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diff --git a/doc/chain.txt b/doc/chain.txt new file mode 100644 index 00000000..6dd0632d --- /dev/null +++ b/doc/chain.txt @@ -0,0 +1,327 @@ + 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] + + DRIVE / PARTITION SPECIFICATION + +Drive can be specified as 'hd#', 'fd#', 'boot', 'mbr', or 'guid'. + +- 'mbr' will select a drive by a signature. +- 'guid' will select a drive by a guid +- '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 happened +to have non-unique guids, they are searched in disk0, partitions of disk0, +disk1 ... order. + +The priority of those options are the same as in the above list. + +If you specify the same value more than once, the last value will be used. + +'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). + +The priority of those options are the same as in the above list. + +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 ommited (e.g. 0x2000::), they default to 0. + + sect=<segment>:<offset>:<ip> + nosect + *sect=0:0x7c00:0x7c00 + 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 ommited (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 + +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 + +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 + +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. + + *nobreak + break + 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. + |