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@@ -0,0 +1,203 @@ + + + +Q: What is this tools aimed at anyway? +A: It is designed to handle ISO images you make from CDs. + + +Q: What is an ISO image anyway? +A: You can create an ISO with free tools like cdrdao in raw mode, or others, + like cdrwin, CloneCD, etc... + + +Q: Are all the ISO formats handled? +A: No. Only raw-2532 images files. Always the file format outputted by CloneCD + and cdrwin, and the format outputted by cdrdao with the --read-raw option. + + +Q: Is Nero's file format supported? +A: No. + + +Q: Why? Nero's a spreaded software! +A: It's a commercial tool. Since I don't use any commercial tool, and that no + free-software generates Nero ISO, it won't be suppored. + + +Q: What is this tool/library able to? +A: First, you can read/write sectors from/to an iso file. You can also read + informations about an iso file. You can extract/insert files from/to + an iso file. Depending upon the mode you'll be using, it will compute + the right CRC/ECC code for the given sector. The whole in the following + (eventually mixed) modes: MODE_1, MODE_2, MODE_2_FORM_1, MODE_2_FORM_2. + Additionnaly, it is able to produce patches (.ppf files) instead modifying + the iso file, saving you time when you use the right softwares. + Actually, it is quite "oriented" on the MODE_2* formats, since it's + the PSX's formats. + + +Q: I've heard CDmage or ECCRegen can also correct the sectors for me. +A: Maybe. Since it only runs on Win32 platforms, I've never tried it. + + +Q: So, what is the goal of this software? +A: To modify (patch) ISO images. Nothing else. And of course I want it free, + opensource, and working on my preffered operating system, Linux. If somebody + can make it working for windows (and I think this is easy to do) I will + please me. I can't do it right now since I don't really have the opportunity + to build Win32 binaries (apart of cygwin's ones) + + +Q: Where does the source code for the CRC/ECC comes from? +A: Originally, it has been taken from cdrdao. Yazoo has given some + modifications to it. Then I've cleaned it up and made some minor + modifications on my self. The source code was called 'yazedc'. + + +Q: Do you have the right to do so? +A: The software is GPL'ed. I've got the right to give modified versions + of it, as long as I don't claim the modificated thing it the original, + and as long as I mantion the original authors in it. + + +Q: What a strange name, 'yazedc' ? +A: I've got my own ideas about the name's origin... The easy solution: + "YAZoo EDC", where EDC is the field name of one of the things it will + recompute. But there is a more... complicated solution I won't give. + + +Q: So, I can modify your code too, create a new tool, and diffuse it? +A: Yes, as long as you give the full source code, that the new software + is also GPL'ed, and that you mantion me as the original writer of the + software, you can. Read the GPL carefully, it's very interesting. + + +Q: What is exactly the format of a CD-Rom? +A: Firstly, when you have a raw sector, you have to understand its primary form. + Secondly, the whole CD has an internal format, called the iso9660. The format + of the iso9660 is easy to find on the internet. Here is one first easy link: + http://www.ccs.neu.edu/home/bchafy/cdb/info/iso9660.txt + Then you have two more difficult documents: + http://www.ecma.ch/ecma1/stand/ecma-119.htm + and + http://www.ecma.ch/ecma1/stand/ecma-130.htm + + All those links were taken from the page + http://www.ccs.neu.edu/home/bchafy/cdb/info/info.html + + The sector format is a bit complicated to find on the internet. Here is what + I've found. + + First you have to know that there is many formats that describe the sector's + organisation. Those are called "Books". There is the Red Book, the Yellow + Book, the Blue Book, the Green Book, the Orange Book, and the White Book. + + The Red Book is for Audio CD. The Yellow for common CD-Roms. The Blue book + for Philips's VideoCD. The Green Book for CD-i and CD-XA. The Orange Book + for CD-R cds. And the White Book seems to be a replacement of the Green one. + + This is quite unclear and you have to actually buy the books since they + aren't in public domain. + + So the informations I'll give comes from various source of various free + softwares. Should I mention two: cdrdao http://cdrdao.sourceforge.net + and ECCRegen http://web.tiscali.it/eccregen as the most useful sources. + + Here is the general form of a CD-Rom sector: + + <--------------------------- sector: 2352 bytes ------------------------------> + <- Header: 16 bytes -><---------------- Datas: 2336 bytes --------------------> + + Let's move to the header description: + + <--------------------------- header: 16 bytes ------------------------------> + <-- sync bytes: 12 bytes --><-- localisation: 3 bytes --><-- mode: 1 byte --> + + The sync bytes are easy: it is always 00 FF FF FF FF FF FF FF FF FF FF 00 + + The localisation is the sector "position" described in time. For example, + the sector 200000 of a CD is at the "time" 44:28:50. The first is the number + of minutes, the second is the number of seconds, in the range 0-59 and the + last is the frame number, in the range 0-74. It means there is 75 frames + into a second for a CD player. Please note that the CD "begins" at 00:02:00. + + Ok now that we know all this, you can feel the way the localisation is + stored. But it is not that easy... + + <-------------------- localisation: 3 bytes --------------------> + <-- minute: 1 byte --><-- second: 1 byte --><-- frame: 1 byte --> + + That's seems to be all right *BUT* the fact is that the bytes are stored + in packed BCD format. You may know what the BCD format is if you are "old" + enough for that. I won't enter into the details so if you want a more + description of the BCD format, look into the net. You only have to know that: + +unsigned char from_BCD(unsigned char x) {return ((x & 15) + (x & 240) * 10));} +unsigned char to_BCD(unsigned char x) {return ((x / 10) << 4) | (x % 10));} +int is_valid_BCD(unsigned char x) {return (((x & 15) < 10) && ((x >> 4) < 10));} + + Last thing: when you look at a BCD packed number, you have to read it in + hexadecimal, and then you will see a "decimal" number. So when you count + in BCD, you'll have this: 0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, + 0x08, 0x09, 0x10, 0x11, 0x12, etc... You see? You've got a "gap": no 0x0a, + 0x0b, 0x0c, etc... So the BCD is only a trick for an easy reading of hexa + dumps of various informations. + + All right. This was for the localisation part. The last part is the mode + byte. It is really simple actually. It is 0 for an empty sector, 1 for a + sector in MODE1, and 2 for a sector in MODE2. Easy as hell. + + Ok here we are: we know the basic form of a CD's sector, and even know the + MODE of the sector. Now the datas depends upon the sector mode. Here you have + the various kinds: + + <-------------- MODE 1 FORM 1 Sector datas: 2336 bytes ----------------------> + <- datas: 2048 bytes -><- EDC: 4 bytes -><- 0s: 8 bytes -><- ECC: 276 bytes -> + + <---------- MODE 1 FORM 2 and also MODE 2 Sector datas: 2336 bytes ----------> + <----------------------------- datas: 2336 bytes ----------------------------> + + <-------------- MODE 2 FORM 1 Sector datas: 2336 bytes ----------------------> + <- SH: 8 bytes -><- datas: 2048 bytes -><- EDC: 4 bytes -><- ECC: 276 bytes -> + + <-------------- MODE 2 FORM 2 Sector datas: 2336 bytes ----------------------> + <- SH: 8 bytes -><---------- datas: 2324 bytes ----------><- spare: 4 bytes -> + + Well, I *really* don't know how to distinguish the different "FORMS" from + each others for the MODE 1. Have to look further for this. + + The ECC and EDC controls blocks. The yazedc code can compute them, so + don't worry about them. + + The 'SH' (SubHeader) field is the most "complicated" one. Those eight little + bits are the only one I'm really not sure of. All of that because you have to + buy the Books to find the information. This SubHeader is only found into + MODE_2_FORM_1 and MODE_2_FORM_2 sectors. + + Here you have the informations I've been able to gather: + + -) The SubHeader has 8 bytes, but it's twice the same 4 bytes. + -) The 4 bytes are described using the following fields: + o) 1st byte: File Number (FN) + o) 2nd byte: Channel Number (CN) + o) 3rd byte: Sub Mode (SM) + o) 4st byte: Coding Info (CI) + -) I've *never* seen any SubHeader with a FN, CN or CI different from 0, + please inform me if you do. + -) The Sub Mode byte is a bit field which seems to be described like this: + 0: End of Record (EOR) + 1: Video + 2: Audio + 3: Data + 4: Trigger + 5: Form 2 + 6: Real Time (RT) + 7: End of File (EOF) + + Very last things to know: of course, the PSX has the CDs in MODE 2... So the + common files are stored in MODE 2 FORM 1, the STR/XA files are stored in + MODE 2 and I've never seen any file stored in MODE 2 FORM 2. And the MODE 2 + FORM 1 and MODE 2 FORM 2 are also called XA-Mode1 and XA-Mode2 or simplier: + XA-1 and XA-2. + + I hope this will help you as it helped me writing this software. |