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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 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 and 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 and 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 / 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 / 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 / BP 9 to BP 12.)
| 'Z' | 'F' | LENGTH | 1 | 'p' | 'z' | HEADER SIZE DIV 4 | LOG2 BLOCK 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
cdrtools with mkisofs
ECMA-119 aka ISO 9660
SUSP 1.12
RRIP 1.12
This text is under
Copyright (c) 2009 - 2010 Thomas Schmitt <>
It shall reflect the effective technical specifications as implemented in
zisofs-tools and the Linux kernel. So please contact mailing list
<> or to the copyright holder in private, if you
want to make changes.
Only if you cannot reach the copyright holder for at least one month it is
permissible to modify and distribute this text under the license "BSD revised".