Documented the zisofs format by H. Peter Anvin.
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doc/zisofs_format.txt
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doc/zisofs_format.txt
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Description of the zisofs Format
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as of zisofs-tools-1.0.8 by H. Peter Anvin
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and cdrtools-2.01.01a39 by Joerg Schilling
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For libburnia-project.org by Thomas Schmitt <scdbackup@gmx.net>
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Apr 7 2009
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The zisofs format was invented by H. Peter Anvin. It compresses data file
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content, marks it by a header and provides a pointer array for coarse random
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access. Within a RRIP enhanced ISO 9660 image the format is additionally marked
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by a System Use entry with signature "ZF".
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The uncompressed size of a single zisofs compressed file is restricted
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to 4 GiB - 1. Larger files shall not be compressed.
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File Header
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The file header has this layout (quoted from zisofs-tools-1.0.8/mkzftree.c):
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Byte offset iso9660 type Contents
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0 (8 bytes) Magic number (37 E4 53 96 C9 DB D6 07)
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8 7.3.1 Uncompressed file size
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12 7.1.1 header_size >> 2 (currently 4)
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13 7.1.1 log2(block_size)
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14 (2 bytes) Reserved, must be zero
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So its size is 16.
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7.3.1 means little endian 4-byte words. 7.1.1. means unsigned single bytes.
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Block Pointers
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There are ceil(input_size / block_size) input resp. output blocks.
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Each input block is of fixed size whereas the output blocks have varying
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size (down to 0). For each output block there is an offset pointer giving
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its byte address in the overall file content. The next block pointer in the
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array tells the start of the next block which begins immediately after the
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end of its predecessor. A final pointer gives the first invalid byte address
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and thus marks the end of the last block.
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The block pointers are stored as an array of 4-byte values which are in ISO
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9660:7.3.1 format directly after the file header, i.e. beginning at byte 16.
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Data Part
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The data part begins immediately after the pointer array. In principle it
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consists of the variable length output blocks as delivered by zlib function
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compress2() when fed with the fixed size input blocks.
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A special case of output block is defined:
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- Zero-length blocks represent a block full of 0-bytes.
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They do not get processed by compress2() but have to be implemented as
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bypass of compress2().
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ZF System Use Entry Format
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ZF may only be applied to files with a single extent and less than 4 GiB of
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uncompressed size.
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The ZF entry follows the general layout of SUSP resp. RRIP.
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Its fields are:
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[1] "BP 1 to BP 2 - Signature Word" shall be (5A)(46) ("ZF").
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[2] "BP 3 - Length" shall specify as an 8-bit number the length in bytes of
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the ZF entry recorded according to ISO 9660:7.1.1.
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This length is 16 decimal.
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[3] "BP 4 - System Use Entry Version" shall be 1 as in ISO 9660:7.1.1.
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[4] "BP 5 to BP 6 - Algorithm" shall be (70)(7A) ("pz") to indicate
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"paged zlib".
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[5] "BP 7 - Header Size" shall specify as an 8-bit number the number of
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4-byte words in the header part of the file data recorded according
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to ISO 9660:7.1.1.
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(This is a copy of header byte 12, resp. header BP 13).
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[6] "BP 8 - Block Size" shall specify as an 8-bit number the binary
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logarithm of the compression block size recorded according to
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ISO 9660:7.1.1.
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(This is a copy of header byte 13, resp. header BP 14.)
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Implementations shall be able to handle values 15, 16 and 17 i.e.
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block sizes 32 kB, 64 kB, and 128 kB.
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[7] "BP 9 to BP 16 - Uncompressed Size" shall tell the number of uncompressed
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bytes represented by the given extent. This field shall be recorded
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according to ISO 9660:7.3.3.
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(This number is the same as in header bytes 8 to 11, resp header BP 9
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to BP 12.)
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| 'Z' | 'F' | LENGTH | 1 | 'p' | 'z' | HEADER SIZE | BLOCK SIZE |
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UNCOMPRESSED SIZE |
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ISO 9660:7.3.3 means 4-byte word in both byte orders, first little endian, then
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big endian. I.e. the decimal number 513 is represented in 8 bytes as:
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{ 0x02, 0x01, 0x00, 0x00, 0x00, 0x00, 0x01, 0x02 }
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Example (uncompressed file size = 1,234,567 bytes):
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{ 'Z', "F', 16, 1, 'p', 'z', 16, 15,
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0x87, 0xD6, 0x12, 0x00, 0x00, 0x12, 0xD6, 0x87 }
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-------------------------------------------------------------------------------
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Revoked specification aspects:
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The comments in zisofs-tools-1.0.8 indicate a special case of output block:
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"a block the length of which is equal to the block size is unencoded."
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This was never implemented and it is questionable whether it can be added to
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the existing implementations in an unambigous way.
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Therefore that rule is not part of this description and shall not be
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implemented.
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-------------------------------------------------------------------------------
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References:
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zisofs-tools
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http://freshmeat.net/projects/zisofs-tools/
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cdrtools with mkisofs
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ftp://ftp.berlios.de/pub/cdrecord/alpha
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ECMA-119 aka ISO 9660
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http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-119.pdf
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SUSP 1.12
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ftp://ftp.ymi.com/pub/rockridge/susp112.ps
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RRIP 1.12
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ftp://ftp.ymi.com/pub/rockridge/rrip112.ps
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zlib:
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/usr/include/zlib.h
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