Description of the zisofs Format as of zisofs-tools-1.0.8 by H. Peter Anvin and cdrtools-2.01.01a39 by Joerg Schilling For libburnia-project.org by Thomas Schmitt - distribute freely , please report any errors or ambiguities - Apr 11 2009 The zisofs format was invented by H. Peter Anvin. It compresses data file content, marks it by a header and provides a pointer array for coarse random access. Within a RRIP enhanced ISO 9660 image the format is additionally marked by a System Use entry with signature "ZF". The uncompressed size of a single zisofs compressed file is restricted to 4 GiB - 1. Larger files shall not be compressed. File Header The file header has this layout (quoted from zisofs-tools-1.0.8/mkzftree.c): Byte offset iso9660 type Contents 0 (8 bytes) Magic number (37 E4 53 96 C9 DB D6 07) 8 7.3.1 Uncompressed file size 12 7.1.1 header_size >> 2 (currently 4) 13 7.1.1 log2(block_size) 14 (2 bytes) Reserved, must be zero So its size is 16. 7.3.1 means little endian 4-byte words. 7.1.1. means unsigned single bytes. Readers shall be able to handle log2(block_size) values 15, 16 and 17 i.e. block sizes 32 kB, 64 kB, and 128 kB. Writers must not use other sizes. Block Pointers There are ceil(input_size / block_size) input resp. output blocks. Each input block is of fixed size whereas the output blocks have varying size (down to 0). For each output block there is an offset pointer giving its byte address in the overall file content. The next block pointer in the array tells the start of the next block which begins immediately after the end of its predecessor. A final pointer gives the first invalid byte address and thus marks the end of the last block. So there are ceil(input_size / block_size) + 1 block pointers. They are stored as an array of 4-byte values which are in ISO 9660:7.3.1 format directly after the file header, i.e. beginning at byte 16. Data Part The data part begins immediately after the pointer array. In principle it consists of the variable length output blocks as delivered by zlib function compress2() when fed with the fixed size input blocks. A special case of input and output block is defined: Zero-length blocks represent a block full of 0-bytes. Such input blocks do not get processed by compress2() but shall be mapped to 0-sized output directly. Vice versa 0-sized blocks have to bypass uncompress() when being read. ZF System Use Entry Format ZF may only be applied to files with a single extent and less than 4 GiB of uncompressed size. The ZF entry follows the general layout of SUSP resp. RRIP. Its fields are: [1] "BP 1 to BP 2 - Signature Word" shall be (5A)(46) ("ZF"). [2] "BP 3 - Length" shall specify as an 8-bit number the length in bytes of the ZF entry recorded according to ISO 9660:7.1.1. This length is 16 decimal. [3] "BP 4 - System Use Entry Version" shall be 1 as in ISO 9660:7.1.1. [4] "BP 5 to BP 6 - Algorithm" shall be (70)(7A) ("pz") to indicate "paged zlib". [5] "BP 7 - Header Size Div 4" shall specify as an 8-bit number the number of 4-byte words in the header part of the file data recorded according to ISO 9660:7.1.1. (This is a copy of header byte 12, resp. header BP 13). [6] "BP 8 - Log2 of Block Size" shall specify as an 8-bit number the binary logarithm of the compression block size recorded according to ISO 9660:7.1.1. (This is a copy of header byte 13, resp. header BP 14. The value has to be 15, 16 or 17 i.e. 32 kiB, 64 kiB, or 128 kiB.) [7] "BP 9 to BP 16 - Uncompressed Size" shall tell the number of uncompressed bytes represented by the given extent. This field shall be recorded according to ISO 9660:7.3.3. (This number is the same as in header bytes 8 to 11, resp header BP 9 to BP 12.) | 'Z' | 'F' | LENGTH | 1 | 'p' | 'z' | HEADER SIZE DIV 4 | LOG2 BLOCK SIZE | UNCOMPRESSED SIZE | ISO 9660:7.3.3 means 4-byte word in both byte orders, first little endian, then big endian. Example (block size 32 kiB, uncompressed file size = 1,234,567 bytes): { 'Z', "F', 16, 1, 'p', 'z', 4, 15, 0x87, 0xD6, 0x12, 0x00, 0x00, 0x12, 0xD6, 0x87 } ------------------------------------------------------------------------------- Revoked specification aspects: A comment in zisofs-tools-1.0.8 indicates a special case of output block: "a block the length of which is equal to the block size is unencoded." This is not implemented in zisofs-tools and in the Linux kernel. Existing zisofs enhanced ISO images might contain encoded blocks which could be mistaken for unencoded blocks. Therefore this rule is not part of this description and must not be implemented. ------------------------------------------------------------------------------- References: zisofs-tools http://freshmeat.net/projects/zisofs-tools/ zlib: /usr/include/zlib.h cdrtools with mkisofs ftp://ftp.berlios.de/pub/cdrecord/alpha ECMA-119 aka ISO 9660 http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-119.pdf SUSP 1.12 ftp://ftp.ymi.com/pub/rockridge/susp112.ps RRIP 1.12 ftp://ftp.ymi.com/pub/rockridge/rrip112.ps