2 # Copyright (c) 1998 Robert Nordier
5 # Redistribution and use in source and binary forms are freely
6 # permitted provided that the above copyright notice and this
7 # paragraph and the following disclaimer are duplicated in all
10 # This software is provided "AS IS" and without any express or
11 # implied warranties, including, without limitation, the implied
12 # warranties of merchantability and fitness for a particular
16 # $FreeBSD: src/sys/boot/i386/boot0/boot0.s,v 1.14.2.6 2000/12/19 00:32:19 rnordier Exp $
18 # A 512-byte boot manager.
20 .set NHRDRV,0x475 # Number of hard drives
21 .set ORIGIN,0x600 # Execution address
22 .set FAKE,0x800 # Partition entry
23 .set LOAD,0x7c00 # Load address
25 .set PRT_OFF,0x1be # Partition table
27 .set TBL0SZ,0x3 # Table 0 size
28 .set TBL1SZ,0xc # Table 1 size
30 .set MAGIC,0xaa55 # Magic: bootable
31 .set B0MAGIC,0xbb66 # Identification
33 .set KEY_ENTER,0x1c # Enter key scan code
34 .set KEY_F1,0x3b # F1 key scan code
37 # Addresses in the sector of embedded data values.
38 # Accessed with negative offsets from the end of the relocated sector (%ebp).
40 .set _NXTDRV,-0x48 # Next drive
41 .set _OPT,-0x47 # Default option
42 .set _SETDRV,-0x46 # Drive to force
43 .set _FLAGS,-0x45 # Flags
44 .set _TICKS,-0x44 # Timeout ticks
45 .set _FAKE,0x0 # Fake partition entry
46 .set _MNUOPT,0xc # Menu options
48 .globl start # Entry point
49 .code16 # This runs in real mode
52 # Initialise segments and registers to known values.
53 # segments start at 0.
54 # The stack is immediately below the address we were loaded to.
56 start: cld # String ops inc
58 movw %ax,%es # Address
61 movw $LOAD,%sp # stack
64 # Copy this code to the address it was linked for
67 movw $start,%di # Destination
68 movw $0x100,%cx # Word count
72 # Set address for variable space beyond code, and clear it.
73 # Notice that this is also used to point to the values embedded in the block,
74 # by using negative offsets.
76 movw %di,%bp # Address variables
77 movb $0x8,%cl # Words to clear
81 # Relocate to the new copy of the code.
83 incb -0xe(%di) # Sector number
84 jmp main-LOAD+ORIGIN # To relocated code
86 # Check what flags were loaded with us, specifically, Use a predefined Drive.
87 # If what the bios gives us is bad, use the '0' in the block instead, as well.
89 main: testb $0x20,_FLAGS(%bp) # Set number drive?
91 testb %dl,%dl # Drive number valid?
92 js main.2 # Possibly (0x80 set)
93 main.1: movb _SETDRV(%bp),%dl # Drive number to use
95 # Whatever we decided to use, now store it into the fake
96 # partition entry that lives in the data space above us.
98 main.2: movb %dl,_FAKE(%bp) # Save drive number
99 callw putn # To new line
100 pushw %dx # Save drive number
102 # Start out with a pointer to the 4th byte of the first table entry
103 # so that after 4 iterations it's beyond the end of the sector.
104 # and beyond a 256 byte boundary and has overflowed 8 bits (see next comment).
105 # (remember that the table starts 2 bytes earlier than you would expect
106 # as the bootable flag is after it in the block)
108 movw $(partbl+0x4),%bx # Partition table (+4)
109 xorw %dx,%dx # Item number
111 # Loop around on the partition table, printing values until we
112 # pass a 256 byte boundary. The end of loop test is at main.5.
114 main.3: movb %ch,-0x4(%bx) # Zero active flag (ch == 0)
115 btw %dx,_FLAGS(%bp) # Entry enabled?
118 # If any of the entries in the table are
119 # the same as the 'type' in the slice table entry,
120 # then this is an empty or non bootable partition. Skip it.
122 movb (%bx),%al # Load type
123 movw $tables,%di # Lookup tables
124 movb $TBL0SZ,%cl # Number of entries
129 # Now scan the table of known types
131 movb $TBL1SZ,%cl # Number of entries
136 # If it matches get the matching element in the
137 # next array. if it doesn't, we are already
138 # pointing at its first element which points to a "?".
140 addw $TBL1SZ,%di # Adjust
141 main.4: movb (%di),%cl # Partition
142 addw %cx,%di # description
143 callw putx # Display it
144 main.5: incw %dx # Next item
145 addb $0x10,%bl # Next entry
146 jnc main.3 # Till done
148 # Passed a 256 byte boundary..
150 # Add one to the drive number and check it is valid,
152 popw %ax # Drive number
153 subb $0x80-0x1,%al # Does next
154 cmpb NHRDRV,%al # drive exist? (from BIOS?)
156 # If not then if there is only one drive,
157 # Don't display drive as an option.
159 decw %ax # Already drive 0?
161 # If it was illegal or we cycled through them,
162 # then go back to drive 0.
164 xorb %al,%al # Drive 0
166 # Whatever drive we selected, make it an ascii digit and save it back
167 # to the "next drive" location in the loaded block in case we
168 # want to save it for next time.
169 # This also is part of the printed drive string so add 0x80 to indicate
172 main.6: addb $'0'|0x80,%al # Save next
173 movb %al,_NXTDRV(%bp) # drive number
174 movw $drive,%di # Display
177 # Now that we've printed the drive (if we needed to), display a prompt.
178 # Get ready for the input byt noting the time.
180 main.7: movw $prompt,%si # Display
181 callw putstr # prompt
182 movb _OPT(%bp),%dl # Display
185 xorb %ah,%ah # BIOS: Get
186 int $0x1a # system time
187 movw %dx,%di # Ticks when
188 addw _TICKS(%bp),%di # timeout
190 # Busy loop, looking for keystrokes but
191 # keeping one eye on the time.
193 main.8: movb $0x1,%ah # BIOS: Check
194 int $0x16 # for keypress
195 jnz main.11 # Have one
196 xorb %ah,%ah # BIOS: Get
197 int $0x1a # system time
198 cmpw %di,%dx # Timeout?
201 # If timed out or defaulting, come here.
203 main.9: movb _OPT(%bp),%al # Load default
204 jmp main.12 # Join common code
206 # User's last try was bad, beep in displeasure.
207 # Since nothing was printed, just continue on as if the user
208 # hadn't done anything. This gives the effect of the user getting a beep
209 # for all bad keystrokes but no action until either the timeout
210 # occurs or the user hits a good key.
212 main.10: movb $0x7,%al # Signal
217 main.11: xorb %ah,%ah # BIOS: Get
219 movb %ah,%al # Scan code
221 # If it's CR act as if timed out.
223 cmpb $KEY_ENTER,%al # Enter pressed?
226 # Otherwise check if legal
229 subb $KEY_F1,%al # Less F1 scan code
230 cmpb $0x4,%al # F1..F5?
233 # We have a selection.
234 # but if it's a bad selection go back to complain.
235 # The bits in MNUOPT were set when the options were printed.
236 # Anything not printed is not an option.
238 main.12: cbtw # Option
239 btw %ax,_MNUOPT(%bp) # enabled?
242 # Save the info in the original tables
243 # for rewriting to the disk.
245 movb %al,_OPT(%bp) # Save option
246 movw $FAKE,%si # Partition for write
247 movb (%si),%dl # Drive number
248 movw %si,%bx # Partition for read
249 cmpb $0x4,%al # F5 pressed?
252 shlb $0x4,%al # Point to
253 addw $partbl,%ax # selected
254 xchgw %bx,%ax # partition
255 movb $0x80,(%bx) # Flag active
257 # If not asked to do a write-back (flags 0x40) don't do one.
259 main.13: pushw %bx # Save
260 testb $0x40,_FLAGS(%bp) # No updates?
262 movw $start,%bx # Data to write
263 movb $0x3,%ah # Write sector
264 callw intx13 # to disk
265 main.14: popw %si # Restore
268 # If going to next drive, replace drive with selected one.
269 # Remember to un-ascii it. Hey 0x80 is already set, cool!
271 jne main.15 # If not F5
272 movb _NXTDRV(%bp),%dl # Next drive
273 subb $'0',%dl # number
275 # load selected bootsector to the LOAD location in RAM.
276 # If it fails to read or isn't marked bootable, treat it
277 # as a bad selection.
278 # XXX what does %si carry?
280 main.15: movw $LOAD,%bx # Address for read
281 movb $0x2,%ah # Read sector
282 callw intx13 # from disk
283 jc main.10 # If error
284 cmpw $MAGIC,0x1fe(%bx) # Bootable?
287 movw $crlf,%si # Leave some
290 jmp *%bx # Invoke bootstrap
295 putkey: movb $'F',%al # Display
297 movb $'1',%al # Prepare
299 jmp putstr.1 # Display the rest
302 # Display the option and note that it is a valid option.
303 # That last point is a bit tricky..
305 putx: btsw %dx,_MNUOPT(%bp) # Enable menu option
306 movw $item,%si # Display
308 movw %di,%si # Display the rest
310 puts: callw putstr # Display string
312 putn: movw $crlf,%si # To next line
314 putstr: lodsb # Get byte
315 testb $0x80,%al # End of string?
317 putstr.1: callw putchr # Display char
318 jmp putstr # Continue
319 putstr.2: andb $~0x80,%al # Clear MSB
321 putchr: pushw %bx # Save
322 movw $0x7,%bx # Page:attribute
323 movb $0xe,%ah # BIOS: Display
324 int $0x10 # character
328 # One-sector disk I/O routine
330 intx13: movb 0x1(%si),%dh # Load head
331 movw 0x2(%si),%cx # Load cylinder:sector
332 movb $0x1,%al # Sector count
335 testb $0x80,_FLAGS(%bp) # Use packet interface?
338 pushl 0x8(%si) # LBA address
339 pushw %es # Set the transfer
340 pushw %bx # buffer address
341 push $0x1 # Block count
342 push $0x10 # Packet size
343 movw %sp,%si # Packet pointer
344 decw %ax # Verify off
345 orb $0x40,%ah # Use disk packet
346 intx13.1: int $0x13 # BIOS: Disk I/O
347 movw %di,%sp # Restore
353 item: .ascii " "; .byte ' '|0x80
354 prompt: .ascii "\nDefault:"; .byte ' '|0x80
355 crlf: .ascii "\r"; .byte '\n'|0x80
357 # Partition type tables
361 # These entries identify invalid or NON BOOT types and partitions.
365 # These values indicate bootable types we know the names of
367 .byte 0x1, 0x4, 0x6, 0xb, 0xc, 0xe, 0x63, 0x83
368 .byte 0x9f, 0xa5, 0xa6, 0xa9
370 # These are offsets that match the known names above and point to the strings
371 # that will be printed.
373 .byte os_misc-. # Unknown
377 .byte os_dos-. # Windows
378 .byte os_dos-. # Windows
379 .byte os_dos-. # Windows
380 .byte os_unix-. # UNIX
381 .byte os_linux-. # Linux
382 .byte os_bsd-. # BSD/OS
383 .byte os_freebsd-. # FreeBSD
384 .byte os_bsd-. # OpenBSD
385 .byte os_bsd-. # NetBSD
387 # And here are the strings themselves. 0x80 or'd into a byte indicates
388 # the end of the string. (not so great for Russians but...)
390 os_misc: .ascii "?"; .byte '?'|0x80
391 os_dos: .ascii "DO"; .byte 'S'|0x80
392 os_unix: .ascii "UNI"; .byte 'X'|0x80
393 os_linux: .ascii "Linu"; .byte 'x'|0x80
394 os_freebsd: .ascii "Free"
395 os_bsd: .ascii "BS"; .byte 'D'|0x80
397 .org PRT_OFF-0xe,0x90
399 .word B0MAGIC # Magic number
402 # These values are sometimes changed before writing back to the drive
403 # Be especially careful that nxtdrv: must come after drive:, as it
404 # is part of the same string.
406 drive: .ascii "Drive "
407 nxtdrv: .byte 0x0 # Next drive number
408 opt: .byte 0x0 # Option
409 setdrv: .byte 0x80 # Drive to force
410 flags: .byte FLAGS # Flags
411 ticks: .word TICKS # Delay
414 # here is the 64 byte partition table that fdisk would fiddle with.
416 partbl: .fill 0x40,0x1,0x0 # Partition table
417 .word MAGIC # Magic number