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Any El Torito walkthroughs or comprehensive docs?
Posted: Thu Jan 29, 2009 2:33 pm
by abachler
I've looked around at various ISO-9660 docs, but every one seems to be missing key details about the encodings. So I was wondering if there is a walkthrough for putting together an ISO-9660 bootable CD at the hex level, not one that uses other utilities. So far I've looked at these -
http://odin.himinbi.org/xp_cds/eltorito_extraction.html
http://en.wikipedia.org/wiki/ISO_9660#I ... ifications
http://alumnus.caltech.edu/~pje/iso9660.html
http://users.telenet.be/it3.consultants ... O9960.html
http://www.mscience.com/logical.html
http://www.ecma-international.org/publi ... ma-119.pdf
and while they give good info, and combined probably provide all the information I will need, I was looking for something more like a tutorial specifc to writing a bootable CD.
Re: Any El Torito walkthroughs or comprehensive docs?
Posted: Thu Jan 29, 2009 3:03 pm
by itisiuk
this works, and use the grub package at the bottom to get stage2_ElTorito
http://wiki.osdev.org/Bootable_El-Torit ... RUB_Legacy
Re: Any El Torito walkthroughs or comprehensive docs?
Posted: Thu Jan 29, 2009 3:47 pm
by abachler
I appreciate the link, but I am specifically looking for documentation to create my own el torito boot disk, i.e. 'not one that uses other utilities', or one that is pre-rolled.
Re: Any El Torito walkthroughs or comprehensive docs?
Posted: Thu Jan 29, 2009 10:07 pm
by Brendan
Hi,
You should only really need the ISO9660 specification (which you've already listed) and the
El Torito specification. Apart from that, it's a good idea to get a hexdump of an existing bootable CD (doesn't matter which one) because some of the fields aren't described well in either specification.
You could also use
my code as an example; although I should mention I've only tested booting it in Bochs (to test if it'll boot), and mounting it as a loopback device (to test if the ISO9660 file-system is sane).
Note: my code uses NASM to generate a bootable CD image, so i'ts not quite "hex level" and a utility is involved.... 
Cheers,
Brendan
Re: Any El Torito walkthroughs or comprehensive docs?
Posted: Fri Jan 30, 2009 1:47 pm
by abachler
Yes I am also using nasm to assemble the iso image. The extra info in your code will come in handy im sure. Thanks.
Re: Any El Torito walkthroughs or comprehensive docs?
Posted: Mon Jul 07, 2014 10:16 am
by SpyderTL
The ISO 9660 wiki page on OSDev describes the descriptor tables pretty well (
http://wiki.osdev.org/ISO_9660), but it only mentions the boot descriptor. (I'll update the wiki page when I get a second.) However, the boot descriptor just tells you what boot specification is used, and where the actual boot information is located.
Code: Select all
ISO 9660 Boot Record Descriptor
char[32] BootSystemID // ASCII encoded. For El Torito, it should be "EL TORITO SPECIFICATION" followed by nine zeros.
byte[32] Reserved
int32 BootCatalogBlock // The physical block where the El Torito boot information is located.
byte[1973] Reserved
Using the BootCatalogBlock value to find the location of the El Torito boot information, you will find the following sequence of tables:
Code: Select all
El Torito Validation Entry
byte Header ID
byte Platform ID
byte[2] Reserved
char[24] ID // ASCI Encoded
int16 Checksum
byte Key1
byte Key2
El Torito Default Entry
byte BootIndicator
byte BootMediaType
int16 LoadSegment
byte SystemType
byte Reserved
int16 SectorCount // The number of 2048 byte blocks to load
int32 FirstBlock // The starting block of the boot loader
byte[20] Reserved
El Torito Boot Section Header Entry (multiple)
byte HeaderIndicator // Zero for terminating entry
byte PlatformID
int16 SectionCount // The number of section entries following this header
char[30] SectionID // ASCII encoded
El Torito Boot Section Entry
byte BootIndicator
byte BootMediaType // Contains flag (bit 5) denoting that this entry is followed by one or more extension entries
int16 LoadSegment
byte SystemType
byte Reserved
int16 SectorCount
int32 FirstBlock
byte SectionCriteriaType
byte[5] Reserved
El Torito Boot Section Extension Entry
byte ExtensionIndicator
byte Flags // Contains flag (bit 5) denoting that this entry is followed by another extension entry
byte[30] SelectionCriteria
This should give you enough information to get you started creating a bootable ISO image (or reading one). I will update the Wiki to include this information shortly.