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Documented the zisofs format by H. Peter Anvin.

Thomas Schmitt 13 years ago
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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 <>
Apr 7 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.
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.
The block pointers 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 output block is defined:
- Zero-length blocks represent a block full of 0-bytes.
They do not get processed by compress2() but have to be implemented as
bypass of compress2().
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" 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 - 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.)
Implementations shall be able to handle values 15, 16 and 17 i.e.
block sizes 32 kB, 64 kB, and 128 kB.
[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 | BLOCK SIZE |
ISO 9660:7.3.3 means 4-byte word in both byte orders, first little endian, then
big endian. I.e. the decimal number 513 is represented in 8 bytes as:
{ 0x02, 0x01, 0x00, 0x00, 0x00, 0x00, 0x01, 0x02 }
Example (uncompressed file size = 1,234,567 bytes):
{ 'Z', "F', 16, 1, 'p', 'z', 16, 15,
0x87, 0xD6, 0x12, 0x00, 0x00, 0x12, 0xD6, 0x87 }
Revoked specification aspects:
The comments in zisofs-tools-1.0.8 indicate a special case of output block:
"a block the length of which is equal to the block size is unencoded."
This was never implemented and it is questionable whether it can be added to
the existing implementations in an unambigous way.
Therefore that rule is not part of this description and shall not be
cdrtools with mkisofs
ECMA-119 aka ISO 9660
SUSP 1.12
RRIP 1.12