1 BOOT(8) BSD System Manager's Manual (i386) BOOT(8)
4 boot, boot.cfg, ldbsd.com, pxeboot - i386 second-stage boot loader
7 The boot system programme, often called ldbsd.com, aims to load the sys-
8 tem kernel into core memory from disc or network and run it, as well as
9 do some auxiliary functions, while dealing with the problems arising from
10 the history of the i386 architecture since 1978, incompatibilities, ex-
11 tensions, bugs, El Torito booting, Intel's Preboot Execution Environment
12 (PXE) for network boot, the Multiboot Specification, etc. MirOS floppies
13 use a specially limited version optimised for size, lacking support for
14 any filesystem other than 4.2FFS and most commands.
16 It can be loaded either directly from the BIOS (most commonly via PXE;
17 earlier versions could also be loaded using El Torito), the bootxx first
18 stage boot loader (from floppy, hard disc, compact flash card, USB stick,
19 and the likes; recently, since bootxx itself was made El Torito capable,
20 this has become the desired method for El Torito boot), any bootloader
21 compliant with the Multiboot specification (as boot is a Multiboot com-
22 pliant OS Kernel image), or after renaming to ldbsd.com by any bootloader
23 implementing the COMBOOT API (specified by SYSLINUX, EXTLINUX, ISOLINUX,
24 PXELINUX) or MS-DOS(R) (unless DOS occupies the HMA). Once loaded, it can
25 be used, in a more or less limited fashion, to boot a MirOS kernel from a
26 supported filesystem (4.2FFS, ISO 9660, FAT12, FAT16, FAT28, TFTP, NFS,
27 and a pseudo filesystem called lmbmfs), inspect the filesystems, get or
28 set machine information, or load other bootloaders (see below for de-
29 tails). It can inflate gzip(1) compressed files (with LZMA compression
30 planned), set up serial console, and provides an interactive prompt.
32 Basic operation procedure is as follows:
36 BIOS We are loaded to 0x07C00. The drive used to load us from
37 is passed in the DL register. The ES:BX and DS:SI regis-
38 ters and the stack contain additional data. While we do
39 not care about the actual address, we expect to be whole.
41 bootxx We are loaded at the final address. The drive used to
42 load us from is passed in a special memory location. The
43 DS:SI registers are set up if we were loaded from a HDD
44 partition. The Master Boot Record (/usr/mdec/mbrldr or
45 /usr/mdec/mbrmgr) takes care to set these up correctly.
47 PXE The NIC's PXE boot ROM initialises the NIC, network
48 driver, UNDI and PXE interfaces, contacts a DHCP server
49 by broadcasting an IPv4 request on the network, gets an
50 IP address and the name of a file to load from the DHCP
51 server, and downloads the file indicated via TFTP to
52 0x07C00. That would be boot. Control is then passed to
53 address 0x07C00 with ES:BX and the stack set up.
55 PXE booting is useful for diskless(8) clients or initial
56 download and execution of the installation kernel,
57 bsd.rd, or for rescue system purposes.
59 DOS We are loaded to xxxx:0100h with no drive or PXE informa-
60 tion set up. The interrupt vectors are hooked by DOS, so
61 if we overwrite any memory in use by DOS, we lose. That
62 would be the case if xxxx is larger than our final ad-
63 dress, any hooks point to an address between our final
64 address and 9000:0000h, the HMA is in use (because that's
65 where the kernel is loaded to), or somesuch. This also
66 implies we cannot chain any other bootloader. Further-
67 more, we require the machine to be in Real Mode, not in
68 VM86 mode, so EMM386.SYS, Win32 or similar must not be
69 active. We ask DOS for the current drive to use this in-
70 formation later. DOS has set up a PSP (Program Segment
71 Prefix) for us, which may contain a command line which we
72 store away for later perusal if it is not empty.
74 COMBOOT We are loaded in a similar way as from DOS, except the
75 machine state is not changed as much from the initial
76 state. After determining that we are in fact loaded via
77 COMBOOT and not DOS, we ask SYSLINUX to terminate after
78 gathering information about the boot drive, partition, or
79 PXE; the UNDI and PXE stacks are kept active if any. The
80 COMBOOT API has set up a PSP like DOS as well, which we
83 Multiboot We request to be loaded to 0x00100000 (the HMA) due to
84 GNU GRUB's limitations, save the boot device off the MBI
85 structure, copy ourselves to the final location, and
86 switch back to Real Mode. Modules passed to us by the
87 Multiboot compliant boot loader will be moved off to just
88 above 20 MiB in memory; their dirnames as well as any
89 command lines to the Multiboot modules are discarded. If
90 a command line is passed to the Multiboot OS kernel, i.e.
91 this bootloader, the pathname of the loader is skipped;
92 it is then cut off after a certain size limit (currently,
93 256 bytes) and stored away for later use. The modules are
94 made available as regular files using the virtual lmbm
95 (Loadable Multiboot Modules) device and filesystem (in-
96 RAM file store, really).
98 During the initial operation, the stack is located about 80 KiB
99 behind the start of our own memory area, and switched to the final
100 location if the position in memory is known to be correct early.
102 2. System information (boot drive, potential partition table entry,
103 PXENV+ and !PXE structure pointers, multiboot module information)
104 are stored in safe locations.
106 3. The code is relocated to the final address once or twice if needed.
107 The final address is 4000:0000h with the stack beginning at
108 3000:FF7Ch. The 128 bytes in between, from 3000:FF80h onwards, con-
109 tains a potentially stored away PSP command line, before it is moved
110 to the buffer after the bss and ssbss sections have been initialised
111 (zeroed out). The stack is shared between Real Mode and 32-bit Vir-
112 tual Protected Address Mode. The code is mostly organised using the
113 small memory model, with everything within 64 KiB (although the real
114 limit is more than 256 bytes less than that due to initial loading
115 issues), except some rather large uninitialised areas and the disc
116 I/O bounce buffer, which begin at 3000:0000 and grow upwards, nick-
117 named the ssbss section. The heap begins after the bss section and
118 grows up to just short of 9000:0000h.
120 4. If the bootloader is compiled to do so, for example on a Live CD, it
121 displays a boot menu unless the shift key is pressed, and retains
122 the numeric return value for later, to replace the cfg suffix of the
123 configuration file with it, unless a (non-empty) command line is
126 5. The system is switched to Protected Mode.
128 6. If a command line was passed, it is moved from its original (Multi-
129 boot, above 1 MiB or way below in the real mode area) or temporary
130 (0x3FF80) location to cmd_buf.
132 7. The hardware is probed:
134 + o Console devices: the default BIOS console (INT 10h, which may be
135 a MDA/MGA/Hercules, CGA/EGA or VGA CRT/LCD, plus DIN or PS/2 or
136 emulated keyboard) as well as up to four serial ports (via the
138 + o Memory: ask the BIOS and probe page by page through the address
139 space, in case the BIOS reports wrong information.
142 + o Loadable Multiboot Modules (if any, they are moved off to safe
143 locations and the virtual lmbm device and lmbmfs are set up)
144 + o Floppy disc drives
145 + o Hard disc drives, including El Torito CD-ROM drives
147 8. If a command line was passed, it is executed. If it contained a boot
148 command, the kernel is attempted to be loaded; otherwise, or if it
149 fails, execution flow continues at the command prompt (see below)
150 with no timeout or auto-boot enabled.
152 9. Unless a control key is held or a command line was passed, the files
153 /x.x.x.x/boot.cfg if loaded via PXE (where x.x.x.x is our own IPv4
154 address) and /boot.cfg, with "cfg" possibly replaced from the Live
155 CD menu, are read and executed as if the commands had been entered
156 on the loader prompt.
158 10. The bootloader prompt
162 is issued, and a command line is read. If no key is pressed within
163 five seconds, the kernels /bsd and /bsd.old are tried, in order, to
164 be booted with the current parameters; if unsuccessful or any key is
165 pressed, the timeout is disabled (it can be manipulated from the
166 configuration file or command line). The system will be unable to
167 boot if no suitable kernel image is found.
169 Commands from the configuration file and the loader prompt are read line
170 by line and executed as read. Empty lines and lines beginning with the
171 comment character, '#', are ignored when reading from the configuration
172 file. Just entering an empty line at the loader prompt, however, will do
173 the default action of booting a kernel with the current parameters. To
174 pass multiple commands on a line, use the U+0060 character, '`', as del-
175 imiter. To pass multiple commands into a macro definition, use the tilde,
176 '~', as delimiter. Leading and trailing whitespace is ignored.
179 The following commands are accepted at the loader prompt:
182 Boots the kernel image specified by image with any options given.
183 If the image file specification, or one of its device or filename
184 parts (see below) is omitted, values from variables will be used.
186 -a Causes the kernel to ask for the root filesystem to use.
188 -c Causes the kernel to go into UKC(8) before performing
191 -d Causes the kernel to drop into ddb(4) at the earliest con-
194 -s Attempts to boot into single-user mode.
197 Displays the file onto the console. Output is paginated every 24
200 echo Displays the arguments onto the console.
202 env On i386, this command is not used.
204 help Prints a list of available commands.
207 Prints the content of the specified directory in long format.
208 Output is paginated every 24 lines.
210 The cd9660, tftp and nfs filesystems do not support this command.
211 They will either always fail or always succeed with sane but
212 unusable results. The FAT12, FAT16 and FAT28 filesystems have
213 hardcoded perms and uid/gid.
216 Issues machine-specific commands:
218 boot dev Load a bootsector (MBR or PBR) from the indicated dev-
219 ice and boot it. Possible devices are fd0 (floppy
220 boot), hd0 (MBR), hd0a, hd0b, hd0c, hd0d (PBR), and
221 some more useless combinations.
223 diskinfo Display a list of probed floppy and hard disc drives
224 including BIOS and geometry information.
226 exec dos Execute INT 21h, AH=4Ch, to return to DOS or SYSLINUX,
227 if possible. This will not always work and may hang
231 Load a bootsector or other bootloader from an image
232 file and execute it. Currently known values for type:
234 grub GNU GRUB 0.9x stage2 file
235 GNU GRUB 0.9x stage2_eltorito file
236 GNU GRUB2 core.img file
238 sector Boot sector or image, loaded to 0000:7C00h
239 MirOS boot second-stage loader
242 Displays the idea boot has about the disklabel of the
243 currently active or the specified device.
246 If used without any arguments, print the current idea
247 boot has about the memory configuration taken from BIOS
248 or probed. Arguments having the form of
252 add (+) or exempt (-) the specified amount of memory.
253 Both size and base address can be specified in decimal,
254 octal or hexadecimal, using standard C prefixes.
256 Memory segments are not required to be adjacent to each
257 other; the only requirement is that there is real phy-
258 sical memory under the range added. The following exam-
259 ple adds 32 MiB of memory right after the first 16 MiB:
261 boot> machine mem +0x2000000@0x01000000
263 Another useful command is to withdraw a range of memory
264 from OS usage, which may have been wrongfully reported
265 as useful by the BIOS. This example excludes the 1516
266 MiB range from the map of useful memory:
268 boot> machine mem -0x100000@0x00F00000
270 regs Debugging command displaying register dumps.
272 oldbios Enable or disable the so-called "Old BIOS / Soekris
273 helper", which restricts boot from loading more than
274 one sector at a time from disc.
276 macro Displays the names of all currently defined macros. Up to four
277 can be defined, holding up to 256 characters.
280 Deletes the macro name, or defines it to cmd.
282 reboot Initiates a warm machine reboot.
285 If invoked without arguments, prints a list of variables and
286 their values. If only a name is given, the value of that variable
287 is displayed. Otherwise, the variable is set to the new value.
288 The following variables are defined:
290 addr Address at which to load the kernel
294 device Boot device name (see below)
296 doboot "0" disables automatic boot on entering an empty line
298 howto Options passed to the loaded kernel, see boot
300 image File name containing the kernel image
302 timeout Number of seconds to wait for human intervention before
305 tty Name of the active console device, for example:
310 stty [device [speed]]
311 Displays or sets the speed for a console device. If the baudrate
312 for the currently active console device is changed, boot offers
313 you five seconds of grace period to switch your terminal to
314 match. If the baudrate for an inactive device is changed, it will
315 only become active on the next switch to a serial console device;
316 it is not used on the PC CRT console.
318 The default baudrate is 9600 bps. boot uses eight data bits, no
319 parity, one stop bit.
321 time Displays the system date and time.
324 An image specification consists of two parts, the device name and a path-
325 name, separated by a colon (':'). In most circumstances, both can be om-
326 itted, and pathnames do not need to begin with a leading slash even if
327 they are absolute. Note that, for some filesystems, you are limited to an
328 8.3 character naming scheme with case insensitive (mapped to lowercase)
329 filenames. Other filesystems may not provide directory listing informa-
330 tion or the ability to stat files (especially remote filesystems).
332 Examples of valid image specifications are:
336 + o hd0a:/ (for "ls")
337 + o lmbm:/ (for "ls")
338 + o cd0a:/boot/grub/stage2
339 + o tftp:/bsd.rd.i386
343 Disklabels are read from hard discs (BIOS drive >= 80h) by searching for
344 a primary MirOS partition first. The default partition type, 0x27, can be
345 changed at installboot(8) time, where it is hardcoded into the partition
346 boot record. If no suitable MBR partition was found or we're on a floppy,
347 the disklabel is searched at the beginning of the drive instead. The la-
348 bel offset for the i386 architecture is one 512-byte sector. On MirOS
349 DuaLive CDs, it may be embedded in the first-stage sparc bootloader. If
350 no disklabel can be read from the disc, one is faked. The device size
351 ('c' slice) defaults to the size of an 1440 KiB floppy disc, but if any
352 MBR primary partitions are found which span more space, their values are
353 used instead. The 'd', 'e', 'f' and 'g' slices are filled with the four
354 MBR primary partitions, if any. The 'a' slice is filled, in this order,
355 with: the partition passed via DS:SI if plausible, the first non-empty
356 MBR partition ('d'-'g' slices), the whole disc ('c' slice).
359 /usr/mdec/bootxx first stage bootloader (PBR)
360 /usr/mdec/boot second stage bootloader
361 /usr/mdec/mbrldr hard disc MBR, simple version
362 /usr/mdec/mbrmgr MBR, bootmanager version
363 /boot usual location of installed loader
364 ldbsd.com alternative name for boot
365 /boot.cfg boot configuration file
366 /bsd standard kernel image
367 /bsd.rd kernel image for installation/recovery
368 /bsd.old alternative kernel image
369 /etc/dhcpd.conf dhcpd(8) configuration file
370 /tftpboot/boot standard location of boot for netboot
371 /tftpboot/boot.cfg common/shared boot configuration file on the TFTP
372 server; /tftpboot/10.11.12.13/boot.cfg contains
373 peer-specific configuration to be used instead
374 /tftpboot/bsd kernel image
375 /tftpboot/pxeboot deprecated, no longer in use
378 A sample configuration file for dhcpd(8) is already contained with MirOS
379 and might look as follows:
381 shared-network KICKSTART {
382 subnet 172.23.42.0 netmask 255.255.255.0 {
383 option routers 172.23.42.1;
385 range 172.23.42.10 172.23.42.199;
389 Boot the default kernel:
393 Remove the 5 second pause at boot-time permanently, causing boot to load
394 the kernel immediately without prompting:
396 # echo "boot" >/boot.cfg
398 Remove the 5 second pause at boot-time permanently, causing boot to do
399 nothing automatically:
401 # echo "set timeout 0" >/boot.cfg
403 Use serial console. A null modem cable should connect the specified seri-
404 al port to a terminal. Useful for debugging.
408 Invoke the serial console at every boot:
410 # echo "set tty com0" >/boot.cfg
412 Multiple commands on one line are useful for machines whose serial con-
413 sole is unusable from within the boot loader, but the only way to talk to
414 the kernel, e.g. for installation on a Soekris/WRAP:
416 boot> stty com0 38400 ` set tty com0 ` boot /bsd.rd
418 Boot the kernel named /bsd from the second hard disc in "User Kernel
419 Configuration" mode (see boot_config(8)). This mechanism allows for the
420 explicit enabling and disabling of devices during the current boot se-
421 quence, as well as the modification of device parameters. Once booted,
422 such changes can be made permanent by using config(8)'s -e option.
424 boot> boot hd1a:/bsd -c
427 gzip(1), compress(3), autoconf(4), ddb(4), dhcpd.conf(5), boot_config(8),
428 boot_i386(8), dhcpd(8), diskless(8), fdisk(8), httpd(8), inetd(8),
429 installboot(8), reboot(8), tftpd(8)
431 Intel Corporation, Preboot Execution Environment (PXE) Specification,
432 Version 2.1, September 20, 1999.
434 Free Software Foundation, Inc., Multiboot Specification, Version 0.6.93.
437 This bootloader is based on code written by Michael Shalayeff for
438 OpenBSD 2.1. The separate pxeboot command first appeared in OpenBSD 3.5,
439 based upon work from NetBSD. In OpenBSD and MirOS #7 and below, the
440 boot.cfg file was called boot.conf, it has been renamed for ISO 9660 and
441 FAT compatibility. A version called cdboot appeared in MirOS #8 and went
442 away for MirOS #10. The separate versions got merged into one bootloader,
443 DOS, COMBOOT, Multiboot support (including modules), pagination, macros,
444 the machine exec and cat commands, working chainbooting of bootsectors
445 and GNU GRUB, faked disklabels (if none exist on disc), FAT filesystem
446 support, and many more things were added or rewritten for MirOS #11 and
447 MirOS bsd4grml by Thorsten Glaser.
450 The default location of the kernels and the boot.cfg file can be changed
453 FILESYSTEM SPECIFIC NOTES
455 ISO 9660 Filesystems above 4 GiB in size definitively do not work. We
456 do not know at this time if there is a 2 GiB or 4 GiB limit,
457 nor if this affects any other filesystems.
459 The ls command does not work.
461 lmbmfs This filesystem and device will only appear if loaded via a
462 Multiboot compliant boot loader, masquerading as Multiboot
463 compliant OS kernel, and when modules are passed. ls is possi-
464 ble only on the root directory ("/", "/."). Files keep the
465 names passed by the Multiboot boot loader, but have the direc-
466 tory part stripped, and can be accessed with any directory
467 leading. If they have no names, they are given an enumerated
470 tftp, nfs These filesystems and devices will only appear of loaded via
471 PXE. They obviously do not support ls.
473 http There are plans to support this if pxebsd.0 is loaded from
474 gPXE, but no code yet. For now, gPXE can load boot using Mul-
475 tiboot, and any passed modules are handled via lmbmfs.
477 ustarfs This filesystem will be implemented soon.
479 MirOS BSD #10-current October 24, 2009 7
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