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✏️ Editing: chain.txt

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

- '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=<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
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=<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, 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=<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 '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.

*gpthcrc
	nogpthcrc

GPT header contains its crc32 checksum. By default the partition iterator
verifies it and aborts in case of mismatch.

*gptlcrc
	nogptlcrc

GPT header contains crc32 checksum of GPT partition list. By default the
partition iterator verifies it and aborts in case of mismatch.

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

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

cmldr=<file>
	sets: file=<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=<file>
	sets: file=<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=<file>
	sets: file=<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=<file>
	msdos=<file>
	sets: file=<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=<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.
TODO/TEST

drmk=<file>
	sets: file=<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=<file> [grubcfg=<config>]
	sets: file=<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=<file>
	sets: file=<file> nohand nosect grldr

Chainloads 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 with 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
- adjusts its BPB values. Everything is made with reference to the selected
disk/partition.

MR SKB