155 lines
6.0 KiB
Plaintext
155 lines
6.0 KiB
Plaintext
|
|
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 <scdbackup@gmx.net>
|
|
- 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
|
|
|
|
-------------------------------------------------------------------------------
|
|
|
|
This text is under
|
|
Copyright (c) 2009 - 2010 Thomas Schmitt <scdbackup@gmx.net>
|
|
It shall reflect the effective technical specifications as implemented in
|
|
zisofs-tools and the Linux kernel. So please contact mailing list
|
|
<libburn-hackers@pykix.org> 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".
|
|
|