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' (or 'nosect') are in effect
45 This is useful for invocations such as:
47 chain.c32 hdN M setbpb save nochain
48 chain.c32 hdN fixchs 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
122 This triplet lets you alter the addresses a sector will use. File is loaded at
123 <segment:offset>, the jump is made to <segment:ip>. This option is mostly used
124 in tandem with 'file=' and 'seg=' options, as some loaders/kernels will expect
125 relocated sector at some particular address (e.g. DRKM).
127 'nosect' will cause sector to not be loaded at all. In plenty cases, when a file
128 is being chainloaded, sector is not necessary.
130 The defaults if option is not specified, are 0:0x7c00:0x7c00.
131 If some of the fields are ommited (e.g. 0x2000::), they default to 0.
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 changed
180 By default, a handover area is always prepared if possible - meaning it doesn't
181 overlap with other areas. It's often not necessary though - usually, a
182 chainloaded file or kernel don't care about it anymore, so a user can disable
183 it explicitly with this option.
188 In case when both file and sector are loaded, ds:si and ds:bp will point to
189 sector address before the chainloading. This option lets user force those
190 registers to point to handover area. This is useful when both the file and the
191 sector are actually a sector's image and the sector is mmapped.
196 This option will install a tiny stub code used to swap drive numbers, if the
197 drive we use during chainloading is not fd0 or hd0.
203 In certain situations it's useful to hide partitions - for example to make sure
204 DOS gets C:. 'hide' will hide hidable primary partitions, except the one we're
205 booting from. Similary, 'hideall' will hide all hidable partitions, except the
206 one we're booting from. Hiding is performed only on the selected drive. Options
207 starting with 'un' will simply unhide every partition (primary ones or all).
208 Writing is only performed, if the os type values actually changed.
213 If you want to make a drive you're booting from totally compatible with current
214 BIOS, you can use this to fix all partitions' CHS numbers. Good to silence e.g.
215 FreeDOS complainig about 'logical CHS differs from physical' of sfdisk about
216 'found (...) expected (...). Functionally seems to be mostly cosmetic, as
217 Microsoft world - in cases it cares about geometry - generally sticks to values
218 written in bootsectors. And the rest of the world generally doesn't care about
219 them at all. Writing is only performed, if the values actually got changed.
224 If you're booting over a network using pxelinux - this lets you keep UNDI
225 stacks in memory (pxelinux only).
230 This option will wait for a keypress right before continuing the chainloading.
231 Useful to see warnings emited by the chain module.
235 nochain sets: nofile nomaps nohand
237 It is possible to trigger a "service-only" run - The chain module will do
238 everything requested as usual, but it will not perform the actual chainloading.
239 'nochain' option disables handover, file loading and sector mapping, as these
240 are pointless in such scenario (although file might be reenabled in some future
241 version, if writing to actual files becomes possible). Mainly useful for
242 options 'fixchs', '[un]hide[all]' and setbpb.
245 sets: file=<file> nohand nosect isolinux
247 Chainload another version/build of the ISOLINUX bootloader and patch the loader
248 with appropriate parameters in memory. This avoids the need for the
249 -eltorito-alt-boot parameter of mkisofs, when you want more than one ISOLINUX
253 sets: file=<file> seg=0x2000 setbpb nohand
255 Prepares to load ntldr directly. You might want to add 'save' option to store
256 corrected BPB values.
259 sets: file=<file> seg=0x2000 setbpb nohand cmldr
261 Prepares to load recovery console directly. In-memory copy of bootsector is
262 patched with "cmdcons\0". Remarks the same as in 'ntldr='.
265 sets: file=<file> seg=0x60 sect=0x1FE0 setbpb nohand
267 Prepares to load freedos kernel directly. You will likely want to add 'save'
268 option, as those kernels seem to require proper geometry written back to disk.
269 Sector address is chosen based on where freedos' bootsectors relocate themselves,
270 although it seems the kernel doesn't rely on it.
272 You might also want to employ 'hide' option, if you have problems with properly
277 sets: file=<file> seg=0x70 sect=0x8000 setbpb nohand
279 Similary to 'freedos=', This prepares to load MSDOS 2.00 - 6.xx or derivatives.
280 Sector address is chosen arbitrarily. Otherwise comments as above.
283 sets: file=<file> seg=0x70::0x200 sect=0x8000 setbpb nohand
285 Only for MSDOS 7+ versions (98se ~ 7.xx, Me ~ 8.xx). Comments as above.
289 sets: file=<file> seg=0x70 sect=0x2000:0:0 setbpb nohand
291 This is used for loading of *only* Dell's DOS derivatives. It does require boot
292 sector at 0x2000 and overall valid BPB values. As in other DOS-ish cases,
293 likely candidates for use are 'save' and 'hide'.
295 grub=<file> [grubcfg=<config>]
296 sets: file=<file> seg=0x800::0x200 nohand nosect grub
298 Chainloads grub legacy's stage2, performing additional corrections on the file
299 in memory. Additionally, alternate config file can be specified through
303 sets: file=<file> nohand nosect grldr
305 Chainloads GRUB4DOS grldr, performing additional corrections on the file
309 sets: file=<file> nomaps setbpb bss
311 This emulates syslinux's native BSS option. This loads both the file and the
312 sector, adjusts BPB values in the loaded sector, then copies all possible BPB
313 fields to the loaded file. Everything is made with reference to the selected
317 sets: file=<file> nosect filebpb
319 This emulates syslinux's native BS option. This loads the file and if possible
320 - adjusts its BPB values. Everything is made with reference to the selected