chain.c32 documentation Although syslinux is capable of (very simple) native chainloading (through .bss and .bs options - see doc/syslinux.txt), it also features a very roboust and rich com32 module designed for such purpose. Chain module can perform few basic tasks: - load and jump to a sector - load and jump to a file (also loading a sector for other purposes) - prepare handover data to use by a file / boot sector - fix different options in a file / sector / partition entries - perform a "service-only" run It can chainload data from both GPT and DOS partitions, as well as boot the first sector from a raw disk. In more details, the rough overview of code is as follows: 1. Parse arguments. 2. Find drive and/or partition to boot from. 3. Perform partition-level patching - for example hiding, unhiding, fixing chs values, etc. 4. Load a file to boot from. 5. Load a sector to boot from, if it doesn't conflict with #5. 6. Prepare handover area, if it doesn't conflict with #5 & #6. 7. Prepare registers. 8. Patch loaded file if necessary. 9. Patch loaded sector if necessary. 10. Chainload. In most basic form, syslinux loads specified boot sector (or mbr, if not specified) at 0:0x7c00, prepares handover area as a standard mbr would do, and jumps to 0:0x7c00. A "service-only" run is possible when either: - 'break' is in effect or - 'nofile' and 'nomaps' (or 'nosect') are in effect This is useful for invocations such as: chain.c32 hdN M setbpb save break chain.c32 hdN fixchs break chain.c32 hdN unhideall break Please see respective options for more details. Module invocation: chain [drive/partition] [options] In case of repeated arguments, rightmost ones take precedence. DRIVE / PARTITION SPECIFICATION Drive can be specified as 'hd#', 'fd#', 'boot', 'mbr', or 'guid'. - 'mbr' will select a drive by its signature. - 'guid' will select a drive by its guid (GPT only). - 'boot' is the drive syslinux was booted from. This is the default value, if nothing else is specified. - 'hd#' and 'fd#' are standard ways to specify drive number as seen by bios, starting from 0. Option 'guid' is shared with partition selection (see below). If you happen to have non-unique guids, they are searched in disk0, partitions of disk0, disk1 ... order. 'mbr' and 'guid' take extra parameter - you should use ':' or '=' as a delimiter. Partition can be specified as '#', 'guid', 'label' or 'fs'. - 'guid' option will select a partition by a guid (not a type guid !) - 'label' will select a partition by a label (searching is done in disk order) - 'fs' will select a partition from which syslinux was executed - '#' is the standard method. Partitions 1-4 are primary, 5+ logical, 0 = boot MBR (default). If you use a number to select a partition it should be specified after a drive using space or comma as delimiters (after 'hd#', 'fd#', 'mbr', 'guid' or 'boot'). OPTIONS file= *nofile It's often convenient to load a file directly and transfer control to it, instead of the sector from the disk. Note, that the must reside on syslinux partition. If you choose this option without specifying any addresses explicitly (see options 'sect=' and 'seg='), the file will cause sector to not be loaded at all (as their memory placement would overlap). seg=:: *seg=0:0x7c00:0x7c00 This triplet lets you alter the addresses a file will use. It's loaded at , the entry point is at . When you chainload some other bootloader or kernel, it's almost always mandatory. The defaults, if option is not specified, are 0:0x7c00:0x7c00 If any of the fields are omitted (e.g. 0x2000::), they default to 0. sect=:: *sect=0:0x7c00:0x7c00 nosect nosect sets: nomaps This triplet lets you alter the addresses a sector will use. It's loaded at , the entry point is at . This option is mostly used in tandem with 'file=' and 'seg=' options, as some loaders/kernels will expect relocated sector at some particular address (e.g. DRKM). 'nosect' will cause sector to not be loaded at all. In plenty cases, when a file is being chainloaded, sector is not necessary. The defaults if option is not specified, are 0:0x7c00:0x7c00. If some of the fields are omitted (e.g. 0x2000::), they default to 0. *maps nomaps In some cases, it's useful to fix BPB values in NTFS/FATxx bootsectors and evntually write them back, but otherwise boot sector itself is not necessary to continue booting. 'nomaps' allows that - a sector will be loaded, but won't be mmapped into real memory. Any overlap tests (vs. handover or file areas) are not performed, being meaningless in such case. setbpb *nosetbpb Microsoft side of the world is paritculary sensitive to certain BPB values. Depending on the system and chainloading method (sector or file), some or all of those fields must match reality - and after e.g. drive clonning or when using usb stick in different computers - that is often not the case. The "reality" means: "hidden sectors" - valid offset of the partition from the beginning of the disk "geometry" - valid disk geometry as reported by BIOS "drive" - valid drive number This option will automatically determine the type of BPB and fix what is possible to fix, relatively to detected BPB. If it's impossible to detect BPB, function will do nothing. filebpb *nofilebpb Chainloaded file can simply be an image of a sector. In such case, it could be useful to also fix its BPB values. save *nosave save sets: strict=2 Fixing BPB values only in memory might not be enough. This option allows writing of the corrected sector. You will probably want to use this option together with 'setbpb'. - this option never applies to a loaded file - chain module will not save anything to disk by default (besides options such as hide or fixchs - so options related directly to partition entries) - writing is only performed, if the values actually changed *hand nohand By default, a handover area is always prepared if possible - meaning it doesn't overlap with other areas. It's often not necessary though - usually, a chainloaded file or kernel don't care about it anymore, so a user can disable it explicitly with this option. hptr *nohptr In case when both file and sector are loaded, ds:si and ds:bp will point to sector address before the chainloading. This option lets user force those registers to point to handover area. This is useful when both the file and the sector are actually a sector's image and the sector is mmapped. swap *noswap This option will install a tiny stub code used to swap drive numbers, if the drive we use during chainloading is not fd0 or hd0. hide[all] unhide[all] *nohide [un]hide[all] sets: strict=2 In certain situations it's useful to hide partitions - for example to make sure DOS gets C:. 'hide' will hide hidable primary partitions, except the one we're booting from. Similary, 'hideall' will hide all hidable partitions, except the one we're booting from. Hiding is performed only on the selected drive. Options starting with 'un' will simply unhide every partition (primary ones or all). Writing is only performed, if the os type values actually changed. fixchs *nofixchs fixchs sets: strict=2 If you want to make a drive you're booting from totally compatible with current BIOS, you can use this to fix all partitions' CHS numbers. Good to silence e.g. FreeDOS complainig about 'logical CHS differs from physical' of sfdisk about 'found (...) expected (...). Functionally seems to be mostly cosmetic, as Microsoft world - in cases it cares about geometry - generally sticks to values written in bootsectors. And the rest of the world generally doesn't care about them at all. Writing is only performed, if the values actually got changed. keepexe *nokeepexe If you're booting over a network using pxelinux - this lets you keep UNDI stacks in memory (pxelinux only). warn *nowarn This option will wait for a keypress right before continuing the chainloading. Useful to see warnings emited by the chain module. prefmbr *noprefmbr In the case of presence of non-standard hybrid MBR/GPT layout, this flag makes chain module prefer MBR layout over GPT. strict[=<0|1|2>] *strict=1 relax Those options control the level of sanity checks used during the traversal of partition table(s). This is useful in buggy corner cases, when the disk size is reported differently across different computers or virtual machines (if it happens at all, the size usually differs by 1 sector). Normally the partition iterator would report an error and abort in such case. Another case scenario is disk corruption in some later EMBR partition. - strict=0 inhibits any checks - strict=1 enables checks, but ignores those that involve disk size - strict=2 enables all checks - relax and nostrict are equivalent to strict=0 - norelax and strict are equivalent to strict=2 break *nobreak break sets: nofile nomaps nohand It is possible to trigger a "service-only" run - The chain module will do everything requested as usual, but it will not perform the actual chainloading. 'break' option disables handover, file loading and sector mapping, as these are pointless in such scenario (although file might be reenabled in some future version, if writing to actual files becomes possible). Mainly useful for options 'fixchs', '[un]hide[all]' and setbpb. isolinux= sets: file= nohand nosect isolinux Chainload another version/build of the ISOLINUX bootloader and patch the loader with appropriate parameters in memory. This avoids the need for the -eltorito-alt-boot parameter of mkisofs, when you want more than one ISOLINUX per CD/DVD. ntldr= sets: file= seg=0x2000 setbpb nohand Prepares to load ntldr directly. You might want to add 'save' option to store corrected BPB values. cmldr= sets: file= seg=0x2000 setbpb nohand cmldr Prepares to load recovery console directly. In-memory copy of bootsector is patched with "cmdcons\0". Remarks the same as in 'ntldr='. reactos= sets: file= seg=0:0x8000:0x8100 setbpb nohand Prepares to load ReactOS's freeldr directly. You might want to add 'save' option to store corrected BPB values. freedos= sets: file= seg=0x60 sect=0x1FE0 setbpb nohand Prepares to load freedos kernel directly. You will likely want to add 'save' option, as those kernels seem to require proper geometry written back to disk. Sector address is chosen based on where freedos' bootsectors relocate themselves, although it seems the kernel doesn't rely on it. You might also want to employ 'hide' option, if you have problems with properly assigned C: drive. pcdos= msdos= sets: file= seg=0x70 sect=0x8000 setbpb nohand Similary to 'freedos=', This prepares to load MSDOS 2.00 - 6.xx or derivatives. Sector address is chosen arbitrarily. Otherwise comments as above. msdos7= sets: file= seg=0x70::0x200 sect=0x8000 setbpb nohand Only for MSDOS 7+ versions (98se ~ 7.xx, Me ~ 8.xx). Comments as above. TODO/TEST drmk= sets: file= seg=0x70 sect=0x2000:0:0 setbpb nohand This is used for loading of *only* Dell's DOS derivatives. It does require boot sector at 0x2000 and overall valid BPB values. As in other DOS-ish cases, likely candidates for use are 'save' and 'hide'. grub= [grubcfg=] sets: file= seg=0x800::0x200 nohand nosect grub Chainloads grub legacy's stage2, performing additional corrections on the file in memory. Additionally, alternate config file can be specified through 'grubcfg=' option grldr= sets: file= nohand nosect grldr Chainloads GRUB4DOS grldr, performing additional corrections on the file in memory. bss= sets: file= nomaps setbpb bss This emulates syslinux's native BSS option. This loads both the file and the sector, adjusts BPB values in the loaded sector, then copies all possible BPB fields to the loaded file. Everything is made with reference to the selected disk/partition. bs= sets: file= nosect filebpb This emulates syslinux's native BS option. This loads the file and if possible - adjusts its BPB values. Everything is made with reference to the selected disk/partition.