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 (options: 'hide', 'hideall', 'mbrchs').
23 4. Load file to boot from (options: 'file', 'seg').
24 5. Load sector to boot from (options: 'sect', 'maps'), if it doesn't conflict
26 6. Prepare handover area (options: 'hand', 'maps'), if it doesn't conflict
27 with #5 & #6, and syslinux is booting a partition.
28 7. Adjust ds:si and ds:bp to point to either a handover, a sector (options:
29 'hptr', 'maps', 'sect', 'file') or nowhere.
30 8. Patch loaded file if necessary ('isolinux', 'grub', 'grldr', 'filebpb')
31 9. Patch loaded sector if necessary ('setdrv', 'sethid', 'setgeo', 'setbpb',
33 10. Chainload (options: 'swap').
35 In most basic form, syslinux loads specified boot sector (or mbr, if not
36 specified) at 0:0x7c00, prepares handover area as a standard mbr would do, and
42 chain [drive/partition] [options]
44 DRIVE / PARTITION SPECIFICATION
46 Drive can be specified as 'hd#', 'fd#', 'boot', 'mbr', or 'guid'.
48 - 'mbr' will select a drive by a signature.
49 - 'guid' will select a drive by a guid
50 - 'boot' is the drive syslinux was booted from. This is the default value, if
51 nothing else is specified.
52 - 'hd#' and 'fd#' are standard ways to specify drive number as seen by bios,
55 Option 'guid' is shared with partition selection (see below). If you happened
56 to have non-unique guids, they are searched in disk0, partitions of disk0,
59 The priority of those options are the same as in the above list.
61 If you specify the same value more than once, the last value will be used.
63 'mbr' and 'guid' take extra parameter - you should use ':' or '=' as a
67 Partition can be specified as '#', 'guid', 'label' or 'fs'.
69 - 'guid' option will select a partition by a guid (not a type guid !)
70 - 'label' will select a partition by a label (searching is done in
72 - 'fs' will select a partition from which syslinux was executed
73 - '#' is the standard method. Partitions 1-4 are primary, 5+ logical, 0 = boot
76 The priority of those options are the same as in the above list.
78 If you use a number to select a partition it should be specified after a drive
79 using space or comma as delimiters (after 'hd#', 'fd#', 'mbr', 'guid' or 'boot').
85 It's often convenient to load a file directly and transfer control to it,
86 instead of the sector from the disk. Note, that the <file> must reside on
89 If you choose this option without specifying any addresses explicitly (see
90 options 'sect=' and 'seg='), the file will cause sector to not be loaded at all
91 (as their memory placement would overlap).
93 seg=<segment>:<offset>:<ip>
96 This triplet lets you alter the addresses a file will use. Loading is done to
97 <segment:offset>, jumping to <segment:ip>. When you chainload some other
98 bootloader or kernel, it's almost always mandatory.
100 The defaults, if option is not specified, are 0:0x7c00:0x7c00
101 If some of the fields are ommited (e.g. 0x2000::), they default to 0.
103 sect=<segment>:<offset>:<ip>
105 *sect=0:0x7c00:0x7c00
107 This triplet lets you alter the addresses a sector will use. File is loaded at
108 <segment:offset>, the jump is made to <segment:ip>. This option is mostly used
109 in tandem with 'file=' and 'seg=' options, as some loaders/kernels will expect
110 relocated sector at some particular address (e.g. DRKM).
112 'nosect' will cause sector to not be loaded at all. In plenty cases, when a file
113 is being chainloaded, sector is not necessary.
115 The defaults if option is not specified, are 0:0x7c00:0x7c00.
116 If some of the fields are ommited (e.g. 0x2000::), segment defaults to 0,
117 offset and ip to 0x7c00.
122 In some cases, it's useful to fix BPB values in NTFS/FATxx bootsectors and
123 evntually write them back, but otherwise boot sector itself is not necessary to
124 continue booting. 'nomaps' allows that - a sector will be loaded, but won't be
125 mmapped into real memory. Any overlap tests (vs. handover or file areas) are
126 not performed, being meaningless in such case.
137 Microsoft side of the world is paritculary bitchy about certain BPB values.
138 Depending on the system and chainloading method (sector or file), some or all
139 of those fields must match reality - and after e.g. drive clonning or
140 when using usb stick in different computers - that is often not the case.
144 "hidden sectors" - valid offset of the partition from the beginning of the disk
145 "geometry" - valid disk geometry as reported by BIOS
146 "drive" - valid drive number
149 updates the partition offset to match the current disk position
151 updates "heads" and "sectors per track" values as reported by BIOS
153 will update the drive value at proper offset. Only 0x24 and 0x40 are
154 accepted as valid values. You can use '@', '=' and ':' as delimiters.
155 If the address is not specified, it's autodetected.
160 Handy shortcut for setdrv, setgeo and sethid.
165 Chainloaded file can simply be an image of a sector. In such case, it could be
166 useful to also fix its BPB values.
171 Fixing BPB values only in memory might not be enough. This option allows
172 writing of the corrected sector. You will probably want to use this option
173 together with 'setbpb' or other ones using that implicitly.
175 - this option never applies to a loaded file
176 - chain module will never save anything to disk by default
177 - writing is only performed, if the values actually changed
182 By default, a handover area is always prepared if possible and potentially
183 useful - meaning it doesn't overlap with other areas, and syslinux chainloads a
184 partition. It's often not necessary though - usually, a chainloaded file or
185 kernel don't care about it anymore, so a user can disable it explicitly with
191 In case when both file and sector are loaded, ds:si and ds:bp will point to
192 sector address before the chainloading. This option lets user force those
193 registers to point to handover area. This is useful when both the file and the
194 sector are actually a sector's image and the sector is mmapped.
199 This option will install a tiny stub code used to swap drive numbers, if the
200 drive we use during chainloading is not fd0 or hd0.
205 In certain situations it's useful to hide partitions - for example to make sure
206 DOS gets C:. 'hide' will hide hidable primary partitions, except the one we're
207 booting from. Similary, 'hideall' will hide all hidable partitions, except the
208 one we're booting from. Hiding is performed only on the boot drive.
209 Writing is only performed, if the 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 physcial' 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 changed.
225 If you're booting over a network using pxelinux - this lets you keep UNDI
231 This option will wait for a keypress right before continuing the chainloading.
232 Useful to see warnings emited by the chain module.
235 sets: file=<file> nohand nosect
237 Chainload another version/build of the ISOLINUX bootloader and patch the loader
238 with appropriate parameters in memory. This avoids the need for the
239 -eltorito-alt-boot parameter of mkisofs, when you want more than one ISOLINUX
243 sets: file=<file> seg=0x2000 setbpb nohand
245 Prepares to load ntldr directly. You might want to add 'save' option to store
246 corrected BPB values.
249 sets: file=<file> seg=0x2000 setbpb nohand
251 Prepares to load recovery console directly. In-memory copy of bootsector is
252 patched with "cmdcons\0". Remarks the same as in 'ntldr='.
255 sets: file=<file> seg=0x60 sect=0x1FE0 setbpb nohand
257 Prepares to load freedos kernel directly. You will likely want to add 'save'
258 option, as those kernels seem to require proper geometry written back to disk.
259 Sector address is chosen based on where freedos' bootsectors relocate themselves,
260 although it seems the kernel doesn't rely on one.
261 You might also want to employ 'hide' option, if you have problems with properly
266 sets: file=<file> seg=0x70 sect=0x8000 setbpb nohand
268 Similary to 'freedos=', This prepares to load MSDOS 2.00 - 6.xx or derivatives.
269 Sector address is chosen arbitrarily. Otherwise comments as above.
272 sets: file=<file> seg=0x70::0x200 sect=0x8000 setbpb nohand
274 Only for MSDOS 7+ versions (98se ~ 7.xx, millenium ~ 8.xx). Comments as above.
278 sets: file=<file> seg=0x70 sect=0x2000:0:0 setbpb nohand
280 This is used for loading of *only* Dell's DOS derivatives. It does require boot
281 sector at 0x2000 and overall valid BPB values. As with other DOSish cases,
282 likely candidates for use are 'save' and 'hide'.
286 sets: file=<file> seg=0x800::0x200 nohand nosect
288 Chainloads grub legacy's stage2, performing additional corrections on the file
289 in memory. Additionally, alternate config file can be specified through
293 sets: file=<file> nohand nosect
295 Chainloads GRUB4DOS grldr, performing additional corrections on the file
299 sets: bss=<file> nomaps setbpb filebpb
301 This attempts to emulate syslinux's native BSS option to certain extent. This
302 loads both the file and the sector and adjusts BPB values in both. As only basic BPB
303 fields are corrected, both the file and the sector should be identical enough
304 (except boot code, drive number, geometry, hidden sectors). Don't forget about
305 'save' and 'swap' options.
307 This will likely get expanded in future.