ZIP File Format Summary
                         =======================

This summarises the ZIP File Format described in various places such as
http://mdfs.net/Docs/Comp/Archiving/Zip/Format and elsewhere. Values in
brackets are values to use to create an generally-compatible archive.


ZIP Data Section
----------------
Files are stored in a ZIP file one after another, with a ZIP local header
before each file:

          0       file header id "PK",&03,&04           4 bytes
          4       version needed to extract (eg &0000)  2 bytes
          6       general purpose bit flag (eg &0000)   2 bytes
          8       compression method                    2 bytes
         10       last modified time in DOS format      2 bytes
         12       last modified date in DOS format      2 bytes
         14       crc-32                                4 bytes
         18       compressed size (n)                   4 bytes
         22       uncompressed size                     4 bytes
         26       filename length (n)                   2 bytes
         28       extra field length (e)                2 bytes
         30       filename                              n bytes
         30+n     extra field                           e bytes
         30+n+e   data                                  l bytes
         30+n+e+l ...

The filename is stored in Unix Format: directories are seperated by '/'s.
Directory entries have '/' at the end of their filename and compressed and
uncompressed sizes of zero. ZIP filenames and BBC filenames are converted
bidirectionally by swapping the following characters:

/ <-> .    ? <-> #    $ <-> <    ^ <-> >    @ <-> =    & <-> +    % <-> ;

On extracting, BBCUnZip converts spaces in filenames to '_'s.

The 'Extra' field holds additional information specific to the data. The
Acorn extra field has the following contents:

         0  extra header id              2 bytes  "AC"
         2  extra header sublength       2 bytes  ------------v
         4  Acorn header id              4 bytes  "ARC0"      4
         8  load address                 4 bytes              8
        12  execution address            4 bytes             12
        16  attributes                   4 bytes             16
        20  &00000000                    4 bytes             20
        24  creation time                2 bytes             22
        26  creation date                2 bytes             24
        28  main account number          2 bytes             26
        30  auxilary account number      2 bytes             28

On extraction, only that data present in the extra field should be written
to the extracted files or directories. If there is no extra field it should
be ignored or suitable defaults from the zip header used:

  extra header sublength:   >27   >25   >23    >15    >11    >7      <8

  load address             load  load  load   load   load   load      0
  execution address        exec  exec  exec   exec   exec   load      0
  attributes               attr  attr  attr   attr    &33    &33     &33
  creation time            ctime ctime ctime  mtime  mtime  mtime   mtime
  creation date            cdate cdate cdate  mdate  mdate  mdate   mdate
  main account number      acc   acc   ignore ignore ignore ignore  ignore
  auxilary account number  aux   acc   ignore ignore ignore ignore  ignore

BBCUnZip only sets file ownership if the -X option is used.

BBCZip can create 'envelopes' where the archive contains all the file
metadata but no contents. This is indicated with compression method 255
and the uncompressed file length is zero.


ZIP Directory (Catalog) Section
-------------------------------
Following all the stored data there is usually a Central Directory, which
repeats most of the ZIP header information:

          0       dir header id "PK",&01,&02            4 bytes
          4       version made by                       2 bytes
          6       version needed to extract             2 bytes -------+
          8       general purpose bit flag              2 bytes        |
         10       compression method                    2 bytes        |
         12       last modiication time, DOS format     2 bytes As in  |
         14       last modification date, DOS format    2 bytes local  |
         16       crc-32                                4 bytes header |
         20       compressed size (l)                   4 bytes        |
         24       uncompressed size                     4 bytes        |
         28       filename length (n)                   2 bytes        |
         30       extra field length (e)                2 bytes -------+
         32       file comment length (c)               2 bytes
         34       disk number start (eg &0000)          2 bytes
         36       internal file attributes (eg &0000)   2 bytes
         38       external file attributes (eg &0000)   4 bytes
         42       relative offset of local header       4 bytes
         46       filename                              n bytes
         46+n     extra field (variable size)           e bytes
         46+n+e   file comment                          c bytes
         46+n+e+c ...

'Version made by' and 'Version needed to extract' can both be zero.


ZIP End Of File
---------------
The final entry is usually an End Of File header:

        0        end of file id, "PK",&05,&06           4 bytes
        4        number of this disk (eg &0000)         2 bytes
        6        number of the disk with the start
                 of the central directory (eg &0000)    2 bytes
        8        total number of entries in
                 the central dir on this disk           2 bytes
        10       total number of entries in
                 the central dir                        2 bytes
        12       size of the central directory          4 bytes
        16       offset of start of central
                 directory with respect to
                 the start of the starting disk         4 bytes
        18       zipfile comment length                 2 bytes
        20       zipfile comment                        c bytes
        20+c


ZIP header IDs
--------------
"PK",&01,&02 - Central directory
"PK",&03,&04 - Local header
"PK",&05,&06 - End of File (end of central directory)
"PK",&07,&08 - Data descriptor
"PK",&05,&05 - Digital signature
"PK",&06,&06 - Zip64 end of central directory
"PK",&06,&07 - Zip64 end of central directory locator
"PK",&06,&08 - Zip64 extra data record
"PK",&30,&30 - Temporary spanning marker
"PK",&0F,&10 - Temporary build header, "OP" - file still open


References
----------
* http://mdfs.net/Docs/Comp/Archiving/Zip/Format
* http://mdfs.net/Docs/Comp/Archiving/Zip/ExtraField
* http://mdfs.net/Apps/Archivers/BBCZip