*** ZIP (PKZip compressed files)
*** Document revision: 1.3
*** Last updated: March 11, 2004
*** Contributors/sources: net documents
The files seen on the C64 are generally PKZIP 1.1-compatible archives,
using the older IMPLODE algorithm, which are decompressible on the C64/C128
using various utilities. All versions of PKUNZIP (and compatible programs)
will also handle the older archives. The explanation I provide below covers
up to the newest version PKZIP at the time of writing this document, 2.04g.
They always start with the 'PK' string at the beginning of the file, and
the first filename follows very closely. The example archive below holds
4-pack ZipCode files, as the filename shows:
00 01 02 03 04 05 06 07 08 09 0A 0B 0C 0D 0E 0F ASCII
----------------------------------------------- ----------------
00000: 50 4B 03 04 14 00 00 00 08 00 00 00 00 00 19 A1 PK..............
00010: EB 0D C2 45 00 00 50 69 00 00 07 00 00 00 31 21 ..............1!
00020: 4D 53 48 4F 57 E4 BC 7B 5C 53 47 DA 38 3E 67 CE MSHOW...........
Bytes: $00-03: PKZIP local file header signature ($50 $4B $03 $04, first
two bytes are ASCII "PK"). This signature is used at the
beginning of *each* compressed file.
04-05: Program version that created archive:
decimal value/10 = major version # (in this case 2)
decimal value%10 = minor version # (in this case .0)
06-07: General purpose bit flags:
bit 0: set - file is encrypted
clear - file is not encrytped
bit 1: if compression method 6 used (imploding)
set - 8K sliding dictionary
clear - 4K sliding dictionary
bit 2: if compression method 6 used (imploding)
set - 3 Shannon-Fano trees were used to encode
the sliding dictionary output
clear - 2 Shannon-Fano trees were used
For method 8 compression (deflate):
bit 2 bit 1
0 0 Normal (-en) compression
0 1 Maximum (-ex) compression
1 0 Fast (-ef) compression
1 1 Super Fast (-es) compression
Note: Bits 1 and 2 are undefined if the
compression method is any other than 6 or 8.
bit 3: if compression method 8 (deflate)
set - the fields crc-32, compressed size and
uncompressed size are set to zero in the
local header. The correct values are put
in the data descriptor immediately
following the compressed data.
The upper three bits are reserved and used internally by
the software when processing the zipfile. The remaining
bits are unused.
08-09: Compression method:
0 - Stored (no compression)
1 - Shrunk
2 - Reduced with compression factor 1
3 - Reduced with compression factor 2
4 - Reduced with compression factor 3
5 - Reduced with compression factor 4
6 - Imploded
7 - Reserved for Tokenizing compression algorithm
8 - Deflated
0A-0B: Last modified file time in MSDOS format
Bits 00-04: Seconds/2 (0-58, only even numbers)
05-10: Minutes (0-59)
11-15: Hours (0-23, no AM or PM)
0C-0D: Last modified file date in MSDOS format
Bits 00-04: Day (1-31)
05-09: Month (1-12)
10-15: Year minus 1980
0E-11: CRC-32 of file (low-high format)
12-15: Compressed size of file (low-high format)
16-19: Uncompressed size of file (low-high format)
1A-1B: Filename length (FL)
1C-1D: Extra field length, description (EFL)
1E-(1E+FL-1): Filename
(1E+FL)-(1E+FL+EFL-1): Extra field
You will notice that in the above byte layout, there is no mention of C64
filetype. That particular field seems to be stored in the central directory
at the end of the ZIP archive.
There are several other signatures used within the ZIP format. The byte
sequence 50 4B 01 02 is used to signify the beginning of the central
directory while the byte sequence 50 4B 05 06 is used to show the end of
the central directory.
The above explanation is only included for completeness. Without source
code, it is almost impossible to work with ZIP archives.
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