185 00007D28 B0F9 mov al,0xf9
186 00007D2A 6658 pop eax
187 00007D2C 6658 pop eax
188 00007D2E 6658 pop eax
189 00007D30 6658 pop eax
190 00007D32 EB2A jmp short 0x7d5e
191 00007D34 6633D2 xor edx,edx
192 00007D37 660FB74E18 movzx ecx,word [bp+SectorsPerTrack]
193 00007D3C 66F7F1 div ecx
195 00007D41 8ACA mov cl,dl
196 00007D43 668BD0 mov edx,eax
197 00007D46 66C1EA10 shr edx,0x10
198 00007D4A F7761A div word [bp+NumberOfHeads]
199 00007D4D 86D6 xchg dl,dh
200 00007D4F 8A5640 mov dl,[bp+BootDrive]
201 00007D52 8AE8 mov ch,al
202 00007D54 C0E406 shl ah,0x6
203 00007D57 0ACC or cl,ah
204 00007D59 B80102 mov ax,0x201
205 00007D5C CD13 int 0x13
207 00007D60 0F8254FF jc near print_disk_error_message
208 00007D64 81C30002 add bx,0x200
209 00007D68 6640 inc eax
211 00007D6B 0F8571FF jnz near read_sectors
218 NTLDR_ERR db 0dh,0ah,'NTLDR is missing',0ffh
219 DISK_ERR db 0dh,0ah,'Disk error',0ffh
220 RESTART_ERR db 0dh,0ah,'Press any key to restart',0dh,0ah
222 more_filler times 16 db 0
224 NTLDR_offset_from_0x7d00 db 0
225 NTLDR_ERR_offset_from_0x7d00 db 0ach
226 DISK_ERR_offset_from_0x7d00 db 0bfh
227 RESTART_ERR_offset_from_0x7d00 db 0cch
238 ; And that ends the code that makes up the traditional boot sector
239 ; From here on out is a disassembly of the extra sector of boot
240 ; code required for a FAT32 volume. Win2k stores this code at
241 ; sector 13 on the file system.
247 00008000 660FB64610 movzx eax,byte [bp+NumberOfFats] ; Put the number of fats into eax
248 00008005 668B4E24 mov ecx,[bp+SectorsPerFatBig] ; Put the count of sectors per fat into ecx
249 00008009 66F7E1 mul ecx ; Multiply them, edx:eax = (eax * ecx)
250 0000800C 6603461C add eax,[bp+HiddenSectors] ; Add the hidden sectors to eax
251 00008010 660FB7560E movzx edx,word [bp+ReservedSectors] ; Put the count of reserved sectors into edx
252 00008015 6603C2 add eax,edx ; Add it to eax
253 00008018 668946FC mov [bp-0x4],eax ; eax now contains the start of the data area, so save it for later
254 0000801C 66C746F4FFFFFFFF mov dword [bp-0xc],0xffffffff ; Save 0xffffffff for later??
255 00008024 668B462C mov eax,[bp+RootDirStartCluster] ; Put the starting cluster of the root directory into eax
256 00008028 6683F802 cmp eax,byte +0x2 ; Check and see if the root directory starts at cluster 2 or above
257 0000802C 0F82A6FC jc near print_ntldr_error_message ; If not exit with error
258 00008030 663DF8FFFF0F cmp eax,0xffffff8 ; Check and see if the root directory start cluster is and end of cluster chain indicator
259 00008036 0F839CFC jnc near print_ntldr_error_message ; If so exit with error
261 search_root_directory_cluster:
262 0000803A 6650 push eax ; Save root directory start cluster on stack
263 0000803C 6683E802 sub eax,byte +0x2 ; Adjust it because the first two fat entries are unused so the third entry marks the first data area cluster
264 00008040 660FB65E0D movzx ebx,byte [bp+SectsPerCluster] ; Put the number of sectors per cluster in ebx
265 00008045 8BF3 mov si,bx ; Now store it also in si register
266 00008047 66F7E3 mul ebx ; Multiply sectors per cluster with root directory start cluster
267 0000804A 660346FC add eax,[bp-0x4] ; Add the start sector of the data area
269 read_directory_sector:
270 0000804E BB0082 mov bx,0x8200 ; We now have the start sector of the root directory, so load it to 0x8200
271 00008051 8BFB mov di,bx ; Put the address of the root directory sector in di also
272 00008053 B90100 mov cx,0x1 ; Read one sector
273 00008056 E887FC call read_sectors ; Perform the read
275 check_entry_for_ntldr:
276 00008059 382D cmp [di],ch ; Check the first byte of the root directory entry for zero
277 0000805B 741E jz ntldr_not_found ; If so then NTLDR is missing so exit with error
278 0000805D B10B mov cl,0xb ; Put the value 11 in cl so we can compare an 11-byte filename
279 0000805F 56 push si ; Save si (which contains the number of sectors per cluster)
280 00008060 BE707D mov si,NTLDR ;0x7d70 ; Check and see if "NTLDR" is the first file entry
281 00008063 F3A6 repe cmpsb ; Do the compare
282 00008065 5E pop si ; Restore sectors per cluster into si
283 00008066 741B jz ntldr_found ; If we found it then continue, else check next entry
284 00008068 03F9 add di,cx ; Add 0 to di? the next entry is 0x15 bytes away
285 0000806A 83C715 add di,byte +0x15 ; Add 0x15 to di
286 0000806D 3BFB cmp di,bx ; Check to see if we have reached the end of our sector we loaded, read_sectors sets bx = end address of data loaded
287 0000806F 72E8 jc check_entry_for_ntldr ; If we haven't reached the end then check the next entry
288 00008071 4E dec si ; decrement si, si holds the number of sectors per cluster
289 00008072 75DA jnz read_directory_sector ; If it's not zero then search the next sector for NTLDR
290 00008074 6658 pop eax ; If we got here that means we didn't find NTLDR in the previous root directory cluster, so restore eax with the start cluster
291 00008076 E86500 call get_fat_entry ; Get the next cluster in the fat chain
292 00008079 72BF jc search_root_directory_cluster ; If we reached end-of-file marker then don't jump, otherwise continue search
295 0000807B 83C404 add sp,byte +0x4
296 0000807E E955FC jmp print_ntldr_error_message
298 ntldr_load_segment_address dw 0x2000
301 00008083 83C404 add sp,byte +0x4 ; Adjust stack to remove root directory start cluster
302 00008086 8B7509 mov si,[di+0x9] ; Put start cluster high word in si
303 00008089 8B7D0F mov di,[di+0xf] ; Put start cluster low word in di
304 0000808C 8BC6 mov ax,si ; Put high word in ax
305 0000808E 66C1E010 shl eax,0x10 ; Shift it into position
306 00008092 8BC7 mov ax,di ; Put low word in ax, now eax contains start cluster of NTLDR
307 00008094 6683F802 cmp eax,byte +0x2 ; Check and see if the start cluster of NTLDR starts at cluster 2 or above
308 00008098 0F823AFC jc near print_ntldr_error_message ; If not exit with error
309 0000809C 663DF8FFFF0F cmp eax,0xffffff8 ; Check and see if the start cluster of NTLDR is and end of cluster chain indicator
310 000080A2 0F8330FC jnc near print_ntldr_error_message ; If so exit with error
312 load_next_ntldr_cluster:
313 000080A6 6650 push eax ; Save NTLDR start cluster for later
314 000080A8 6683E802 sub eax,byte +0x2 ; Adjust it because the first two fat entries are unused so the third entry marks the first data area cluster
315 000080AC 660FB64E0D movzx ecx,byte [bp+SectsPerCluster] ; Put the sectors per cluster into ecx
316 000080B1 66F7E1 mul ecx ; Multiply sectors per cluster by the start cluster, we now have the logical start sector
317 000080B4 660346FC add eax,[bp-0x4] ; Add the start of the data area logical sector
318 000080B8 BB0000 mov bx,0x0 ; Load NTLDR to offset zero
319 000080BB 06 push es ; Save es
320 000080BC 8E068180 mov es,[ntldr_load_segment_address] ; Get the segment address to load NTLDR to
321 000080C0 E81DFC call read_sectors ; Load the first cluster
322 000080C3 07 pop es ; Restore es
323 000080C4 6658 pop eax ; Restore eax to NTLDR start cluster
324 000080C6 C1EB04 shr bx,0x4 ; bx contains the amount of data we transferred, so divide it by 16
325 000080C9 011E8180 add [ntldr_load_segment_address],bx ; Add that value to the segment
326 000080CD E80E00 call get_fat_entry ; Get the next cluster in eax
327 000080D0 0F830200 jnc near jump_to_ntldr ; If we have reached the end of file then lets get to NTLDR
328 000080D4 72D0 jc load_next_ntldr_cluster ; If not, then load another cluster
331 000080D6 8A5640 mov dl,[bp+BootDrive] ; Put the boot drive in dl
332 000080D9 EA00000020 jmp 0x2000:0x0 ; Jump to NTLDR
335 000080DE 66C1E002 shl eax,0x2 ; Multiply cluster by 4
336 000080E2 E81100 call load_fat_sector ; Load the fat sector
337 000080E5 26668B01 mov eax,[es:bx+di] ; Get the fat entry
338 000080E9 6625FFFFFF0F and eax,0xfffffff ; Mask off the most significant 4 bits
339 000080EF 663DF8FFFF0F cmp eax,0xffffff8 ; Compare it to end of file marker to set the flags correctly
340 000080F5 C3 ret ; Return to caller
343 000080F6 BF007E mov di,0x7e00 ; We will load the fat sector to 0x7e00
344 000080F9 660FB74E0B movzx ecx,word [bp+SectsPerCluster] ; Get the sectors per cluster
345 000080FE 6633D2 xor edx,edx ; We will divide (cluster * 4) / sectorspercluster
346 00008101 66F7F1 div ecx ; eax is already set before we get to this routine
347 00008104 663B46F4 cmp eax,[bp-0xc] ; Compare eax to 0xffffffff (initially, we set this value later)
348 00008108 743A jz load_fat_sector_end ; If it is the same return
349 0000810A 668946F4 mov [bp-0xc],eax ; Update that value
350 0000810E 6603461C add eax,[bp+HiddenSectors] ; Add the hidden sectors
351 00008112 660FB74E0E movzx ecx,word [bp+ReservedSectors] ; Add the reserved sectors
352 00008117 6603C1 add eax,ecx ; To the hidden sectors + the value we computed earlier
353 0000811A 660FB75E28 movzx ebx,word [bp+ExtendedFlags] ; Get extended flags and put into ebx
354 0000811F 83E30F and bx,byte +0xf ; Mask off upper 8 bits
355 00008122 7416 jz load_fat_sector_into_memory ; If fat is mirrored then skip fat calcs
356 00008124 3A5E10 cmp bl,[bp+NumberOfFats] ; Compare bl to number of fats
357 00008127 0F83ABFB jnc near print_ntldr_error_message ; If bl is bigger than numfats exit with error
358 0000812B 52 push dx ; Save dx
359 0000812C 668BC8 mov ecx,eax ; Put the current fat sector offset into ecx
360 0000812F 668B4624 mov eax,[bp+SectorsPerFatBig] ; Get the number of sectors occupied by one fat
361 00008133 66F7E3 mul ebx ; Multiplied by the active fat index
362 00008136 6603C1 add eax,ecx ; Add the current fat sector offset
363 00008139 5A pop dx ; Restore dx
364 load_fat_sector_into_memory:
365 0000813A 52 push dx ; Save dx, what is so important in dx??
366 0000813B 8BDF mov bx,di ; Put 0x7e00 in bx
367 0000813D B90100 mov cx,0x1 ; Load one sector
368 00008140 E89DFB call read_sectors ; Perform the read
369 00008143 5A pop dx ; Restore dx
371 00008144 8BDA mov bx,dx ; Put it into bx, what is this value??
372 00008146 C3 ret ; Return
375 00008147 0000 add [bx+si],al
376 00008149 0000 add [bx+si],al
377 0000814B 0000 add [bx+si],al
378 0000814D 0000 add [bx+si],al
379 0000814F 0000 add [bx+si],al
380 00008151 0000 add [bx+si],al
381 00008153 0000 add [bx+si],al
382 00008155 0000 add [bx+si],al
383 00008157 0000 add [bx+si],al
384 00008159 0000 add [bx+si],al
385 0000815B 0000 add [bx+si],al
386 0000815D 0000 add [bx+si],al
387 0000815F 0000 add [bx+si],al
388 00008161 0000 add [bx+si],al
389 00008163 0000 add [bx+si],al
390 00008165 0000 add [bx+si],al
391 00008167 0000 add [bx+si],al
392 00008169 0000 add [bx+si],al
393 0000816B 0000 add [bx+si],al
394 0000816D 0000 add [bx+si],al
395 0000816F 0000 add [bx+si],al
396 00008171 0000 add [bx+si],al
397 00008173 0000 add [bx+si],al
398 00008175 0000 add [bx+si],al
399 00008177 0000 add [bx+si],al
400 00008179 0000 add [bx+si],al
401 0000817B 0000 add [bx+si],al
402 0000817D 0000 add [bx+si],al
403 0000817F 0000 add [bx+si],al
404 00008181 0000 add [bx+si],al
405 00008183 0000 add [bx+si],al
406 00008185 0000 add [bx+si],al
407 00008187 0000 add [bx+si],al
408 00008189 0000 add [bx+si],al
409 0000818B 0000 add [bx+si],al
410 0000818D 0000 add [bx+si],al
411 0000818F 0000 add [bx+si],al
412 00008191 0000 add [bx+si],al
413 00008193 0000 add [bx+si],al
414 00008195 0000 add [bx+si],al
415 00008197 0000 add [bx+si],al
416 00008199 0000 add [bx+si],al
417 0000819B 0000 add [bx+si],al
418 0000819D 0000 add [bx+si],al
419 0000819F 0000 add [bx+si],al
420 000081A1 0000 add [bx+si],al
421 000081A3 0000 add [bx+si],al
422 000081A5 0000 add [bx+si],al
423 000081A7 0000 add [bx+si],al
424 000081A9 0000 add [bx+si],al
425 000081AB 0000 add [bx+si],al
426 000081AD 0000 add [bx+si],al
427 000081AF 0000 add [bx+si],al
428 000081B1 0000 add [bx+si],al
429 000081B3 0000 add [bx+si],al
430 000081B5 0000 add [bx+si],al
431 000081B7 0000 add [bx+si],al
432 000081B9 0000 add [bx+si],al
433 000081BB 0000 add [bx+si],al
434 000081BD 0000 add [bx+si],al
435 000081BF 0000 add [bx+si],al
436 000081C1 0000 add [bx+si],al
437 000081C3 0000 add [bx+si],al
438 000081C5 0000 add [bx+si],al
439 000081C7 0000 add [bx+si],al
440 000081C9 0000 add [bx+si],al
441 000081CB 0000 add [bx+si],al
442 000081CD 0000 add [bx+si],al
443 000081CF 0000 add [bx+si],al
444 000081D1 0000 add [bx+si],al
445 000081D3 0000 add [bx+si],al
446 000081D5 0000 add [bx+si],al
447 000081D7 0000 add [bx+si],al
448 000081D9 0000 add [bx+si],al
449 000081DB 0000 add [bx+si],al
450 000081DD 0000 add [bx+si],al
451 000081DF 0000 add [bx+si],al
452 000081E1 0000 add [bx+si],al
453 000081E3 0000 add [bx+si],al
454 000081E5 0000 add [bx+si],al
455 000081E7 0000 add [bx+si],al
456 000081E9 0000 add [bx+si],al
457 000081EB 0000 add [bx+si],al
458 000081ED 0000 add [bx+si],al
459 000081EF 0000 add [bx+si],al
460 000081F1 0000 add [bx+si],al
461 000081F3 0000 add [bx+si],al
462 000081F5 0000 add [bx+si],al
463 000081F7 0000 add [bx+si],al
464 000081F9 0000 add [bx+si],al
465 000081FB 0000 add [bx+si],al
466 000081FD 0055AA add [di-0x56],dl ; We can't forget the infamous boot signature
projects / reactos.git / blob