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
14 It can chainload data from both GPT and DOS partitions, as well as boot the
15 first sector from a raw disk.
17 In more details, the flow of code is as follows:
20 2. Find drive and/or partition to boot from.
21 3. Hide / unhide systems and/or fix chs values in partition entries on the
22 drive syslinux is booting from.
23 4. Load a file to boot from.
24 5. Load a sector to boot from, if it doesn't conflict with #5.
25 6. Prepare handover area, if it doesn't conflict with #5 & #6, and syslinux is
28 8. Patch loaded file if necessary.
29 9. Patch loaded sector if necessary.
32 In most basic form, syslinux loads specified boot sector (or mbr, if not
33 specified) at 0:0x7c00, prepares handover area as a standard mbr would do, and
39 chain [drive/partition] [options]
41 DRIVE / PARTITION SPECIFICATION
43 Drive can be specified as 'hd#', 'fd#', 'boot', 'mbr', or 'guid'.
45 - 'mbr' will select a drive by a signature.
46 - 'guid' will select a drive by a guid
47 - 'boot' is the drive syslinux was booted from. This is the default value, if
48 nothing else is specified.
49 - 'hd#' and 'fd#' are standard ways to specify drive number as seen by bios,
52 Option 'guid' is shared with partition selection (see below). If you happened
53 to have non-unique guids, they are searched in disk0, partitions of disk0,
56 The priority of those options are the same as in the above list.
58 If you specify the same value more than once, the last value will be used.
60 'mbr' and 'guid' take extra parameter - you should use ':' or '=' as a
64 Partition can be specified as '#', 'guid', 'label' or 'fs'.
66 - 'guid' option will select a partition by a guid (not a type guid !)
67 - 'label' will select a partition by a label (searching is done in
69 - 'fs' will select a partition from which syslinux was executed
70 - '#' is the standard method. Partitions 1-4 are primary, 5+ logical, 0 = boot
73 The priority of those options are the same as in the above list.
75 If you use a number to select a partition it should be specified after a drive
76 using space or comma as delimiters (after 'hd#', 'fd#', 'mbr', 'guid' or 'boot').
82 It's often convenient to load a file directly and transfer control to it,
83 instead of the sector from the disk. Note, that the <file> must reside on
86 If you choose this option without specifying any addresses explicitly (see
87 options 'sect=' and 'seg='), the file will cause sector to not be loaded at all
88 (as their memory placement would overlap).
90 seg=<segment>:<offset>:<ip>
93 This triplet lets you alter the addresses a file will use. Loading is done to
94 <segment:offset>, jumping to <segment:ip>. When you chainload some other
95 bootloader or kernel, it's almost always mandatory.
97 The defaults, if option is not specified, are 0:0x7c00:0x7c00
98 If any of the fields are ommited (e.g. 0x2000::), they default to 0.
100 sect=<segment>:<offset>:<ip>
102 *sect=0:0x7c00:0x7c00
104 This triplet lets you alter the addresses a sector will use. File is loaded at
105 <segment:offset>, the jump is made to <segment:ip>. This option is mostly used
106 in tandem with 'file=' and 'seg=' options, as some loaders/kernels will expect
107 relocated sector at some particular address (e.g. DRKM).
109 'nosect' will cause sector to not be loaded at all. In plenty cases, when a file
110 is being chainloaded, sector is not necessary.
112 The defaults if option is not specified, are 0:0x7c00:0x7c00.
113 If some of the fields are ommited (e.g. 0x2000::), segment defaults to 0,
114 offset and ip to 0x7c00.
119 In some cases, it's useful to fix BPB values in NTFS/FATxx bootsectors and
120 evntually write them back, but otherwise boot sector itself is not necessary to
121 continue booting. 'nomaps' allows that - a sector will be loaded, but won't be
122 mmapped into real memory. Any overlap tests (vs. handover or file areas) are
123 not performed, being meaningless in such case.
128 Microsoft side of the world is paritculary bitchy about certain BPB values.
129 Depending on the system and chainloading method (sector or file), some or all
130 of those fields must match reality - and after e.g. drive clonning or
131 when using usb stick in different computers - that is often not the case.
135 "hidden sectors" - valid offset of the partition from the beginning of the disk
136 "geometry" - valid disk geometry as reported by BIOS
137 "drive" - valid drive number
139 This option will automatically determine the type of BPB and fix what is possible
140 to fix, relatively to detected BPB. If it's impossible to detect BPB, function
146 Chainloaded file can simply be an image of a sector. In such case, it could be
147 useful to also fix its BPB values.
152 Fixing BPB values only in memory might not be enough. This option allows
153 writing of the corrected sector. You will probably want to use this option
154 together with 'setbpb' or other ones using that implicitly.
156 - this option never applies to a loaded file
157 - chain module will never save anything to disk by default
158 - writing is only performed, if the values actually got changed
163 By default, a handover area is always prepared if possible and potentially
164 useful - meaning it doesn't overlap with other areas, and syslinux chainloads a
165 partition. It's often not necessary though - usually, a chainloaded file or
166 kernel don't care about it anymore, so a user can disable it explicitly with
172 In case when both file and sector are loaded, ds:si and ds:bp will point to
173 sector address before the chainloading. This option lets user force those
174 registers to point to handover area. This is useful when both the file and the
175 sector are actually a sector's image and the sector is mmapped.
180 This option will install a tiny stub code used to swap drive numbers, if the
181 drive we use during chainloading is not fd0 or hd0.
187 In certain situations it's useful to hide partitions - for example to make sure
188 DOS gets C:. 'hide' will hide hidable primary partitions, except the one we're
189 booting from. Similary, 'hideall' will hide all hidable partitions, except the
190 one we're booting from. Hiding is performed only on the booting drive. Options
191 starting with 'un' will simply unhide every partition (primary ones or all).
192 Writing is only performed, if the os type values actually changed.
197 If you want to make a drive you're booting from totally compatible with current
198 BIOS, you can use this to fix all partitions' CHS numbers. Good to silence e.g.
199 FreeDOS complainig about 'logical CHS differs from physcial' of sfdisk about
200 'found (...) expected (...). Functionally seems to be mostly cosmetic, as
201 Microsoft world - in cases it cares about geometry - generally sticks to values
202 written in bootsectors. And the rest of the world generally doesn't care about
203 them at all. Writing is only performed, if the values actually got changed.
208 If you're booting over a network using pxelinux - this lets you keep UNDI
209 stacks in memory (pxelinux only).
214 This option will wait for a keypress right before continuing the chainloading.
215 Useful to see warnings emited by the chain module.
218 sets: file=<file> nohand nosect isolinux
220 Chainload another version/build of the ISOLINUX bootloader and patch the loader
221 with appropriate parameters in memory. This avoids the need for the
222 -eltorito-alt-boot parameter of mkisofs, when you want more than one ISOLINUX
226 sets: file=<file> seg=0x2000 setbpb nohand
228 Prepares to load ntldr directly. You might want to add 'save' option to store
229 corrected BPB values.
232 sets: file=<file> seg=0x2000 setbpb nohand cmldr
234 Prepares to load recovery console directly. In-memory copy of bootsector is
235 patched with "cmdcons\0". Remarks the same as in 'ntldr='.
238 sets: file=<file> seg=0x60 sect=0x1FE0 setbpb nohand
240 Prepares to load freedos kernel directly. You will likely want to add 'save'
241 option, as those kernels seem to require proper geometry written back to disk.
242 Sector address is chosen based on where freedos' bootsectors relocate themselves,
243 although it seems the kernel doesn't rely on it.
245 You might also want to employ 'hide' option, if you have problems with properly
250 sets: file=<file> seg=0x70 sect=0x8000 setbpb nohand
252 Similary to 'freedos=', This prepares to load MSDOS 2.00 - 6.xx or derivatives.
253 Sector address is chosen arbitrarily. Otherwise comments as above.
256 sets: file=<file> seg=0x70::0x200 sect=0x8000 setbpb nohand
258 Only for MSDOS 7+ versions (98se ~ 7.xx, Me ~ 8.xx). Comments as above.
262 sets: file=<file> seg=0x70 sect=0x2000:0:0 setbpb nohand
264 This is used for loading of *only* Dell's DOS derivatives. It does require boot
265 sector at 0x2000 and overall valid BPB values. As in other DOS-ish cases,
266 likely candidates for use are 'save' and 'hide'.
268 grub=<file> [grubcfg=<config>]
269 sets: file=<file> seg=0x800::0x200 nohand nosect grub
271 Chainloads grub legacy's stage2, performing additional corrections on the file
272 in memory. Additionally, alternate config file can be specified through
276 sets: file=<file> nohand nosect grldr
278 Chainloads GRUB4DOS grldr, performing additional corrections on the file
282 sets: bss=<file> nomaps setbpb bss
284 This emulates syslinux's native BSS option. This loads both the file and the
285 sector, adjusts BPB values in the loaded sector, then copies all possible BPB
286 fields to the loaded file. Everything is made with reference to the selected
290 sets: bs=<file> nosect filebpb
292 This emulates syslinux's native BS option. This loads the file and if possible
293 - adjusts its BPB values. Everything is made with reference to the selected