Doc/chain

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Introduction

Although Syslinux is capable of (very simple) native chainloading (through .bss and .bs options - see doc/syslinux), it also features a very robust and rich com32 module designed for such purpose.

The chain.c32 module can perform some 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.

A rough overview of the code is as follows:

  1. Parse arguments.
  2. Find drive and/or partition to boot from.
  3. Perform partition-level patching - for example (un)hiding, fixing CHS values, etc.
  4. Load a file to boot from.
  5. Load a sector to boot from, if it doesn't conflict with #4.
  6. Prepare handover area, if it doesn't conflict with #4 and #5.
  7. Prepare registers.
  8. Patch loaded file if necessary.
  9. Patch loaded sector if necessary.
  10. Chainload.

In its most basic form, SYSLINUX loads a 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 the 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 either 'hd#', 'fd#', 'boot', 'mbr', or 'guid':

  • 'mbr' selects a drive by its signature;
  • 'guid' selects 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 an extra parameter; you should use either ':' or '=' as delimiter.

Partition can be specified as '#', 'guid', 'label' or 'fs':

  • 'guid' selects a partition by a specified guid value (not by type guid !);
  • 'label' selects a partition by label (searching by disk order);
  • 'fs' selects the partition from which SYSLINUX was executed;
  • '#' is the standard method.

Partitions 1-4 are primary, 5+ are logical, 0 = boot MBR (default).

If you use a number to select a partition, it should be specified after a drive by using either a space or a comma character as delimiter (after 'hd#', 'fd#', 'mbr', 'guid' or 'boot').


OPTIONS



        file=<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 <file> must reside on the SYSLINUX's partition.

If you choose this option without explicitly specifying any addresses (see options 'sect=' and 'seg='), the file will cause the sector to not be loaded at all (as their memory placement would overlap).



        seg=<segment>:<offset>:<ip>
        *seg=0:0x7c00:0x7c00

This triplet lets you alter the addresses a file will use. It's loaded at <segment:offset>; the entry point is at <segment:ip>. When you chainload some other bootloader or kernel, this option is almost always mandatory.

If this option is not specified, the defaults are 0:0x7c00:0x7c00. If any field is omitted (e.g. 0x2000::), it defaults to 0.



        sect=<segment>:<offset>:<ip>
        *sect=0:0x7c00:0x7c00
        nosect
        nosect sets: nomaps

This triplet lets you alter the addresses a sector will use. It's loaded at <segment:offset>; the entry point is at <segment:ip>. This option is mostly used in tandem with the 'file=' and 'seg=' options, as some loaders/kernels expect a relocated sector at some particular address (e.g. DRKM).

'nosect' will cause the sector to not be loaded at all. In many cases, when a file is being chainloaded, specifying a sector is not necessary.

If this option is not specified, the defaults are 0:0x7c00:0x7c00. If any field is omitted (e.g. 0x2000::), it defaults to 0.



        *maps
        nomaps

In some cases, it's useful to fix BPB values in NTFS/FATxx boot sectors and eventually write them back, but otherwise the boot sector itself is not necessary so to continue the booting process. 'nomaps' allows such situation: a sector will be loaded, but it won't be mapped into real memory. Any overlap tests (vs. handover or file areas) are not performed, being meaningless in such case.



        setbpb
        *nosetbpb

Microsoft's side of the world is particularly sensitive to certain BPB values. Depending on the system and the chainloading method (i.e. sector or file), some or all of those fields must match "reality"; but after performing certain operations such as drive cloning, or when using a USB stick in different computers, that's often not the case (i.e. some BPB values might not match the real media).

Matching the "reality" means evaluating:

"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 it can, according to the detected BPB. If the detection of the BPB is not possible, this function does nothing.



        filebpb
        *nofilebpb

A 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 the corrected values to the sector. You would probably want to use this option together with 'setbpb'.

  • this option never applies to a loaded file.
  • by default, the chain module will not save anything (such as BPB values) to disk (other than options such as 'hide' or 'fixchs', which are directly related to partition entries).
  • the writing is only performed if the values actually changed.


        *hand
        nohand

By default, a handover area is prepared if possible - meaning it doesn't overlap with other areas. This is often not necessary though; usually, a chainloaded file or kernel doesn't care about it anymore, so a user can disable it explicitly with this option.



        hptr
        *nohptr

In case both, file and sector, are loaded, then ds:si and ds:bp will point to the sector address before the chainloading. This option forces those registers to point to the handover area. This is useful when both the file and the sector are actually a sector's image and the sector is mapped.



        swap
        *noswap

This option installs 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. Similarly, '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). The 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 complaining about logical CHS differs from physical, or sfdisk about found (...) expected (...). Functionally, it seems to be mostly cosmetic, as Microsoft world (in those 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. The writing is only performed if the values actually changed.



        keepexe
        *nokeepexe

If you are booting over a network using PXELINUX, this option lets you keep UNDI stacks in memory (PXELINUX only).



        warn
        *nowarn

This option will wait for a key-press right before continuing the chainloading. Useful to see warnings emitted by the chain module.



        prefmbr
        *noprefmbr

In case non-standard hybrid MBR/GPT layout is present, this flag makes the chain module prefer MBR layout over GPT.



        strict[=<0|1|2>] 
        *strict=1
        relax

        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 

These options control the level of sanity checks used during the traversal of partition table(s). This is potentially useful in (buggy) corner cases, for example 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.



        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. The 'break' option disables the handover, file loading and sector mapping, as these are pointless in such scenario (although the file might be re-enabled in some future version, if the writing to actual files becomes possible). Mainly useful for options such as 'fixchs', '[un]hide[all]' and 'setbpb'.



        isolinux=<file>
        sets: file=<file> nohand nosect isolinux

Chainload to 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=<file>
        sets: file=<file> seg=0x2000 setbpb nohand

Prepares to load ntldr directly. You might want to add the 'save' option so to store corrected BPB values.



        cmldr=<file>
        sets: file=<file> seg=0x2000 setbpb nohand cmldr

Prepares to load recovery console directly. The in-memory copy of the bootsector is patched with cmdcons\0 . Same remarks as in 'ntldr='.



        reactos=<file>
        sets: file=<file> seg=0:0x8000:0x8100 setbpb nohand

Prepares to load ReactOS's freeldr directly. You might want to add the 'save' option so to store corrected BPB values.



        freedos=<file>
        sets: file=<file> seg=0x60 sect=0x1FE0 setbpb nohand

Prepares to load FreeDOS kernel directly. You will likely want to add the 'save' option, as those kernels seem to require a proper geometry written back to disk. The 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 the 'hide' option, if you have problems with properly assigning the C: drive.



        pcdos=<file>
        msdos=<file>
        sets: file=<file> seg=0x70 sect=0x8000 setbpb nohand

Similarly to 'freedos=', this prepares to load MSDOS 2.00 - 6.xx or derivatives. The sector address is chosen arbitrarily. Otherwise, comments as above.



        msdos7=<file>
        sets: file=<file> seg=0x70::0x200 sect=0x8000 setbpb nohand

Only for MSDOS 7+ versions (98se ~ 7.xx, Me ~ 8.xx). Comments as above.





        drmk=<file>
        sets: file=<file> seg=0x70 sect=0x2000:0:0 setbpb nohand

This is used for loading of Dell's DOS derivatives only. It does require a boot sector at 0x2000 and overall valid BPB values. As in other DOS-ish cases, likely candidates for use are the 'save' and 'hide' options.



        grub=<file> [grubcfg=<config>]
        sets: file=<file> seg=0x800::0x200 nohand nosect grub

Chainloads to grub legacy's stage2, performing additional corrections on the file in memory. Additionally, an alternate config file can be specified through the 'grubcfg=' option.



        grldr=<file>
        sets: file=<file> nohand nosect grldr

Chainloads to GRUB4DOS grldr, performing additional corrections on the file in memory.



        bss=<file>
        sets: file=<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 in reference to the selected disk/partition.



        bs=<file>
        sets: file=<file> nosect filebpb

This emulates SYSLINUX's native BS option. This loads the file and, if possible, also adjusts its BPB values. Everything is made in reference to the selected disk/partition.


See Also