1 chain.c32 documentation
3 Although syslinux is capable of (very simple) native chainloading (through .bss
4 and .bs options - see doc/syslinux.txt), it also features a very roboust and
5 rich com32 module designed for such purpose.
7 Chain module can perform few basic tasks:
9 - load and jump to a sector
10 - load and jump to a file (also loading a sector for other purposes)
11 - prepare handover data to use by a file / boot sector
12 - fix different options in a file / sector / partition entries
13 - perform a "service-only" run
15 It can chainload data from both GPT and DOS partitions, as well as boot the
16 first sector from a raw disk.
18 In more details, the flow of code is as follows:
21 2. Find drive and/or partition to boot from.
22 3. Hide / unhide systems and/or fix chs values in partition entries on the
23 drive syslinux is booting from.
24 4. Load a file to boot from.
25 5. Load a sector to boot from, if it doesn't conflict with #5.
26 6. Prepare handover area, if it doesn't conflict with #5 & #6, and syslinux is
29 8. Patch loaded file if necessary.
30 9. Patch loaded sector if necessary.
33 In most basic form, syslinux loads specified boot sector (or mbr, if not
34 specified) at 0:0x7c00, prepares handover area as a standard mbr would do, and
37 A "service-only" run is possible when either:
39 - 'nochain' is in effect
43 - 'nofile' and 'nomaps' are in effect
45 This is useful for invocations such as:
47 chain.c32 hdN M setbpb save nochain
48 chain.c32 hdN mbrchs nochain
49 chain.c32 hdN unhideall nochain
51 Please see respective options for more details.
56 chain [drive/partition] [options]
58 DRIVE / PARTITION SPECIFICATION
60 Drive can be specified as 'hd#', 'fd#', 'boot', 'mbr', or 'guid'.
62 - 'mbr' will select a drive by a signature.
63 - 'guid' will select a drive by a guid
64 - 'boot' is the drive syslinux was booted from. This is the default value, if
65 nothing else is specified.
66 - 'hd#' and 'fd#' are standard ways to specify drive number as seen by bios,
69 Option 'guid' is shared with partition selection (see below). If you happened
70 to have non-unique guids, they are searched in disk0, partitions of disk0,
73 The priority of those options are the same as in the above list.
75 If you specify the same value more than once, the last value will be used.
77 'mbr' and 'guid' take extra parameter - you should use ':' or '=' as a
81 Partition can be specified as '#', 'guid', 'label' or 'fs'.
83 - 'guid' option will select a partition by a guid (not a type guid !)
84 - 'label' will select a partition by a label (searching is done in
86 - 'fs' will select a partition from which syslinux was executed
87 - '#' is the standard method. Partitions 1-4 are primary, 5+ logical, 0 = boot
90 The priority of those options are the same as in the above list.
92 If you use a number to select a partition it should be specified after a drive
93 using space or comma as delimiters (after 'hd#', 'fd#', 'mbr', 'guid' or 'boot').
99 It's often convenient to load a file directly and transfer control to it,
100 instead of the sector from the disk. Note, that the <file> must reside on
103 If you choose this option without specifying any addresses explicitly (see
104 options 'sect=' and 'seg='), the file will cause sector to not be loaded at all
105 (as their memory placement would overlap).
107 seg=<segment>:<offset>:<ip>
110 This triplet lets you alter the addresses a file will use. Loading is done to
111 <segment:offset>, jumping to <segment:ip>. When you chainload some other
112 bootloader or kernel, it's almost always mandatory.
114 The defaults, if option is not specified, are 0:0x7c00:0x7c00
115 If any of the fields are ommited (e.g. 0x2000::), they default to 0.
117 sect=<segment>:<offset>:<ip>
119 *sect=0:0x7c00:0x7c00
121 This triplet lets you alter the addresses a sector will use. File is loaded at
122 <segment:offset>, the jump is made to <segment:ip>. This option is mostly used
123 in tandem with 'file=' and 'seg=' options, as some loaders/kernels will expect
124 relocated sector at some particular address (e.g. DRKM).
126 'nosect' will cause sector to not be loaded at all. In plenty cases, when a file
127 is being chainloaded, sector is not necessary.
129 The defaults if option is not specified, are 0:0x7c00:0x7c00.
130 If some of the fields are ommited (e.g. 0x2000::), segment defaults to 0,
131 offset and ip to 0x7c00.
136 In some cases, it's useful to fix BPB values in NTFS/FATxx bootsectors and
137 evntually write them back, but otherwise boot sector itself is not necessary to
138 continue booting. 'nomaps' allows that - a sector will be loaded, but won't be
139 mmapped into real memory. Any overlap tests (vs. handover or file areas) are
140 not performed, being meaningless in such case.
145 Microsoft side of the world is paritculary bitchy about certain BPB values.
146 Depending on the system and chainloading method (sector or file), some or all
147 of those fields must match reality - and after e.g. drive clonning or
148 when using usb stick in different computers - that is often not the case.
152 "hidden sectors" - valid offset of the partition from the beginning of the disk
153 "geometry" - valid disk geometry as reported by BIOS
154 "drive" - valid drive number
156 This option will automatically determine the type of BPB and fix what is possible
157 to fix, relatively to detected BPB. If it's impossible to detect BPB, function
163 Chainloaded file can simply be an image of a sector. In such case, it could be
164 useful to also fix its BPB values.
169 Fixing BPB values only in memory might not be enough. This option allows
170 writing of the corrected sector. You will probably want to use this option
171 together with 'setbpb' or other ones using that implicitly.
173 - this option never applies to a loaded file
174 - chain module will never save anything to disk by default
175 - writing is only performed, if the values actually got changed
180 By default, a handover area is always prepared if possible and potentially
181 useful - meaning it doesn't overlap with other areas, and syslinux chainloads a
182 partition. It's often not necessary though - usually, a chainloaded file or
183 kernel don't care about it anymore, so a user can disable it explicitly with
189 In case when both file and sector are loaded, ds:si and ds:bp will point to
190 sector address before the chainloading. This option lets user force those
191 registers to point to handover area. This is useful when both the file and the
192 sector are actually a sector's image and the sector is mmapped.
197 This option will install a tiny stub code used to swap drive numbers, if the
198 drive we use during chainloading is not fd0 or hd0.
204 In certain situations it's useful to hide partitions - for example to make sure
205 DOS gets C:. 'hide' will hide hidable primary partitions, except the one we're
206 booting from. Similary, 'hideall' will hide all hidable partitions, except the
207 one we're booting from. Hiding is performed only on the selected drive. Options
208 starting with 'un' will simply unhide every partition (primary ones or all).
209 Writing is only performed, if the os type values actually changed.
214 If you want to make a drive you're booting from totally compatible with current
215 BIOS, you can use this to fix all partitions' CHS numbers. Good to silence e.g.
216 FreeDOS complainig about 'logical CHS differs from physical' of sfdisk about
217 'found (...) expected (...). Functionally seems to be mostly cosmetic, as
218 Microsoft world - in cases it cares about geometry - generally sticks to values
219 written in bootsectors. And the rest of the world generally doesn't care about
220 them at all. Writing is only performed, if the values actually got changed.
225 If you're booting over a network using pxelinux - this lets you keep UNDI
226 stacks in memory (pxelinux only).
231 This option will wait for a keypress right before continuing the chainloading.
232 Useful to see warnings emited by the chain module.
236 nochain sets: nofile nomaps nohand
238 It is possible to trigger a "service-only" run - The chain module will do
239 everything requested as usual, but it will not perform the actual chainloading.
240 'nochain' option disables handover, file loading and sector mapping, as these
241 are pointless in such scenario (although file might be reenabled in some future
242 version, if writing to actual files becomes possible). Mainly useful for
243 options 'mbrchs', '[un]hide[all]' and setbpb.
246 sets: file=<file> nohand nosect isolinux
248 Chainload another version/build of the ISOLINUX bootloader and patch the loader
249 with appropriate parameters in memory. This avoids the need for the
250 -eltorito-alt-boot parameter of mkisofs, when you want more than one ISOLINUX
254 sets: file=<file> seg=0x2000 setbpb nohand
256 Prepares to load ntldr directly. You might want to add 'save' option to store
257 corrected BPB values.
260 sets: file=<file> seg=0x2000 setbpb nohand cmldr
262 Prepares to load recovery console directly. In-memory copy of bootsector is
263 patched with "cmdcons\0". Remarks the same as in 'ntldr='.
266 sets: file=<file> seg=0x60 sect=0x1FE0 setbpb nohand
268 Prepares to load freedos kernel directly. You will likely want to add 'save'
269 option, as those kernels seem to require proper geometry written back to disk.
270 Sector address is chosen based on where freedos' bootsectors relocate themselves,
271 although it seems the kernel doesn't rely on it.
273 You might also want to employ 'hide' option, if you have problems with properly
278 sets: file=<file> seg=0x70 sect=0x8000 setbpb nohand
280 Similary to 'freedos=', This prepares to load MSDOS 2.00 - 6.xx or derivatives.
281 Sector address is chosen arbitrarily. Otherwise comments as above.
284 sets: file=<file> seg=0x70::0x200 sect=0x8000 setbpb nohand
286 Only for MSDOS 7+ versions (98se ~ 7.xx, Me ~ 8.xx). Comments as above.
290 sets: file=<file> seg=0x70 sect=0x2000:0:0 setbpb nohand
292 This is used for loading of *only* Dell's DOS derivatives. It does require boot
293 sector at 0x2000 and overall valid BPB values. As in other DOS-ish cases,
294 likely candidates for use are 'save' and 'hide'.
296 grub=<file> [grubcfg=<config>]
297 sets: file=<file> seg=0x800::0x200 nohand nosect grub
299 Chainloads grub legacy's stage2, performing additional corrections on the file
300 in memory. Additionally, alternate config file can be specified through
304 sets: file=<file> nohand nosect grldr
306 Chainloads GRUB4DOS grldr, performing additional corrections on the file
310 sets: file=<file> nomaps setbpb bss
312 This emulates syslinux's native BSS option. This loads both the file and the
313 sector, adjusts BPB values in the loaded sector, then copies all possible BPB
314 fields to the loaded file. Everything is made with reference to the selected
318 sets: file=<file> nosect filebpb
320 This emulates syslinux's native BS option. This loads the file and if possible
321 - adjusts its BPB values. Everything is made with reference to the selected